Python: Perform repetitive tasks using Iteration or Loop

For and While loop constructs in Python enable us to perform repetitive tasks or help us iterate over a string or a list or any iterable object

January 6, 2013 - 4 minute read -
python python-loop python-iteration

Iterations in programs allow us to perform repetitive tasks in a loop. Let’s have look at the following flow diagram.

Iteration or Looping in Python

Observing the flow diagram:

  • A test (expression that evaluates to True or False)
  • If the test is True, then execute the loop body once and go back to re-evaluate the test
  • Repeat until test evaluates to False, after which code following iteration statement is executed

Table of Contents

A simple example of iteration using while construct in Python:

x = 0
result = 0
upper_limit = 10
while (x <= upper_limit): # Looping condition
    result = result + x
    x = x+1 # Incrementing x by 1
print 'Sum of first ' + str(upper_limit) + ' numbers is ' + str(result)

On execution, the above program prints:

Sum of first 10 numbers is 55

Analysis of the above code:

  • First, we declare 3 integer variables, x, result and upper_limit
  • In the while loop we have a condition, and the code block inside the loop gets executed as long as the condition is True
  • Inside the code block we add x to result and increment x by 1
    • We are actually trying to find the sum of first 10 numbers
  • As soon as x becomes 11, the condition inside while loop evaluated to False and hence the loop breaks
  • Lastly we print out the sum of first 10 numbers

In programming, a code performing similar task can be written in multiple ways. For instance, we can write the above code using for construct as well.

A simple example of iteration using for construct in Python:

result = 0
for x in range(1,11): # range() function explained below
    result = result + x
print 'Sum of first 10 numbers is ' + str(result)

Analysis of the above code:

  • Let’s first see how range() works
>>> range(10) #returns a list that contains numbers from 0 to (10-1)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]  # This is a 'list' of integers
>>> range(1, 10) # Here we force range() to start from 1 and end before 10
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(1, 11) # This is what we used in the for loop above
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> range(1, 11, 2) # Third argument 2 says that it will jump 2 steps. 2 is called 'step argument'
[1, 3, 5, 7, 9]
>>> range(1, 11, 3) # 3 makes it jump 3 steps at a time
[1, 4, 7, 10]
  • Now that we know what range does, for simple iterates over each value in the the list returned by range and assigns it to x
  • Inside the for loop we add x to result
  • Finally, we print out the result

See? How easy it was to perform the same task in a different way.

What to use & When?

  • Use while when you don’t know the number of times the loop should repeat. It’s only used when you know the condition when loop should break.
  • Use for when you know the fixed number of times the loop should repeat.

Next we will apply the knowledge gained so far to solve an interesting problem. How to find square root of any number?


Note: This is a part of what I learned in an online Open Course Ware offered by MIT on edX. Its for my personal reference & also for those who would like to revisit the course.

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