`while` loops

When we want to do something over and over again, we use a loop.
Python has two constructs for loops, while statements and for statements.
A while loop has this form:
```
while condition:
    statements
```
As with other compound statements, the first line of a while statement must be followed by a colon, the remainder must be indented with respect to the first line, and it may be a simple statement or a block.
As long as the condition is true, the statements are executed.

For example,

>>> n = 0
>>> while n < 5:
	print(n)
	n = n + 1
	
0
1
2
3
4

Note that we can abbreviate expressions like n = n + 1 and n = n * 3 using the operators +=, -=, *= and /=, for example, n += 1, n *= 3
An infinite loop is one that never halts; be careful to insure that the condition for a while loop fails to be satisfied or that there is a way to escape from the loop (see the next slide). (You can interrupt an infinite loop in IDLE with "control-c".)
Practice
- Define a function that takes two integers and repeatedly increases the first and decreases the second, printing them out each time, until they are equal.
- Define a function that takes two coordinates, increments in each direction, and a bound and keeps updating the coordinates with the increments until either 0 or the bound is reached in either direction. The function should print the final coordinates.

Interrupting a loop

Execution of a loop is interrupted by a return statement.

def square_until(x, bound):
    """Keep squaring x till it goes over bound, returning that number."""
    while True:
        if x > bound:
            return x
        x = x**2

Another way to interrupt a loop is with a break statement.

def squares_until(x, bound):
    """Keep squaring x till it goes over bound, printing out all the results."""
    while True:
        if x > bound:
            break
        print(x)
        x = x**2

Practice
- Define a function that takes a bound between 0 and 1 and prints out random numbers until one of them is less than the bound.
- Define a function that takes a character (a string) and asks for a word from the user containing that string. It keeps trying until the user complies and then returns the final submitted string.
- Define a version of the last function that is impatient and stops trying after the user fails to comply five times.

while loops

Interrupting a loop

Answers

`while` loops