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Home Lesson-3.3

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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 have a different kind of a loop now, the for loop:

xxxxxxxxxx
3
1
for i in range(5):
2
    print(i)
3
# A dummy line

The output is:

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5
1
0
2
1
3
2
4
3
5
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:

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

 

range()

range(5) represents the following sequence: 0, 1, 2, 3, 4. In general, range(n) represents 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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2
1
for i in range(10, 100):
2
    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 numbers greater than 0:

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2
1
for i in range(10, 100, 2):
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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2
1
for i in range(98, 9, -2):
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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2
1
for i in range(5, 5):
2
    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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2
1
for i in range(10, 5):
2
    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.

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4
1
##### Alarm! Wrong code snippet! #####
2
for i in range(0.0, 10.0):
3
    print(i)
4
##### 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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3
1
word = 'good'
2
for char in word:
3
    print(char)

The output is:

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4
1
g
2
o
3
o
4
d

We can add some more code to enrich the output:

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5
1
word = 'good'
2
count = 1
3
for char in word:
4
    print(char, 'occurs at position', count, 'in the string', word)
5
    count = count + 1

The output is:

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