675 lines
15 KiB
Markdown
675 lines
15 KiB
Markdown
# Codecademy's Python 2 Course
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https://www.codecademy.com/courses/learn-python/lessons/python-syntax/
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It's gratis and accepts Python 3 syntax.
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# Object-oriented
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"The main goal of an object oriented language is to make code reusable – we do this through the use of classes and objects. If we want to design a new type of car, we can start with what they all have in common: wheels, seats, a frame. Now that we’ve determined what cars have in common, we can more easily implement any type of car we want by starting from that basic blueprint."
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https://discuss.codecademy.com/t/what-does-it-mean-that-python-is-an-object-oriented-language/297314
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# Errors (ex6, CH1, P3)
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"SyntaxError means there is something wrong with the way your program is written — punctuation that does not belong, a command where it is not expected, or a missing parenthesis can all trigger a SyntaxError.
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A NameError occurs when the Python interpreter sees a word it does not recognize. Code that contains something that looks like a variable but was never defined will throw a NameError."
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SyntaxError example: `SyntaxError: EOL while scanning string literal`
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# Linear Regression
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One of the projects in my Python 3 course on Codecademy was to calculate the linear regression of any given line in a set. This is my journey of doing just that, following their instructions. The goal, is to calculate the bounciness of different balls with the least error possible.
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Starting with `y = m*x + b`
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We can determine the y of a point pretty easily if we have the slope of the line (m) and the intercept (b). Thus, we can write a basic function to calculate the y:
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```
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def get_y(m, b, x):
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y = m*x + b
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return y
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print(get_y(1, 0, 7) == 7)
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print(get_y(5, 10, 3) == 25)
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```
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We can then use that to calculate the linear regression of a line:
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```
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def calculate_error(m, b, point):
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x_point = point[0]
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y_point = point[1]
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y2 = get_y(m, b, x_point)
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y_diff = y_point - y2
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y_diff = abs(y_diff)
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return y_diff
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```
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To get a more accurate result, we need a function that will parse several points at a time:
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```
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def calculate_all_error(m, b, points):
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totalerror = 0
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for point in points:
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totalerror += calculate_error(m, b, point)
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return abs(totalerror)
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```
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We can test it using their examples:
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```
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#every point in this dataset lies upon y=x, so the total error should be zero:
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datapoints = [(1, 1), (3, 3), (5, 5), (-1, -1)]
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print(calculate_all_error(1, 0, datapoints))
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#every point in this dataset is 1 unit away from y = x + 1, so the total error should be 4:
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datapoints = [(1, 1), (3, 3), (5, 5), (-1, -1)]
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print(calculate_all_error(1, 1, datapoints))
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#every point in this dataset is 1 unit away from y = x - 1, so the total error should be 4:
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datapoints = [(1, 1), (3, 3), (5, 5), (-1, -1)]
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print(calculate_all_error(1, -1, datapoints))
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#the points in this dataset are 1, 5, 9, and 3 units away from y = -x + 1, respectively, so total error should be
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# 1 + 5 + 9 + 3 = 18
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datapoints = [(1, 1), (3, 3), (5, 5), (-1, -1)]
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print(calculate_all_error(-1, 1, datapoints))
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```
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You can save all possible m and b values between -10 and 10 (m values), as well as -20 and 20 (b values) using:
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```
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possible_ms = [mv * 0.1 for mv in range(-100, 100)] #your list comprehension here
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possible_bs = [bv * 0.1 for bv in range(-200, 200)] #your list comprehension here
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```
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We can find the combination that produces the least error, which is:
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```
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m = 0.3
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b = 1.7
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x = 6
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```
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The goal was to calculate the bounciness of different balls with the least error possible. With this data, we can calculate how far a given ball would bounce. For example, a 6 cm ball would bounce 3.5 cm. We know this because we can plug in the numbers like this:
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```
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get_y(0.3, 1.7, 6)
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```
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# Math (ex6)
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```
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mirthful_addition = 12381 + 91817
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amazing_subtraction = 981 - 312
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trippy_multiplication = 38 * 902
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happy_division = 540 / 45
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sassy_combinations = 129 * 1345 + 120 / 6 - 12
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exponents = (16 ** 0.5) # 16 to the 1/2th power. (4)
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remainder = (15 % 2) # The remainder (and thus the result) equals 1
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```
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## Find the remainder of a number using %
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```
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is_this_number_odd = 15 % 2
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is_this_number_divisible_by_seven = 133 % 7
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```
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# Updating variables / operators.
