Intro To Lists Loops And Functions

Lists

Lists are one of the most common data structures in Python.

Features:

• Mutable

• Members do not have to be same type

• Ordered

sample_list = ['hello', 4, False]

Useful operations:

• len(list) returns number of members

• list[0] returns member at index

  • Note: Accessing members at an index that doesn't exist will throw an IndexError

• list.index('dog') returns the index of the value

• list.append(item) adds item to the end

• list.pop() removes and returns last item

• list.pop(1) removes and returns item at given index

• item in list checks if item is a member of the list and returns boolean

Loops

Loops repeat a process/operation multiple times.

for loops run x amount of times (whatever you decide), and while loops run indefinitely as long as a condition is met.

for

Iterating over a list:

numbers = [1, 2, 3, 4, 5]
for number in numbers:
  print(number)

Iterating over a string:

word = "Hello"
for letter in word:
  print(letter)

Iterating over a range of integers (useful when you want to run a code block a specific number of times):

for x in range(10):
  print(x) # prints 0 to 9
  
for x in range(1, 7):
  print(x) # prints 1 to 6
  
for x in range(0, 31, 5):
  print(x) # prints 5, 10, 15, 20, 25, 30
  
for x in range(5, -1, -1):
  print(x) # prints 5 to 0

while

Just be careful with a while loop because if the condition is never met, it never runs. And if the condition is always met, you'll run into an infinite loop that crashes your program.

Pro tip: A common use case is to continually request a specific input, re-asking until the user actually provides it.

userInput = ""
while userInput != "hello":
  userInput = input("Please say 'hello'")

print("You said hello! Thank you.")

To exit a while loop completely, simply use the break keyword.

To move onto the next iteration in a while loop, simply use the continue keyword.

Functions

Functions are just blocks of code that can accept an input, performs some computation, and then can return an output. They are useful for code reuse and help make your application more modular.

Built-in functions

Functions that are part of the core language. Python just hands them to you automatically.

User-defined functions

To create your own function, here's the basic syntax:

def function_name(param1, param2, paramN):
  optionalOutput = ""
  # computations occur here
  return optionalOutput

Pro tip: It's recommended to include a docstring as the first comment in your user-defined function. This description is accessible to any other dev via help(fn) or fn.__doc__.

def fn():
  """This is where the docstring lives"""
  # ...

The main function

Every Python module (i.e. file) has a special __name__ variable that's accessible.

• If the file is being run as the main program, __name__ is set to "__main__"

• For all other modules, __name__ is set to the module's filename

Pro tip: Best practice when running a main program is to use the following code pattern:

def main():
  # Entry point for program
  
if __name__ == "__main__":
  main()

Documentation (for users and programmers)