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

For Loop Visual Representation

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

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)

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