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```
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sandwich_price += sales_tax
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```
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is the same as:
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```
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sandwich_price = sandwich_price + sales_tax
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```
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but is much shorter.
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## Comments
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Are indicated by # or """This is not for running"""
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# Numbers
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An integer is like `5`, a float is a number with a decimal point like `5.0`. They can also be in scientific notation like `2.3e7`
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In Python 2, you need to make sure math like `7/2` = `3.5` is correct is by inputting it into Python like `7./2.` or `float(7)/2`
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## Limitations of floats
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Floats are limited by the number of digits. For example `1/3 = 0.3`
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```
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>>> format(math.pi, '.12g') # give 12 significant digits
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'3.14159265359'
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>>> format(math.pi, '.2f') # give 2 digits after the point
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'3.14'
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```
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# Strings
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Multi-line strings are marked by
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```"""
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Mulit-
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line
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strings"""
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```
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# Booleans (True/False)
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True = int(1)
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False = int(0)
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## Boolean Expressions
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`and` evaluates true if both are true.
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`or` evaluates true if either are true.
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`not` return the opposite boolean value.
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## Relational Operators (ch.4, ex. 3)
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`==` returns `True` if is is equal
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`!=` returns `True` if is is NOT equal
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Here are more of he same kind of operator:
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`>` greater than
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`<` less than
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`>=` greater than or equal to
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`<=` less than or equal to
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# if, else, if else
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`elif` = if else
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# Datatypes
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Force treating as a string: str(7)
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Force treating as an integer: int("7")
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Force treating as a float: float(7)
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## Check Datatypes
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Check datatypes using type(var)
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# Escaping Characters
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Simply add a `\` to escape a character that would otherwise cause issues.
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# Arrays / Indexes
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`cows = "cows"[0]`
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This sets the variable `cows` to the 0th letter of the string `"cows"` which is `c`. These indexes start at 0, not 1.
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# Strings
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## String Methods
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`len(var)` Get length of string.
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`var.lower()` Force lowercase
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`var.upper()` Force uppercase
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`str(var)` Force treating variable as a string.
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If it uses dot notation like `.lower()`, it works exclusively on strings.
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## Concatenation
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`"Ten times a cow is equal to " + result + " with 10 times as many breeding opportunities."`
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or
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`print(var, var2, var3)`
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or
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`string1 += string2`
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## String Formatting with %
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`"%s %s - 2020" % (month, day) # Replace %s with a variable. First the month, then the day.`
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`Add %03d to specify a signed integer padded 2 places with zeros. For example, 2 becomes 02.`
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This is super useful for displaying dates like this: `print("%02d-%02d-%02d") % (now.month, now.day, now.year)` or time like this: `print '%02d:%02d:%04d' % (now.hour, now.minute, now.second)` (Ch3, Ex. 4)
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## Date and Time (Ch3)
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Grab the current time:
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```
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from datetime import datetime
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now = datetime.now()
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year = now.year
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month = now.month
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day = now.day
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```
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# Function P3 Ch.2
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## Defining a Function
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```
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def greet_customer():
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print("Welcome!")
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```
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## Calling Functions
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```
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greet_customer()
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```
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or if it has parameters:
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```
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greet_customer(1,ten)
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```
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# Passing Arguments
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```
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greet_customer(special_item):
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print(special_item)
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greet_customer(beef)
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```
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Result:
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```
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beef
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```
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## Using Keyword Arguments
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Keyword arguments are nice for specifying a default but changeable argument.
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Here's an example from P3, Ch2, ex7
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```
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def create_spreadsheet(title, row_count = 1000):
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row_count = str(row_count)
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print("Creating a spreadsheet called " + title + " with " + row_count +" rows.")
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create_spreadsheet("Applications", row_count = 10)
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```
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```
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row_count = 1000
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```
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is the default
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```
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row_count = 10
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```
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is the passed argument and thus what is used for a result:
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```
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Creating a spreadsheet called Applications with 10 rows.
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```
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## Returning Stuff
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You can return stuff like this to store for later:
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```
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def addfour(number, cow):
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addedfour = number + 4
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cat = number - 4
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return addedfour, cat # All returned arguments must be on the same return call.
