CPSC120A
Fundamentals of Computer Science

Day 16 Notes

While Loops

Reading Questions

Quiz

While Loop

Another name is conditional loop, a loop that runs while a condition is true
```
i = 0
while i < 10:
    print(i)
    i = i + 1
```
Loop will run while x is less than 10

What happens if we get the condition wrong?

i = 0
while i > 10:
    print(i)
    i = i + 1

The loop won’t run because 0 is not greater than 10

What if we get the change in x wrong?

i = 0
while i < 10:
    print(i)
    i = i - 1

Inifinte loop, hold control and press c to kill in Idle
Any for loop can be written as a while loop
The above loop as a for loop is
```
for i in range(10):
    print(i)
```
Which is much more concise with fewer opportunities for errors
So why use a while loop?
Because not all while loops can be written as a for loop
For loops are for when you know the number of times the loop will run
Sometimes you have inputs, user or function parameters that will make predicting the number of iteration impossible
```
user_input = input('> ')
while user_input != 'bye':
    print(user_input)
    user_input = input('> ')
```
Note, there are two prompts because we need user for the first while test

Could get around this by reordering the loop body

user_input = ''
while user_input != 'bye':
    user_input = input('> ')
    print(user_input)

It’s also worth emphasizing that the loop condition is true if it should continue to run
But when writing loops you often think in terms of when you want the loop to stop
In that case, just negate the stopping condition
Want to stop when user_input == 'bye', so the stopping condition is user_input != 'bye'
To negate any comparison use this table:

a == b	a != b
a != b	a == b
a < b	a >= b
a > b	a <= b
a <= b	a > b
a >= b	a < b

To negate a logical expression use DeMorgan’s Law:

p and q	not p or not q
p or q	not p and not q

Lab