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.

Figure 1.3: A while-loop flowchart

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++;
}

Exercise 1.6

Create a similar while-loop flowchart for counting backward from 100 to 1.

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.

Figure 1.4: An until-loop flowchart

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 1.8

What are the initialization, condition, post-body processing code, and body of the “count to 10” example rewritten with a for-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.

Figure 1.5: A for-loop flowchart