Looping - Or the art of repeating oneself!

What will we cover?
How to use loops to cut down on repetitive typing. Different types of loop and when to use them.

In the last exercise we printed out part of the 12 times table. But it took a lot of typing and if we needed to extend it, it would be very time consuming. Fortunately there is a better way and its where we start to see the real power that programming languages offer us.

FOR Loops

What we are going to do is get the programming language to do the repetition, substituting a variable which increases in value each time it repeats. In Python it looks like this:

>>>for i in range(1,13):
...    print "%d x 12 = %d" % (i, i*12)

Note 1: We need the range(1,13) to specify 13 because range() generates from the first number up to, but not including, the second number. This may seem somewhat bizarre at first but there are reasons and you get used to it.

Note 2: The for operator in Python is actually a foreach operator in that it applies the subsequent code sequence to each member of a collection. In this case the collection is the list of numbers generated by range(). You can prove that by typing print range(1,13) at the python prompt and seeing what gets printed.

Note 3: The print line is indented further than the for line above it. That is a very important point since it's how Python knows that the print is the bit to repeat. It doesn't matter how much indentation you use so long as it's consistent.

Note 4: In the interactive interpreter you need to hit return twice to get the program to run. The reason is that the Python interpreter can't tell whether the first one is another line about to be added to the loop code or not. When you hit Enter a second time Python assumes your finished entering code and runs the program.

So how does the program work? Let's step through it.
First of all, python uses the range function to create a list of numbers from 1 to 12.

Next python makes i equal to the first value in the list, in this case 1. It then executes the bit of code that is indented, using the value i = 1:

   print "%d x 12 = %d" % (1, 1*12)

Python then goes back to the for line and sets i to the next value in the list, this time 2. It again executes the indented code, this time with i = 2:

   print "%d x 12 = %d" % (2, 2*12)

It keeps repeating this sequence until it has set i to all the values in the list. At that point it moves to the next command that is not indented - in this case there aren't any more commands so the program stops.

Here's the same loop in BASIC:

FOR I = 1 to 12
    PRINT I, " x 12 = ", I*12

This is much more explicit and easier to see what is happening. However the Python version is more flexible in that we can loop over a set of numbers, the items in a list or any other collection (e.g. a string).

And in Tcl

Tcl uses a for construct that is common in many programming languages, being modelled on C. It looks like this:

for {set i 1} {$i <= 12} {incr i} {
    puts [format "%d x 12 = %d" $i [expr $i*12]]

Note: This construct has 3 parts:

The loop body will only execute if the test part is true. Each of these parts can contain arbitrary code but the test part must evaluate to a boolean value (which in Tcl means zero or non-zero). Note that although I have shown the loop body indented, this is purely to aid understanding. Tcl does not require me to indent the block, rather the curly braces are used to mark the beginning and end.

Tcl also has a foreach construct that can be applied to lists.


FOR loops are not the only type of looping construct available. Which is just as well since FOR loops require us to know, or be able to calculate in advance, the number of iterations that we want to perform. So what happens when we want to keep doing a specific task until something happens but we don't know when that something will be? For example, we might want to read and process data from a file, but we don't know in advance how many data items the file contains. We just want to keep on p[rocessing data until we reach the end of the file. That's possible but difficult in a FOR loop.

To solve this problem we have another type of loop: the WHILE loop. It looks like this in BASIC:

J = 1
WHILE J <= 12
    PRINT J, " x 12 = ", J*12
    J = J + 1

This produces the same result as before but uses a while loop instead of a for loop. Notice the structure is while, followed by an expression which evaluates to a Boolean value (true or false, remember?). If the expression is true, the code inside the loop is executed.

