3.1.2.4 Loops in Python | for

Looping your code with for

Another kind of loop available in Python comes from the observation that sometimes it's more important to count the "turns" of the loop than to check the conditions.

Imagine that a loop's body needs to be executed exactly one hundred times. If you would like to use the while loop to do it, it may look like this:

i = 0 while i < 100: # do_something() i += 1

It would be nice if somebody could do this boring counting for you. Is that possible?

Of course it is - there's a special loop for these kinds of tasks, and it is named for.

Actually, the for loop is designed to do more complicated tasks - it can "browse" large collections of data item by item. We'll show you how to do that soon, but right now we're going to present a simpler variant of its application.

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Take a look at the snippet:

for i in range(100): # do_something() pass

There are some new elements. Let us tell you about them:

• the for keyword opens the for loop; note - there's no condition after it; you don't have to think about conditions, as they're checked internally, without any intervention;
• any variable after the for keyword is the control variable of the loop; it counts the loop's turns, and does it automatically;
• the in keyword introduces a syntax element describing the range of possible values being assigned to the control variable;
• the range() function (this is a very special function) is responsible for generating all the desired values of the control variable; in our example, the function will create (we can even say that it will feed the loop with) subsequent values from the following set: 0, 1, 2 .. 97, 98, 99; note: in this case, the range() function starts its job from 0 and finishes it one step (one integer number) before the value of its argument;
• note the pass keyword inside the loop body - it does nothing at all; it's an empty instruction - we put it here because the for loop's syntax demands at least one instruction inside the body (by the way - if, elif, else and while express the same thing)

Our next examples will be a bit more modest in the number of loop repetitions.

Take a look at the snippet below. Can you predict its output?

for i in range(10): print("The value of i is currently", i)

Run the code to check if you were right.

Note:

• the loop has been executed ten times (it's the range() function's argument)
• the last control variable's value is 9 (not 10, as it starts from 0, not from 1)

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The range() function invocation may be equipped with two arguments, not just one:

for i in range(2, 8): print("The value of i is currently", i)

In this case, the first argument determines the initial (first) value of the control variable.

The last argument shows the first value the control variable will not be assigned.

Note: the range() function accepts only integers as its arguments, and generates sequences of integers.

Can you guess the output of the program? Run it to check if you were right now, too.

The first value shown is 2 (taken from the range()'s first argument.)

The last is 7 (although the range()'s second argument is 8).