Programming made easy – nested loops

We’re not quite done with loops yet. There are still a few things to learn, although some can be left for later. There are certain situations where it is appropriate to use a loop inside another loop. Such a loop is called a nested loop. A nested loop is executed, essentially, from the inside out. Each loop is like a layer and has its own loop control, its own condition and its own loop body. Below is a schematic of three nested loops.

A loop inside a loop, inside a loop.

There are many different kinds of nested loops, and they are largely dependent on the underlying algorithm. Below are some examples of the types of nested loops that are possible.

Various examples of nested loops

We will consider how a nested loop can be used to create a times table. It basically works by iterating the inner loop 12 times for every iteration of the outer loop. So, 12 × 12 = 144 iterations. The outer loop is highlighted in blue, and the inner loop in red. Here is a sample output:

 1   2   3   4   5   6   7   8   9  10  11  12
 2   4   6   8  10  12  14  16  18  20  22  24
 3   6   9  12  15  18  21  24  27  30  33  36
 4   8  12  16  20  24  28  32  36  40  44  48
 5  10  15  20  25  30  35  40  45  50  55  60
 6  12  18  24  30  36  42  48  54  60  66  72
 7  14  21  28  35  42  49  56  63  70  77  84
 8  16  24  32  40  48  56  64  72  80  88  96
 9  18  27  36  45  54  63  72  81  90  99 108
10  20  30  40  50  60  70  80  90 100 110 120
11  22  33  44  55  66  77  88  99 110 121 132
12  24  36  48  60  72  84  96 108 120 132 144

Ada

with ada.Text_IO; use Ada.Text_IO;
with ada.Integer_Text_IO; use ada.Integer_Text_IO;
procedure nested is 
begin
   for x in 1..12 loop
      for y in 1..12 loop
         put(x*y, width=>4);
      end loop;
      new_line;
   end loop;
end nested;

Fortran

program nested
   integer :: x, y
   do x = 1, 12
      do y = 1, 12
         write(*,10,advance='no') xy
         10 format(I4)
      end do
      write(*,*)
   end do
end program nested

C

#include <stdio.h>
int main(void){
   int x, y;
   for (x=1; x<=12; x=x+1)
   {
      for (y=1; y<=12; y=y+1)
         printf("%4d", x*y);
      printf("\n");
   }
   return 0;
}

Julia

for x = 1:12
   for y = 1:12
      @printf("%4d", x*y)
   end
   println()
end
println(h)