Learn to Program with Small Basic

Nested Loops
The statements in the body of a For loop can be any Small Basic statement,
including another For loop. Nesting is when you put one For loop inside
another For loop (no birds are involved). Using nested loops allows you to
create iterations in two or more dimensions. This technique is important,
and you can use it to solve a wide range of programming problems.
To understand the idea of nested For loops, you’ll examine a program
that causes your computer to “jump” four times and “clap” three times after
each jump. Because the program needs to count two actions (jumps and
claps), it needs to use two loops, as shown in Listing 13-7. The counter for
the outer loop, j, runs from 1 to 4. The counter for the inner loop, c, runs
from 1 to 3.
1 ' NestedLoops.sb
2 For j = 1 To 4 ' The jump counter
3 TextWindow.Write("Jump " + j + ": ")
4 For c = 1 To 3 ' The clap counter
5 TextWindow.Write("Clap " + c + " ")
6 EndFor
7 TextWindow.WriteLine("")
8 EndFor
Listing 13-7: Nesting For loops
In the first iteration of the outer loop (where j = 1), the inner loop
repeats three times (for the three values of c); each time, it writes the word
Clap followed by a space, the current value of c, and another space (line 5).
When you nest For loops like this, the inner loop goes through all its iterations
for each iteration of the outer loop. So the first iteration of the outer
loop makes the program display Jump 1: Clap 1 Clap 2 Clap 3. When the
inner loop ends, the program outputs an empty line (line 7) to move the
cursor to the beginning of the next line, and the second iteration of the
outer loop starts with j = 2. The inner loop runs again for c = 1, 2, and 3.
This causes the program to display Jump 2: Clap 1 Clap 2 Clap 3. This continues,
so the program displays Jump 3: Clap 1 Clap 2 Clap 3 and then Jump 4:
Clap 1 Clap 2 Clap 3. Then the program ends. Perhaps your computer wants
to be a cheerleader!
Figure 13-6 helps to explain how the program works. The outer circle
represents each time the outer loop runs: for example, at the top of the
outer circle, when j = 1 in the outer loop, the inner loop runs three times,
where c = 1, c = 2, and c = 3. Follow the outer loop and think through each
inner loop. Continue until you get all the way around the outer circle.
Repeating For Loops 189