for loops

for loops allow one to iterate (repeat) over a collection or sequence. Abstractly all for loops share this structure:

for item in collection:
  do something

But its often easier to understand them in action.

For Example

x = "Mary had a little lamb"
x = x.split(' ')

for i in x:
    print(i, len(i))

Above we store a sample sentence at x and then split that sentence by space, producing a list of words that we then store at x (overwriting the original sentence in the process). Next we enter the for loop, which could be read as follows: for every word (i) in the list (x) print the word (i) and its length (or number of letters).

for loops with range()

One can also use a for loop to count by iterating across a range.

A single value in range is interpreted as the stop value (the start value defaults to 0 in this case).

For Example

for i in range(5):
  print(i)

range(x,y), however, allows one to specify the start value and the stop value.

For Example

for i in range(5, 10):
  print(i)

Optionally one can specify the step value by setting the third parameter.

For Example

for i in range(0,15,3):
   print(i)

In the above example we increment, or add, by 3 until we reach the number 15 (starting from 0).

Decrementing, or counting down, is accomplished by setting the start value high and step to a negative value.

For Example

for i in range(1024,-1,-1):
   print(i)

Above we start at 1024 and count down, by -1, until we reach -1.