6.2. The while statement¶
Computers are often used to automate repetitive tasks. Repeating identical or similar tasks without making errors is something that computers do well and people do poorly. Because iteration is so common, Python provides several language features to make it easier.
One form of iteration in Python is the while statement.
Here is a simple program that counts down from five and then says
“Blastoff!”.
Activity: CodeLens 6.2.1 (codelens511)
You can almost read the while statement as if it were
English. It means, “While n is greater than 0, display the
value of n and then reduce the value of n by 1. When you get
to 0, exit the while statement and display the word Blastoff!
- The program has five iterations but does not print anything.
- Incorrect! The program both prints things and has more than five iterations. Try again.
- The loop will repeat forever and results in an infinite loop.
- Correct! Because the loop is incrementing, rather than decrementing, n will always be greater than 0 and the loop will never end.
- The program compiles and prints "1 2 3 4 5 Blastoff!" where each space is a new line.
- Incorrect! The program will compile, but will not start with 1 and won't ever reach "Blastoff!". Try again.
- The program compiles and prints "5 4 3 2 1 Blastoff!" where each space is a new line.
- Incorrect! The program will compile, but it won't ever reach "Blastoff!". Try again.
Q-2: Consider the code block below. What happens when you run this program?
n = 5
while n > 0:
print(n)
n = n + 1
print('Blastoff!')
More formally, here is the flow of execution for a while
statement:
Evaluate the condition, yielding
TrueorFalse.If the condition is false, exit the
whilestatement and continue execution at the next statement.If the condition is true, execute the body and then go back to step 1.
This type of flow is called a loop because the third step loops back around to the top. We call each time we execute the body of the loop an iteration. For the above loop, we would say, “It had five iterations”, which means that the body of the loop was executed five times.
The body of the loop should change the value of one or more variables so that eventually the condition becomes false and the loop terminates. We call the variable that changes each time the loop executes and controls when the loop finishes the iteration variable. If there is no iteration variable, the loop will repeat forever, resulting in an infinite loop.
- x = 2; y = 5
- Incorrect! These were the values of x and y at first, but they changed by the time the loop finished executing. Try again.
- x = 5; y = 2
- Incorrect! The while loop will finish executing before x and y reach these values. Try again.
- x = 4; y = 3
- Correct! The loop will terminate at x = 4 and y = 3 because at this point, x is not less than y.
- x = 4; y = 2
- Incorrect! The way the loop modifies x and y, it is impossible for y to be 2 while x is 4. Try again.
Q-4: Consider the code block below. What are the values of x and y when this while loop finishes executing?
x = 2
y = 5
while (y > 2 and x < y):
x = x + 1
y = y - 1