Loops in Python

for Loop

Let us look at a simple problem of printing numbers. We would like to print the first 5 non-negative integers using a for loop:

Code

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

Output

0
1
2
3
4

In this example:

• 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.

Understanding the Code

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

Visual Representation

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

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range()

The range(5) function represents the following sequence: 0, 1, 2, 3, 4. In general, range(n) represents the sequence: 0, 1, ..., n - 1.

The range function is quite versatile. For example, the following code prints all two-digit numbers greater than zero:

Code

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.

Example: Even Two-Digit Numbers

Let us add another level of complexity. The following code prints all even two-digit numbers greater than zero:

Code

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.

Equivalents

The following are equivalent:

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

Decreasing Sequences

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:

Code

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.

Empty Sequences

Now, consider the following code:

Code

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:

Code

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

Neither of these snippets produces any error.

for i in range(0.0, 10.0):
    print(i)

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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:

Code

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

Output

g
o
o
d

We can add some more code to enrich the output:

Code

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

Output

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