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7.7 — Intro to loops and while statements

Introduction to loops

And now the real fun begins -- in the next set of lessons, we’ll cover loops. Loops are control flow constructs that allow a piece of code to execute repeatedly until some condition is met. Loops add a significant amount of flexibility into your programming toolkit, allowing you to do many things that would otherwise be difficult.

For example, let’s say you wanted to print all the numbers between 1 and 10. Without loops, you might try something like this:

While that’s doable, it becomes increasingly less so as you want to print more numbers: what if you wanted to print all the numbers between 1 and 1000? That would be quite a bit of typing! But such a program is writable in this way because we know at compile time how many numbers we want to print.

Now, let’s change the parameters a bit. What if we wanted to ask the user to enter a number and then print all the numbers between 1 and the number the user entered? The number the user will enter isn’t knowable at compile-time. So how might we go about solving this?

While statements

The while statement (also called a while loop) is the simplest of the three loop types that C++ provides, and it has a definition very similar to that of an if statement:

while (condition)
    statement;

A while statement is declared using the while keyword. When a while statement is executed, the condition is evaluated. If the condition evaluates to true, the associated statement executes.

However, unlike an if statement, once the statement has finished executing, control returns to the top of the while statement and the process is repeated. This means a while statement will keep looping for as long as the condition evaluates to true.

Let’s take a look at a simple while loop that prints all the numbers from 1 to 10:

This outputs:

1 2 3 4 5 6 7 8 9 10 done!

Let’s take a closer look at what this program is doing. First, count is initialized to 1, which is the first number we’ll print. The condition count <= 10 is true, so the statement executes. In this case, our statement is a block, so all the statements in the block will execute. The first statement in the block prints 1 and a space, and the second increments count to 2. Control now return back to the top of the while statement, and the condition is evaluated again. 2 <= 10 evaluates to true, so the code block is executed again. The loop will repeatedly execute until count is 11, at which point 11 <= 10 will evaluate to false, and the statement associated with the loop will be skipped. At this point, the loop is done.

While this program is a bit more code than typing all the numbers between 1 and 10, consider how easy it would be to modify the program to print all the numbers between 1 and 1000: all you'd need to do is change count <= 10 to count <= 1000.

While statements that evaluate to false initially

Note that if the condition initially evaluates to false, the associated statement will not execute at all. Consider the following program:

The condition 15 <= 10 evaluates to false, so the associated statement is skipped. The program continues, and the only thing printed is done!.

Infinite loops

On the other hand, if the expression always evaluates to true, the while loop will execute forever. This is called an infinite loop. Here is an example of an infinite loop:

Because count is never incremented in this program, count <= 10 will always be true. Consequently, the loop will never terminate, and the program will print "1 1 1 1 1"... forever.

Intentional infinite loops

We can declare an intentional infinite loop like this:

The only way to exit an infinite loop is through a return statement, a break statement, an exit statement, a goto statement, an exception being thrown, or the user killing the program.

Here's a silly example demonstrating this:

This program will continuously loop until the user enters n as input, at which point the if statement will evaluate to true and the associated return 0; will cause function main() to exit, terminating the program.

It is common to see this kind of loop in web server applications that run continuously and service web requests.

Best practice

Favor while(true) for intentional infinite loops.

Loop variables

Often, we want a loop to execute a certain number of times. To do this, it is common to use a loop variable, often called a counter. A loop variable is an integer that used to count how many times a loop has executed. In the examples above, the variable count is a loop variable.

Loop variables are often given simple names, such as i, j, or k. However, if you want to know where in your program a loop variable is used, and you use the search function on i, j, or k, the search function will return half your program! For this reason, some developer prefer loop variable names like iii, jjj, or kkk. Because these names are more unique, this makes searching for loop variables much easier, and helps them stand out as loop variables. An even better idea is to use "real" variable names, such as count, or a name that gives more detail about what you're counting (e.g. userCount).

Loop variables should be signed

Loop variables should almost always be signed, as unsigned integers can lead to unexpected issues. Consider the following code:

Take a look at the above example and see if you can spot the error. It's not very obvious.

It turns out, this program is an infinite loop. It starts out by printing 10 9 8 7 6 5 4 3 2 1 blastoff! as desired, but then goes off the rails, and starts counting down from 4294967295. Why? Because the loop condition count >= 0 will never be false! When count is 0, 0 >= 0 is true. Then --count is executed, and count wraps around back to 4294967295 (Assuming 32-bit integers). And since 4294967295 >= 0 is true, the program continues. Because count is unsigned, it can never be negative, and because it can never be negative, the loop won't terminate.

Best practice

Loop variables should be of type (signed) int.

Doing something every N iterations

Each time a loop executes, it is called an iteration.

Often, we want to do something every 2nd, 3rd, or 4th iteration, such as print a newline. This can easily be done by using the modulus operator on our counter:

This program produces the result:

01 02 03 04 05 06 07 08 09 10
11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40
41 42 43 44 45 46 47 48 49 50

Nested loops

It is also possible to nest loops inside of other loops. In the following example, the inner loop and outer loop each have their own counters. However, note that the loop expression for the inner loop makes use of the outer loop's counter as well!

This program prints:

1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

Quiz time

Question #1


In the above program, why is variable inner declared inside the while block instead of immediately following the declaration of outer?

Show Solution [1]

Question #2


Write a program that prints out the letters a through z along with their ASCII codes. Hint: to print characters as integers, you have to use a static_cast.

Show Solution [1]

Question #3


Invert the nested loops example so it prints the following:

5 4 3 2 1
4 3 2 1
3 2 1
2 1
1

Show Solution [1]

Question #4


Now make the numbers print like this:

        1
      2 1
    3 2 1
  4 3 2 1
5 4 3 2 1

Hint: Figure out how to make it print like this first:

X X X X 1
X X X 2 1
X X 3 2 1
X 4 3 2 1
5 4 3 2 1

Show Solution [1]


7.8 -- Do while statements [2]
Index [3]
7.6 -- Goto statements [4]