MacLochlainns Weblog

It’s always interesting to explain a new programming language to students. Python does presents some challenges to that learning process. I think for-loops can be a bit of a challenge until you understand them. Many students are most familiar with the traditional for loop like Java:

for (i = 0; i < 5; i++) { ... }

Python supports three types of for-loops – a range for loop, a for-each expression, and a for-loop with enumeration. Below are examples of each of these loops.

1. A range for-loop goes from a low numerical value to a high numerical value, like:

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

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

It prints the following range values:

0 1 2

0 1 2

2. A for-each loop goes from the first to the last item while ignoring indexes, like:

list = ['a','b','c'] for i in list: print i

list = ['a','b','c'] for i in list: print i

It prints the following elements of the list:

a b c

a b c

3. A for-loop with enumeration goes from the first to the last item while ignoring indexes, like:

list = ['a','b','c'] for i, e in enumerate(list): print "[" + str(i) + "][" + list[i] + "]"

list = ['a','b','c'] for i, e in enumerate(list): print "[" + str(i) + "][" + list[i] + "]"

The i represents the index values and the e represents the elements of a list. The str() function casts the numeric value to a string.

It prints the following:

[0][a] [1][b] [2][c]

[0][a] [1][b] [2][c]

This should help my students and I hope it helps you if you’re trying to sort out how to use for loops in Python.