Nested For Loops
Kenneth Leroy Busbee
Overview
Nested for loops places one for loop inside another for loop. The inner loop is repeated for each iteration of the outer loop.
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 10 by 10 multiplication table:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | ------------------------------------------------------------- 1 ! 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 2 ! 2 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 3 ! 3 | 6 | 9 | 12 | 15 | 18 | 21 | 24 | 27 | 30 | 4 ! 4 | 8 | 12 | 16 | 20 | 24 | 28 | 32 | 36 | 40 | 5 ! 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 6 ! 6 | 12 | 18 | 24 | 30 | 36 | 42 | 48 | 54 | 60 | 7 ! 7 | 14 | 21 | 28 | 35 | 42 | 49 | 56 | 63 | 70 | 8 ! 8 | 16 | 24 | 32 | 40 | 48 | 56 | 64 | 72 | 80 | 9 ! 9 | 18 | 27 | 36 | 45 | 54 | 63 | 72 | 81 | 90 | 10 ! 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |
We might also see that the answers could be designed as a collection of cells (each cell being exactly six spaces wide). The pseudocode to produce part of the table is:
For row = 1, row <= 3, row += 1 For column = 1, column <= 3, column += 1 Output row * column Output "\t" Output "\n"
Key Terms
- complex logic
- Often solved with nested control structures.