Programming Fundamentals/Nested Loops
Overview
[edit | edit source]Nested for loops places one for loop inside another for loop. The inner loop is repeated for each iteration of the outer loop.
Discussion
[edit | edit source]Nested Control Structures
[edit | edit source]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
[edit | edit source]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"
An Example - Nested Do While For Loops
[edit | edit source]Please input what number you want to multiply 3 What number do you want constant 2 2 x 1 = 2 2 x 2 = 4 2 x 3 = 6 Would you like another set? Type yes no Ok bye
Here is a pseudo code for the example above
Function Main
Declare Integer answer
Declare Integer multiply
Declare Integer constant
Declare String choice
Loop
Output "Please input what number you want to multiply"
Input multiply
Output "What number do you want constant"
Input constant
Declare Integer i
Assign i = 0
For i = 1 to multiply
Assign answer = i * constant
Output constant & " x " & i & " = " & answer
End
Output "Would you like another set? Type yes"
Input choice
Do choice == "Yes"
Output "Ok bye"
End
Key Terms
[edit | edit source]- complex logic
- Often solved with nested control structures.