12 Loop

12.1 For Loop

Common iterables that can be used in loop:

range
str: iterate through each characters
list

for i in "python":
  print(i)
#> p
#> y
#> t
#> h
#> o
#> n

for i in [1, 2]:
  print(i)
#> 1
#> 2

range() is very helpful

range(start, stop, step)

start is inclusion
stop is exclusive

type(range(1))
#> <class 'range'>

list(range(2))
#> [0, 1]
list(range(1, 3))
#> [1, 2]
list(range(1, 10, 2))
#> [1, 3, 5, 7, 9]

for number in range(3):
  print(number)
#> 0
#> 1
#> 2

for number in range(1, 4):
  print("Attempt: ", number, number * ".")
#> Attempt:  1 .
#> Attempt:  2 ..
#> Attempt:  3 ...

12.1.1 For else

The break statement, breaks out of the innermost enclosing for or while loop

else is executed when the loop terminates through exhaustion of the iterable, but not when the loop is terminated by a break statement.

From: Ref

successful = False

for number in range(1, 4):
  print("Attempt: ", number)
  if successful:
      print("Success")
      break
else:
  print("Attempt 3 times and failed")
#> Attempt:  1
#> Attempt:  2
#> Attempt:  3
#> Attempt 3 times and failed

12.1.2 Nested Loop

for x in range(3):
  for y in range(2):
    print(f"({x},{y})")
#> (0,0)
#> (0,1)
#> (1,0)
#> (1,1)
#> (2,0)
#> (2,1)

count = 0

for n in range(1, 10):
  if (n % 2) == 0:
    print(n)
    count += 1
#> 2
#> 4
#> 6
#> 8
    
print(f"We have {count} even numbers")
#> We have 4 even numbers

10 % 2
#> 0
10 % 3
#> 1

12.1.3 Enumerate

flavor_list = ["vanilla", "chocolate", "pecan", "strawberry"]

Using range(len())

for i in range(len(flavor_list)):
    flavor = flavor_list[i]
    print(f"{i + 1}: {flavor}")
#> 1: vanilla
#> 2: chocolate
#> 3: pecan
#> 4: strawberry

Using enumerate() (better)

for i, flavor in enumerate(flavor_list):
    print(f"{i + 1}: {flavor}")
#> 1: vanilla
#> 2: chocolate
#> 3: pecan
#> 4: strawberry

Prefer enumerate instead of looping over a range and indexing into a sequence.
You can supply a second, optional parameter to enumerate that specifies the beginning number for counting (zero is the default).

12.2 While Loop

number = 100
while number > 0:
  number //= 2
  print(number)
#> 50
#> 25
#> 12
#> 6
#> 3
#> 1
#> 0