401 lines
8.4 KiB
Markdown
401 lines
8.4 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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# 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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## 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) Use `print(list(var))` when printing.
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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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### 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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# Fun Projects
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Design a shop using Ex7 and Ex9 as a frame:
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7:
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```
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money_in_wallet = 40
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sandwich_price = 7.50
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sales_tax = .08 * sandwich_price
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sandwich_price += sales_tax
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money_in_wallet -= sandwich_price
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```
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9:
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```
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cucumbers = 1
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price_per_cucumber = 3.25
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total_cost = cucumbers * price_per_cucumber
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print(total_cost)
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```
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`total_price += nice_sweater`
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Cool concept from Ch2 Ex15:
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```
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name = raw_input("What is your name? ")
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quest = raw_input("What is your quest? ")
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color = raw_input("What is your favorite color? ")
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print "Ah, so your name is %s, your quest is %s, " \
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"and your favorite color is %s." % (name, quest, color)
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```
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