As an alternative we'll look at the Tcl version:

set j  1
while {$j <= 12} {
   puts [format "%d x 12 = %s"  $j [expr $j*12]]
   set j [expr $j + 1]

As you see the structure is pretty similar just some curly brackets or braces instead of the WEND in BASIC. But what's that mess inside the loop? Remember format strings in Python? format is Tcl's equivalent. The $j just means the value of j (rather than the letter 'j'!) and the expr just says 'calculate the next bit as an expression'. The square brackets tell Tcl which bits to do first. Tcl is an unusual language in that it attempts to interpret its code in one go, so without the brackets it would try to print the word 'expr' then see some more values and give up with an error message. We need to tell it to do the sums, then format the string, then print the result. Confused? Don't worry about it. As I said Tcl is an unusual language with a few uniquely good points and a lot of strangeness.

Now look at Python:

>>> j = 1
>>> while j <= 12:
...    print "%d x 12 = %d" % (j, j*12)
...    j = j + 1

By now that should look pretty straightforward. Just one thing to point out - do you see the colon (:) at the end of the while and for lines above? That just tells Python that there's a chunk of code (a block) coming up. Most languages have an end of block marker (like BASIC's WEND or Tcl's braces) but Python uses indentation to indicate the structure. This means that its important to indent all of the lines inside the loop by the same amount, this is good practice anyway since it's easier to read!

More Flexible Loops

Coming back to our 12 times table at the beginning of this section. The loop we created is all very well for printing out the 12 times table. But what about other values? Can you modify the loop to make it do the 7 times table say? It should look like this:

>>> for j in range(1,13):
...    print "%d x 7 = %d" % (j,j*7)

Now this means we have to change the 12 to a 7 twice. And if we want another value we have to change it again. Wouldn't it be better if we could enter the multiplier that we want?

We can do that by replacing the values in the print string with another variable. Then set that variable before we run the loop:

>>> multiplier = 12
>>> for j in range(1,13):
...    print "%d x %d = %d" % (j, multiplier, j*multiplier)

That's our old friend the 12 times table. But now to change to the seven times, we only need to change the value of 'multiplier'.

Note that we have here combined sequencing and loops. We have first a single command, multiplier = 12 followed, in sequence by a for loop.

Looping the loop

Let's take the previous example one stage further. Suppose we want to print out all of the times tables from 2 to 12 (1 is too trivial to bother with). All we really need to do is set the multiplier variable as part of a loop, like this:

>>> for multiplier in range(2,13):
...    for j in range(1,13):
...       print "%d x %d = %d" % (j,multiplier,j*multiplier)

Notice that the part indented inside the first for loop is exactly the same loop that we started out with. It works as follows:
We set multiplier to the first value (2) then go round the second loop.
Then we set multiplier to the next value (3) and go round the inner loop again,
and so on. This technique is known as nesting loops.

One snag is that all the tables merge together, we could fix that by just printing out a separator line at the end of the first loop, like this:

>>> for multiplier in range(2,13):
...    for j in range(1,13):
...       print "%d x %d = %d" % (j,multiplier,j*multiplier)
...    print "------------------- "

Note that the second print statement lines up with the second 'for', it is the second statement in the loop sequence. Remember, the indenting level is very important in Python.

Experiment with getting the separator to indicate which table it follows, in effect to provide a caption. Hint: You probably need to use the multiplier variable and a format string.

Other loops

Some languages provide more looping constructs but some kind of for and while are usually there. (Modula 2 and Oberon only provide while loops since they can simulate for loops - as we saw above.) Other loops you might see are:

Same as a while but the test is at the end so the loop always executes at least once.

Similar to above but the logic of the test is reversed.

Mainly seen in older languages, these usually set a marker in the code and then explicitly jump directly to it.

Points to remember
  • FOR loops repeat a set of commands for a fixed number of iterations.
  • WHILE loops repeat a set of commands until some terminating condition is met. They may never execute the body of the loop idf the terminating condition is false to start with.
  • Other types of loops exist but FOR and WHILE are nearly always provided.
  • Python for loops are really foreach loops - they operate on a list of items.
  • Loops may be nested one inside another.
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If you have any questions or feedback on this page send me mail at: alan.gauld@btinternet.com