16.4: Nested For Loops

General Discussion

Nested Control Structures

We are going to first introduce the concept of nested control structures. Nesting is a concept that places one item inside of another. Consider:

if expression
  true action
else
  false action

This is the basic form of the if then else control structure. Now consider:

if age is less than 18
  you can't vote
  if age is less than 16
    you can't drive
  else
    you can drive
else
  you can vote
  if age is less than 21
    you can't drink
  else
    you can drink

As you can see we simply included as part of the "true action" a statement and another if then else control structure. We did the same (nested another if then else) for the "false action". In our example we nested if then else control structures. Nesting could have an if then else within a while loop. Thus, the concept of nesting allows the mixing of the different categories of control structures.

Many complex logic problems require using nested control structures. By nesting control structures (or placing one inside another) we can accomplish almost any complex logic problem.

An Example - Nested for loops

Here is an example of a 12 by 12 multiplication table:

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

We might also see that the answers could be designed as a collection of cells (each cell being exactly six spaces wide). The C++ source code to produce the above is:

C++ source code: nested for loops - multiplication table
cout << "      ";
for(across=1; across <13; across++)
  {
  cout << setw(4) << across << " |";
  }
cout << endl;

cout << "      ";
for(across=1; across <13; across++)
  {
  cout << "------";
  }
cout << endl;

for(down=1; down <13; down++)
  {
  cout << setw(4) << down << " !";
  for(across=1; across <13; across++)
    {
    cout << setw(4) << down*across << " |";
    }
  cout << endl;
  }  

Figure \(\PageIndex{1}\) Colorized Code - multiplication table

Figure \(\PageIndex{2}\) Colorized Output - multiplication table

Demonstration Program in C++

Creating a Folder or Sub-Folder for Source Code Files

Depending on your compiler/IDE, you should decide where to download and store source code files for processing. Prudence dictates that you create these folders as needed prior to downloading source code files. A suggested sub-folder for the Bloodshed Dev-C++ 5 compiler/IDE might be named:

• Demo_Programs

If you have not done so, please create the folder(s) and/or sub-folder(s) as appropriate.

Download the Demo Program

Download and store the following file(s) to your storage device in the appropriate folder(s). Following the methods of your compiler/IDE, compile and run the program(s). Study the source code file(s) in conjunction with other learning materials.

Download from Connexions: Demo_Nested_For_Loops.cpp

Definitions

Complex Logic

Often solved with nested control structures.