Python 101 - Part 6 - Repetition Structures - For

Hi!

One more post here teaching you Python and today we will talk about for.

In case you haven’t seen the previous tutorials, we already have:

• Introduction to Python
• Python Code Blocks
• Conditional Structures in Python - 1
• Conditional Structures in Python - 2
• Repetition Structures - while

Let’s start with a simple example:

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l = [1, 2, 3, 4, 5]
for i in l:
    print(i)

Here, initially, we start with something we haven’t seen yet: lists.

We start by creating l as a list that contains a sequence of numbers.

So this list is used inside the for.

In other languages, we determine the initial value of the counter variable, its limit and the iteration step. In python this does not happen. Here, for only iterates over sequences. This has several advantages, such as iterating over strings, or lists of different elements.

It will be shown later because this for is not at a disadvantage compared to the for of other languages.

Then, at each iteration, the iteration variable, in this case i, receives the element of a position in the list.

Thus, this snippet above prints on the screen all the values ​​contained in the list l.

Now let’s see this code snippet:

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d = ['rice', 'beans', 'meat', 'potato']
for i in d:
    print('I'm cooking ', i)

This example demonstrates the use of for with lists that contain strings, showing that you can use lists with any object in a for.

In for we can also use break and continue. Thus, we can further control the behavior of the loop. Also, there is a feature in Python’s for that makes it very powerful: we can use an else at the end of the for as in the following example:

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for i in ['a', 'b', 'c', 'd', 'e', ​​'f', 'g']:
    if i == 'h':
        break
    print(i)
else:
    print('"h" is not in the list but I print anyway')

Using else at the end, we can control what will be executed at the end of a complete for execution and what will be executed if the loop is broken by a break. When the loop is completely traversed without a break, execution proceeds into else!

In this example, the output will look something like this:

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b
c
d
e
f
g
"h" is not in the list but I print it anyway

This is very important, as it avoids having to do complicated tests to detect when a for is broken by a break that will be called under complex conditions.

Finally, many may be wondering: “How can this for be so powerful if we have to define the lists manually?”

The fact is that we don’t have to do it manually if it’s a list of numbers (just like we don’t need to in other languages).

There is a Python function, range, which returns a generator (object that can be iterated over like a list, but generates the values ​​at runtime) of numbers in the specified range. See the example:

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for i in range(50):
    if i%2 == 0:
        print(i, 'is even')
    else:
        print(i, 'is odd')

When we do range(50), we will have a generator that will work as a list of integers from 0 to 49. (try it in the shell).

Well folks! That’s it for today!

Now it’s not long before we start effective programming in Python!

In the next posts, native Python data types!

Be sure to post questions and opinions in the comments below!

See you in the next post!