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Lesson-3.2

Loops

for loop

Let us look at a simple problem of printing numbers. We would like to print the first 5 non-negative integers. We could do this using a while loop but let's try a different kind of a loop now, the for loop:

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for i in range(5):
    print(i)
# A dummy line

The output is:

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2
3
4
5
0
1
2
3
4

for and in are keywords in Python. range is an object that represents a sequence of numbers. Line-2 is the body of the loop. An intuitive understanding of the code given above is as follows:

  • In each iteration of the loop, an element in the sequence is picked up and is printed to the console.
  • Assuming that the sequence is ordered from left to right, the leftmost element is the first to be picked up.
  • The sequence is processed from left to right.
  • Once the rightmost element has been printed to the console, control returns to line 1 for one last time. Since there are no more elements to be read in the sequence, the control exits the loop and moves to line 3.

A visual representation is given below:

control flow of for loop

Similar to while loops and if-else blocks, the body of a for loop should be indented.

range()

Now let's dive a bit deeper into this range() function that we have been using. This function predictably returns a sequence, or a range if you will, of numbers. range(5) results in the following sequence: \(0, 1, 2, 3, 4\). In general, range(n) creates the sequence: \(0, 1, ..., n - 1\). range is quite versatile. The following code prints all two digit numbers greater than zero:

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for i in range(10, 100):
    print(i)

range(10, 100) represents the sequence \(10, 11, ..., 99\). In general, range(start, stop) represents the sequence start, start + 1, ..., stop - 1. Let us add another level of complexity. The following code prints all even two digit natural numbers:

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for i in range(10, 100, 2):
    print(i)

range(10, 100, 2) represents the sequence 10, 12, ..., 98. In general, range(start, stop, step) represents the sequence start, start + step, start + 2 * step, ..., last, where last is the largest element in this sequence that is less than stop. This is true when the step parameter is positive.

The following are equivalent:

  • range(n)
  • range(0, n)
  • range(0, n, 1)

So far we have seen only increasing sequences. With the help of a negative step size, we can also come up with decreasing sequences. The following code prints all two-digit even numbers greater than zero in descending order:

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for i in range(98, 9, -2):
    print(i)

For a negative step value, range(start, stop, step) represents the sequence start, start + step, start + 2 * step, ..., last, where last is the smallest element in the sequence greater than stop.

Now, consider the following code:

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for i in range(5, 5):
    print(i)

range(5, 5) is an empty sequence. So, the above code will not print anything. Another instance of an empty sequence:

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for i in range(10, 5):
    print(i)

The point to note is that neither of these code snippets produces any error. Finally, try executing the following snippet and observe the output.

##### Alarm! Wrong code snippet! #####
for i in range(0.0, 10.0):
    print(i)
##### Alarm! Wrong code snippet! #####

Iterating through Strings

Since a string is a sequence of characters, we can use the for loop to iterate through strings. The following code will print each character of the string x in one line:

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word = 'good'
for char in word:
    print(char)

The output is:

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g
o
o
d

We can add some more code to enrich the output:

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word = 'good'
count = 1
for char in word:
    print(char, 'occurs at position', count, 'in the string', word)
    count = count + 1

The output is:

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g occurs at position 1 in the string good
o occurs at position 2 in the string good
o occurs at position 3 in the string good
d occurs at position 4 in the string good