Learning
Objectives: While Loops
Explain while loop syntax
Identify the causes of an infinite while loop
Describe the break statement as it relates to a while loop
While Loops
While Loop Syntax
while loops, like for loops, use curly braces {} and indents for all
commands that should be repeated. However, for loops generally contain 3
elements (an initialized variable, a boolean expression involving that
variable, and a change in the value of that variable) while a while loop
usually contains just a boolean expression. The for and while loops below
produce the same results.
for (int i = 0; i < 5; i++) { int i = 0;
cout << "Loop#: " << i << while (i < 5) {
endl;
cout << "Loop# " << i << endl
}
i++;
}
Note that the variable declaration and initialization happen before the start
of the while loop and any changes that occur to the variable happen within
the body of the curly braces {}. On the other hand, everything happens in
one step within parentheses () when using a for loop.
Here is another example of a while loop that prints Hello based on the
value of the variable count.
int count = 5; // some random number set by user
while (count > 0) {
cout << "Hello" << endl;
count--;
}
Code Visualizer
challenge
What happens if you:
Change the while statement to while (count > -1 * count)?
Replace count-- in the code above with count = count - 2?
Change the while statement to while (count < 10)?
How does while (count > -1 * count) work?
To understand how the loop works, it’s best to substitute values in for the
variable count. In the first iteration, we have while (5 > -1 * 5), this
statement is true so the print command is executed. Since count gets
decremented by 1 with each iteration, the while loop condition changes
slightly every time like as follow:
while (4 > -1 * 4)
while (3 > -1 * 3)
while (2 > -1 * 2)
while (1 > -1 * 1)
Once the condition gets to while (0 > -1 * 0), it no longer holds true and
the while loop ends. The result is 5 Hellos being printed to the screen.
Infinite Loops
Infinite loops are loops that do not have a test condition that causes them to
stop. The following is a common mistake that results in an infinite loop:
int count = 5; // some random number set by user
while (count > 0) {
cout << "Hello" << endl;
}
Since the variable count never gets decremented. It remains at 5, and 5 will
forever be greater than 0, so the loop will never stop.
warning
Copy the code above and TRY IT to see what happens. C++ will
eventually stop the loop due to an output limit, but it may take some
time before this happens.
Why Use a While Loop?
If a while loop does the same thing as a for loop, then what is the purpose
of having both? while loops are actually more useful when you are waiting
for a certain event to occur. Imagine you are making a video game. The
game should continue until the player loses all of their lives. You don’t
know how long this will take, so a while loop is more appropriate. On the
other hand, if you have more specific loop parameters, a for loop will be
better.
int player_lives = 3;
while (player_lives > 0) {
// video game code
// goes here
}
Turtle Coding: While Loop
Instead of a for loop, recreate the images below using a while loop.
Turtle Graphics Review
tina.forward(n) - Where n represents the number of pixels.
tina.backward(n) - Where n represents the number of pixels.
tina.right(d) - Where d represents the number of degrees.
tina.left(d) - Where d represents the number of degrees.
tina.pencolor({“COLOR”}) - Where COLOR represents the track or line
color you want tina to leave behind.
tina.width(W) - Where W represents how wide (in pixels) tina’s track is.
tina.shape(“SHAPE”) - Where SHAPE represents the shape tina takes.
tina.speed(SPEED) - Where SPEED represents how fast tina moves
Challenge 1
.guides/img/TurtleChallenge1
Hint
The pattern is still the same:
1. Go forward (creating a long line).
2. Make a right turn.
3. Go forward (creating a small line).
4. Make a right turn.
5. Go forward (creating another small line).
6. Make a right turn.
7. Go forward (creating a final small line).
8. Repeat steps #1 through #7 three more times for a total of four
iterations.
However, a while loop usually contains only a boolean expression(s) in its
header. Thus, you must initialize a counting variable before the start of the
while loop. Also, that counting variable should be incremented inside the
body of the loop. The pattern you are trying to iterate is still the same:
Challenge 2
.guides/img/TurtleChallenge2
Hint
Since a circle has 360 degrees, you will need a loop that repeats 360 times.
Be careful about how far the turtle moves forward and turns. The circle
can get very big, very quickly.
Challenge 3
.guides/img/TurtleChallenge3
Hint
The pattern here is to move forward and make a right turn.
The trick lies within the fact that the distance the turtle moves has to get
larger as the loop advances. Think of some operators that you can use to
make the loop iterator variable get bigger during each iteration.
Sample Solutions
Here are some sample solutions using while loops:
tina.pencolor({"blue"});
tina.width(2);
tina.shape("arrow");
tina.speed(TS_SLOWEST);
int i = 0;
while (i < 4) {
tina.forward(75);
tina.right(90);
tina.forward(25);
tina.right(90);
tina.forward(25);
tina.right(90);
tina.forward(25);
i++;
}
tina.pencolor({"red"});
tina.width(2);
tina.shape("square");
tina.speed(TS_FASTEST);
int i = 0;
while (i < 360) {
tina.forward(1);
tina.right(1);
i++;
}
tina.pencolor({"green"});
tina.width(2);
tina.shape("triangle");
tina.speed(TS_NORMAL);
int i = 10;
while (i <= 200) {
tina.forward(i);
tina.right(90);
i+=10;
}
info
NOTE
In most cases, for loops and while loops can be used interchangeably.
It is up to you to decide which one is better suited for your task.
Break Statement
Infinite Loops Are Bad, Right?
Well, that depends. If an infinite loop is caused because the counting
variable isn’t incremented, then it’s a bug. However, some programmers
purposely create a condition that will always evaluate to true. Therefore,
the loop will always run. In such cases, a break statement is used to stop the
loop at a particular point in the program.
Take a look at the following program (also shown in the text editor on the
left).
#include <iostream>
using namespace std;
int main() {
srand(time(NULL)); // start randomizer every time program runs
while (true) {
cout << "This is an infinite loop" << endl;
int randNum = rand() % 100 + 1; // generate random number
between 1 and 100
if (randNum > 75) {
cout << "The loop ends" << endl;
break; // stop the loop
} // end if condition
} // end while loop
cout << "The program ends" << endl;
return 0;
Then click on the TRY IT button below a few times to run the code and see
the resulting outputs. You can also click on the ++Code Visualizer++ link
below to see how the code runs behind-the-scenes.
Code Visualizer
Even though while (true) will always evaluate as a true statement, the
loop never becomes infinite because of the break statement.
challenge
What happens if you:
Remove break; from the program?
Add break; to the line before cout << "The loop ends" << endl;?
Code Visualizer
Comparing While Loops
The while loops introduced on the previous pages look different from the
while loop covered on this page; however, they both have the same
components and behave similarly.
.guides/img/WhileLoopComparison
