Python Open Labs: March 26, 2018

Over the past few weeks, students have been learning how to iterate through items – whether they may be in strings, lists, tuples, or dictionaries. Students have mainly been using for loops to grab the value of each item, and lots of progress has been in regards to writing them with little to no instruction.

In this blog post, I wanted to go over two different ways to write for loops that I presented in class. One method uses the loop to take on the literal value of an item, whereas the other method uses the loop to take on the index of the item.

Let’s say that we have a list:

lst = [1, 2, 3, 4, 5, 6]

If I wanted to iterate through the list such that my iterator takes on the literal value of the list, then I would write my for loop like this:

for item in lst:

>>>>print(item)

The output of this program would simply be:

1
2
3
4
5
6

If I wanted my for loop to instead take on the index value of each item within the list, then I would write my for loop like this:

for item in range(len(lst)):

>>>>print(item)

The output of the program would now be:

0
1
2
3
4
5

The only difference between this for loop and the previous one is the additional use of the keywords range and len, which allow the iterator to take on an item’s index value.

Note that using the for loop structure with these keywords also allow you to take on the literal value of the list when you index the list within the loop.

Here is example of what that looks like:

for item in range(len(lst)):

>>>>print(item, lst[item])

The output of this program is:

0 1
1 2
2 3
3 4
4 5
5 6

Writing for loops either way is acceptable, but it’s important to know which one might be most relevant to your program. If I were simply looking to add one to every item in the list and print its output, using the for loop without range or len is just fine. If I had multiple lists to iterate over that all happen to be the same length, I might want to incorporate the keywords to save time and efficiency in my program.

For instance, let’s say I had two lists that were of the same length.

animals = [“dog”, “bird”, “horse”]

nums = [2, 11, 19]

I would only need to iterate through one list to grab the values from both by using range and len.

Here is what that looks like:

for item in range(len(animals)):

>>>>print(animals[item], nums[item])

My output would look like:

dog 2
bird 11
horse 19

I hope you found this blog post about for loop structure and output helpful and are confident enough to know which ones to use in your own programs!

Navie Narula

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