Break and Continue: While Loops

[Steven D'Aprano]

On Thursday 23 June 2016 14:17, Elizabeth Weiss wrote:

> CODE #2:
> 
> i=0
> while True:
>    i=i+1
>   if i==2:
>      print("Skipping 2")
>      continue
>   if i==5:
>      print("Breaking")
>      break
>    print(i)
> 
> ------
> 
> Questions:
> 1. what does the word True have to do with anything here?

Consider:

"while the pasta is too hard to eat, keep boiling it"

"while you are feeling sick, stay in bed and drink plenty of fluids"

"While" repeatedly takes a condition, decides if it is true or false, and then 
decides what to do. If you give it a condition True, that's *always* true, so 
it repeats forever unless you use "break" to escape from the loop.


> 2. i=i+1- I never understand this. Why isn't it i=i+2?

i = i + 1 increase i by one each time around the loop:

i = 0, then 1, then 2, then 3, then 4...

If you used i = i+2, it would increase by two each time around the loop:

i = 0, then 2, then 4, then 6, then ...


> 3. Do the results not include 2 of 5 because we wrote if i==2 and if i==5?
> 4. How is i equal to 2 or 5 if i=0?

i starts off as 0. But then you increase it by 1 each time around the loop.


Can I ask, have you learned about for loops yet? There seems to be a fashion 
among some teachers and educators to teach while loops before for loops. I 
think that is a terrible idea, while loops are so much more complicated than 
for loops.

Try this example instead:

for i in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]:
    if i == 2:
        print("skipping")
        continue
    if i == 5:
        print("breaking")
        break
    print("i =", i)


The main thing you need to know to understand this is that the line "for i in 
..." sets i=0, then runs the indented block under it, then sets i=1 and runs 
the indented block, then sets i=2, and so forth. Run the code and see if it 
makes sense to you. Feel free to ask as many questions as you like!


The line "for i in [0, 1, 2, ..." is a bit wordy and verbose. Can you imagine 
if you wanted to loop 1000 times? Fortunately Python has an abbreviated 
version:

for i in range(10):
    if i == 2:
        print("skipping")
        continue
    if i == 5:
        print("breaking")
        break
    print("i =", i)



-- 
Steve