1.7 Loops

If I ask you to close your eyes, count to 10, then open them, the steps look like this:

1. Close your eyes
2. Set count to 1
3. While count is not equal 10, increment count by 1
4. Open your eyes

Steps 2 and 3 together constitute a loop. In this loop, we repeatedly do something while a condition is met. We do not move to step 4 from step 3 while the test returns true.

Compare the simplicity of Figure 1.3 to

1. Close your eyes
2. Set count to 1
3. Increment count to 2
4. Increment count to 3
5. …
6. Increment count to 10
7. Open your eyes

If that doesn’t convince you, imagine if I asked you to count to 200. Here is how we do it in C++:

int n = 1;

while( n < 201 ) {
    n++;
}

That loop was a while-loop, but this is an until-loop:

1. Close your eyes
2. Set count to 1
3. Increment count by 1 until count equals 10
4. Open your eyes

Many languages do not have until-loops, but VBA does:

n = 1
Do Until n>200
    n = n + 1
Loop

We saw earlier that the code within a function’s definition is its body. The repeated code in a loop is also called its body.

Exercise 1.7

What is different between while-loops and until-loops regarding when you test the condition? Compare this until-loop flowchart with the previous while-loop example.

Our next example is a for-loop, so named because of the keyword that many programming languages use. A for-loop is very useful when you want to repeat something a specified number of times. This example uses the Go language:

sum := 0
for n := 1; n <= 50; n++ {
    sum += n
}

It adds all the numbers between 1 and 50, storing the result in sum. Here, := is an assignment, n++ means “replace the value of n by its previous value plus 1,” and sum += n means “replace the value of sum by its previous value plus the value of n.”

There are four parts to this particular syntax for the for-loop:

• the initialization: n := 1
• the condition: n <= 50
• the post-body processing code: n++
• the body: sum += n

The sequence is: do the initialization once; test the condition and, if true, execute the body; execute the post-body code; test the condition again and repeat. If the condition is ever false, the loop stops, and we move to whatever code follows the loop.

Exercise 1.9

Draw a flowchart of a for-loop, including the initialization, condition, post-body processing code, and the body. Use the template in Figure 1.5.