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```
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I'll make it add four to 456
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```
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yo, cow = addfour(456)
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print ("456 + 4 equals " + str(yo) )
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```
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```
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460
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```
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You can also do this with multiple arguments:
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```
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x_squared, y_squared = square_point(1, 3)
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```
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# Lists
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You can put either strings, integers or other lists in a list, probably other things too.
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`heights = [['Jenny', 61], ['Alexus', 70], ['Sam', 67], ['Grace', 64]]`
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Use zip to combine elements from two lists: `zip(names, dogs_names)` would match dogs and their owners.
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But don't forget to turn it into a list before printing `print(list(zipvar))`
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You can append values to lists like this:
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```
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orders = ['daisies', 'periwinkle']
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orders.append('tulips')
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```
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You can add lists like this: `['list1', 'stuff'] + ['list2', 'stuff']` but need to use `lst1 + lst2` and not `[lst1] + [lst2]`
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# List Comprehensions
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Say you only want to allow people over the height of 161 to ride your roller coaster for safety reasons. You could do this to create a list of only those heights about 161.
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```
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heights = [161, 164, 156, 144, 158, 170, 163, 163, 157]
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can_ride_coaster = []
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can_ride_coaster = [height for height in heights if height > 161]
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print(can_ride_coaster)
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```
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You can manipulate lists with list comprehensions as well:
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```
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fahrenheit = [celsius * (9 / 5) + 32 for celsius in celsius]
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```
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### Print every other element
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```
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odd = [odd for odd in lst if lst.index(odd) % 2 == 1]
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return odd
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```
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### Print manipulated numbers in for loop
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```
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single_digits = range(10)
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squares = []
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cubes = [(num ** 3) for num in single_digits]
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for single_digits in single_digits:
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print(single_digits)
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squares.append(single_digits ** 2)
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print(squares)
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print(cubes)
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```
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# For example, printing all cuts under $30
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```
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cuts_under_30 = [hairstyles[i] for i in range(0, (len(hairstyles) - 1)) if new_prices[i] < 30]
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```
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### Another example
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```
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[lst.append("hi, " + name)]
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```
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## Make sure starting number is uneven
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```
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def delete_starting_evens(lst):
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while len(lst) > 0 and lst[0] % 2 == 0:
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lst.remove(lst[0])
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return lst
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#Uncomment the lines below when your function is done
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print(delete_starting_evens([4, 8, 10, 11, 12, 15]))
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print(delete_starting_evens([4, 8, 10]))
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```
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## Ranges
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You can get a range of numbers using `range()`
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It will give you all numbers under the number you input. For example, `range(8)` would give you 0-7, `range(1, 8)` would give you 1-7, and `range(1, 8, 2)` would give you 1,3,5,7 (2 is the interval and needs no padding) Use `print(list(var))` when printing.
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### Ranges and list manipulation
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You can combine `range()` with list manipulation like this:
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```
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for index in range(1, len(lst), 2):
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new_list.append(lst[index])
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```
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## length
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Grab length with `len(list)`
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# Selecting List elements
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Use: `listname[indexnumber]`
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The index starts at 0. Grab the last in an index using `list[-1]`
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## Giving values to list elements
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`lst[index] = indexlst`
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## Cutting the middle of a list (ex3, functions + lists, ch.4)
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```
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def remove_middle(lst, start, end):
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end = end + 1
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return lst[:start] + lst[end:]
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print(remove_middle([4, 8, 15, 16, 23, 42], 1, 3))
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```
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This prints `[4, 23, 42]` removing the index 1-3.
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# If something occurs more than N times, return True
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```
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def more_than_n(lst, item, n):
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if lst.count(item) > n:
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return True
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else:
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return False
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print(more_than_n([2, 4, 6, 2, 3, 2, 1, 2], 2, 3))
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```
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## More frequency analysis
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```
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def more_frequent_item(lst, item1, item2):
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if (lst.count(item1)) > (lst.count(item2)):
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return item1
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if lst.count(item2) > lst.count(item1):
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return item2
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if lst.count(item1) == lst.count(item2):
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return item1
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#Uncomment the line below when your function is done
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print(more_frequent_item([2, 3, 3, 2, 3, 2, 3, 2, 3], 2, 3))
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```
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## A creative original solution to grabbing the middle of an index (solving their problem with their names)
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```
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def middle_element(lst):
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if len(lst) % 2 > 0:
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rounded = round(len(lst) / 2)
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rounded = int(rounded)
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print(rounded)
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return lst[rounded]
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elif len(lst) % 2 == 0:
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position = int((len(lst) / 2) - 1)
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position2 = int(len(lst) / 2)
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both = (lst[position] + lst[position2]) / 2
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return both
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print(middle_element([5, 2, -10, -4, 4, 5, 7]))
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```
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### Add the last two elements, append them. (Ex.8)
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```
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def append_sum(lst):
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trip = 0
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while trip < 3:
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lasttwo = lst[-2:]
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added = lasttwo[0] + lasttwo[1]
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print(added)
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lst.append(added)
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trip = trip + 1
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return lst
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#Uncomment the line below when your function is done
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print(append_sum([1, 1, 2]))
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```
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### Sublists
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Grab a subset of a list using `sublist = letters[1:6]` This would give you index **1-5**.
