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16.4: Nested For Loops

  • Page ID
    10341
  • 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.