Working With Loops In Python

by HSG on Oct 30, 2012 in Python Tutorial

Sometimes we have to repeat ourselves before we are heard. Then again there are times where we have to perform a certain action the same way several times before we can carry on with what we want to do.

Repetition is the keyword here and for humans that is something we generally try to avoid. Yet our digital friends love repetition. They never get tired and they never get bored of doing the same thing over and over again countless times.

So it’s little wonder then that all modern programming languages give us various ways in which we can perform a certain action as many times as we need.

In python we have the for statement which gives us the power to loop over large collections of data very quickly and efficiently.

If you’re trying to imagine a use for loops then consider this:

Your application requires that the user enters a set of four numbers. One of those numbers must be the number 20 before it will continue to execute.

Because it is only four numbers you could use a bunch of if statements to check for the correct number, but that would not only be time consuming but rather inefficient.

The best way to go would be to use a loop, even for such a simple case. Check out the code below and then I will explain what’s going on:

myNumbers = [1,2,20,32]

  for number in myNumbers:

    print number

On line one, we create a variable called myNumbers which is actually a list, or in other languages it would be called an array.

The second line is where we start the for loop. You will notice that we create a variable called number which is accessible only within the loop and then we use the in keyword followed by the name of our list variable.

In plain English the line would read as: for each number in myNumbers.

Then on the third line we print out the loop variable, which will contain the value of the current index being accessed in our myNumbers variable.

So, in simple terms the code will access each number in our list and print it back to us.

Now I did mention that our imaginary application would also check for the correct number within a list provided by the user, and so here is the extended version of our loop which contains an if statement which checks for the number 20.

 myNumber = [1,2,4,20]

 for number in myNumbers:

          if number == 20:

              print “Found the correct number!”

Don’t worry if you haven’t got as far as learning conditional statements, all you need to keep in mind is that the code above will iterate over each number in our list variable. If it finds the number 20 then it will print the congratulatory message to the screen.

If you’re feeling adventurous you can extend the above example to include an else statement which will perform an action if the wrong number is encountered.

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