Python Loops

for-loops

• used for iterating: going through an iterable one by one

'for-each' loop example

name = 'Ted'
for character in name:
    print(character)

will print

T
e
d

• can also be used for loops, tuples, and dictionaries (will print the keys)
• can use to change a mutable iterable
• can use to move data between lists
• 'for-each' example:

  tv = ['Got', 'Narcos', 'Vice']
  coms = ['Arrested Development', 'Friends', 'Always Sunny']
  all_shows = []
  
  for show in tv:
    show = show.upper()
    all_shows.append(show)  
  
  for show in coms:
    show = show.upper()
    all_shows.append(show)

  now all_shows is now ['GOT','NARCOS', 'VICE', 'ARRESTED DEVELOPMENT', 'FRIENDS', 'ALWAYS SUNNY']

###'for' loop example

for i in range(1,11):
print(i)

prints:

1
2
3
4
5
6
7
8
9
10

• if starting at index 0, you only have to give it the ending index
• second param is not inclusive

while-loops

• executes code as long as an expression evaluates to True
• example:

  • x = 10
    while x > 0:
        print('{}'.format(x))
        x -= 1
    print('Happy New Year!'

    prints

    10
    9
    8
    7
    6
    5
    4
    3
    2
    1
    Happy New Year!

• a loop with an expression that is always 'True' is an infinite loop

break and continue

• break makes a loop end immediately
  • often used in if statements so the loop will continuously from until a particular condition occurs
• continue ends current iteration of loop and moves to the next iteration
  • example:

    for i in range(1,6):
        if i == 3:
            continue
        print(i)

    prints

    1
    2
    4
    5

Nested Loops

• loops that are inside other loops
• called inner and outer loops
• want to limit the number of loops that are nested, it can easily get very complicated
• example:

  for i in range(1, 3):
    print(i)
    for letter in ['a', 'b', 'c']
        print(letter)

  prints

  1
  a
  b
  c
  2
  a
  b
  c

• example:

  list1 = [1, 2, 3, 4]
  list2 = [5, 6, 7, 8]
  added = []
  for i in list1:
      for j in list2:
          added.append(i + j)
  print(added)

  prints

  [6, 7, 8, 9, 7, 8, 9, 10, 8, 9, 10, 11, 9, 10, 11, 12]

• example:

  while input('y or n?') != 'n':
    for i in range(1,6):
        print(i)

  prints

  y or n?y
  1
  2
  3
  4
  5
  y or n?y
  1
  2
  3
  4
  5
  y or n?n