For Loops

One of the most fundamental processes in a computer program is to repeat statements many (perhaps many, many, many) times. Computers never run out of patience.

Like most languages, Python has two major types of loops.

For loops execute for a fixed number of iterations. It is possible to exit early, but this must be explicitly added to the code.

While loops do not start with a predetermined number of iterations. They terminate when some condition becomes False.

For Loops in Python

for item in iterator:

The else clause is optional and not frequently used; it is executed if the loop completes all the steps. Colons introduce a code block and are required as indicated. The code blocks must be indented. The item is a variable which successively takes on the values in the iterator.

Iterables and Iterators

An iterable is any data structure that can step through items one at a time. An iterator is the structure by which an iterable is traversed. Some data structures, such as range, are direct iterators, whereas others, such as strings, are iterables with the ability to be iterated in a for loop.


The range iterator is used to step over a sequence of numbers and is very frequently used with for loops. It takes up to three arguments.

  • range(10) : 0,1,2,3,4,5,6,7,8,9
  • range(1,10) : 1,2,3,4,5,6,7,8,9
  • range(0,10,2) : 0,2,4,6,8
  • range(10,0,-2) : 10,8,6,4,2 (note that zero is not included)

The interval is often called a stride. If it is present the lower bound must also be present even if it is the default, 0. Otherwise the lower bound may be omitted. If the stride is omitted it is 1. The last value is never reached.

In Python 3 the range function returns an iterator object and is not directly accessible. To see the values or assign them to a variable, convert it to a list



Execute the following for loop:

for i in range(10):

Modify this loop to print the values of i for


Modify your loop to print the first N integers. Be sure that N is set to a value before you try to run the loop.

Write a loop that will sum the first N integers. Hint: you will need a variable called an accumulator whose value is assigned outside the loop to 0.

Example solution


for i in range(N+1):

for i in range(1,N+1):

print("The sum of 1 to ",N," is ",sum_it)

Other Iterables

Any ordered sequence can be iterated. Common types used with for loops are lists and strings.

for player in cast:
    print("One of the Pythons:",player)

Another example:

for ch in "HelloEverybody":


Sometimes we need both the item and its index. We can use the enumerate function for this purpose. Enumerate returns an iterator each element of which is a tuple consisting of a count, by default starting at 0, and the corresponding value of its argument.

for i,v in enumerate(velocity):


The zip() function is also handy, especially but not limited to, for loops. Zip accepts multiple iterables and returns a new iterable whose elements are tuples of corresponding items from its arguments.

x = [20.,9.,6.,5.,6.,8.3]
for t in zip(x,velocity):

When using zip there is a risk that the iterables will not be the same length. The default is for the function to stop when it reaches the end of its shortest argument, but this often masks a bug. Python 3.10 introduced the strict parameter to zip. If the strict=True option is added, it will throw an error if the lengths do not match.

#Must be 3.10 or greater
x = [20.,9.,6.,5.,6.,8.3,12.3]
for t in zip(x,velocity,strict=True):