Python, -1 return value

Question

So guys, I have a funky error going on and I'm wondering if you could help me out. I have a function that's supposed to find the way to give change using the least coins.

def change_counter(cost, paid):
    changefor = paid - cost

and i have a a variable, changefor which is basically how much money you need to make change for. Then I declared some variables, penny = 0 and such that track how many of each coin are being given out. Then I made a while loop, that cycles until changefor hits zero.

while(changefor > 0):
    if changefor >= 100:
        changefor - 100
        ++hundreddollar
    elif,etc

The rest is really self explanatory, and just printing the amounts of each coin used, etc. Does anyone see what could be causing a problem? I get a return value of -1, and nothing from the while loop seems to be doing anything.

http://www.geeksforgeeks.org/dynamic-programming-set-7-coin-change/ — brunsgaard, Mar 20 '15 at 04:28
Thanks for the link, I'll definitely read that but do you have any idea what could be causing an error from the code I posted? — user3250420, Mar 20 '15 at 04:29
Not sure.. if you throw me a gist on the whole script i will tell you why ;) — brunsgaard, Mar 20 '15 at 04:33

score 2 · Accepted Answer · edited May 23 '17 at 12:06

Because you're computing things but not storing them anywhere

First, thank you for teaching me something new.

++x

does compile, and also does nothing. Look here, Why are there no ++ and -- operators in Python?, but there are no increment/decrement operators in python. Similarly,

changefor - 100

Doesn't do anything. Sure, it evaluates changefor and subtracts 100, but then it doesn't store that value anywhere. Pitfall of a dynamic language.

changefor = changefor - 100

or

changefor -= 100

will do you better

Edit: I was about to ask why the decrement syntax is accepted and found that this person Behaviour of increment and decrement operators in Python already ran into this oddity.

You beat me to this - I was trying to figure out what on earth this was doing. — Joel, Mar 20 '15 at 04:35

Python, -1 return value

1 Answers1