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You can also do `[:5]` for all up to index 4, and `[5:]` for all after index 5. And, you can do `[-3:]` for the last 3 in an index.
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## Counting frequency of elements in a list.
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`var.count('stringtofind')`
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## Sorting strings alphabetically
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```
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var.sort()
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print(var)
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```
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or use
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```
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sortedvar = sorted(var)
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```
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to produce a new list with sorted contents without changing the original variable.
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## Sorting strings based on an element in a sublist
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Replace 1 with the index you want to sort by.
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`pizzas.sort(key=lambda x: x[1])``
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# Catch Errors
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```
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try:
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command_may_cause_error
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except NameOfError:
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print("Nice little message to user.")
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```
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# Loops
|
||
The following assigns an arbitrary name to a temporary variable (tempvar) to the number of elements in `sport_games` For every element in `sport_games`, it will print `print(tempvar)`
|
||
|
||
```
|
||
for tempvar in sport_games:
|
||
print(tempvar)
|
||
```
|
||
|
||
You can use `range()` to get lists of arbitrary lengths.
|
||
|
||
You can also add lists like this:
|
||
|
||
```
|
||
students_period_A = ["Alex", "Briana", "Cheri", "Daniele"]
|
||
students_period_B = ["Dora", "Minerva", "Alexa", "Obie"]
|
||
|
||
for student in students_period_A:
|
||
students_period_B.append(student)
|
||
```
|
||
|
||
## Iterating until we find something
|
||
|
||
```
|
||
for thing in stuff:
|
||
if thing == thing_I_want:
|
||
print("Found it!")
|
||
break # Stops the loop, executes code outside.
|
||
```
|
||
|
||
## Skip a value in a list.
|
||
|
||
Ex.5
|
||
|
||
```
|
||
for age in ages:
|
||
if age < 21:
|
||
continue
|
||
else:
|
||
print(age)
|
||
```
|
||
|
||
## Move from list to list
|
||
Ex.7
|
||
```
|
||
while len(students_in_poetry) < 6:
|
||
student = all_students.pop(-1) # Remove last element
|
||
students_in_poetry.append(student) # Add it to students_in_poetry
|
||
|
||
print(students_in_poetry)
|
||
```
|
||
|
||
# Nested Loops
|
||
|
||
```
|
||
for location in sales_data:
|
||
print(location)
|
||
for sub in location:
|
||
scoops_sold += sub
|
||
```
|
||
|
||
# Fun Projects
|
||
|
||
Design a shop using Ex7 and Ex9 as a frame:
|
||
|
||
7:
|
||
```
|
||
money_in_wallet = 40
|
||
sandwich_price = 7.50
|
||
sales_tax = .08 * sandwich_price
|
||
|
||
sandwich_price += sales_tax
|
||
money_in_wallet -= sandwich_price
|
||
```
|
||
|
||
9:
|
||
|
||
```
|
||
cucumbers = 1
|
||
price_per_cucumber = 3.25
|
||
total_cost = cucumbers * price_per_cucumber
|
||
print(total_cost)
|
||
```
|
||
|
||
`total_price += nice_sweater`
|
||
|
||
Cool concept from Ch2 Ex15:
|
||
|
||
```
|
||
name = raw_input("What is your name? ")
|
||
quest = raw_input("What is your quest? ")
|
||
color = raw_input("What is your favorite color? ")
|
||
|
||
print "Ah, so your name is %s, your quest is %s, " \
|
||
"and your favorite color is %s." % (name, quest, color)
|
||
```
|