CS50 Greedy Need Advice

Question

I am doing the cs50 problem set \"Greedy\". Basically asking the user for how much change is owed and then outputting the minimum amount of coins that can equal the inputed amou

Answer 1

Use the round function to round up your numbers. In this part of your code, you need to add the round function.

int cents = (int)(n * 100);

It has to be like this:

cents = round(n * 100);

If you have other such problems, you can use a debugger such as debug50, in which you put a breakpoint by clicking in the right of the line number, then in your terminal window(the one where you execute clang) you should type:

~/pset1/cash/ $ debug50 ./cash

Or whatever your program is called. This will open the debugger and inside it you will find all the variables and what they are equal to. Press on the button next to the play button to move one line of code forward.

Answer 2

It looks like this is an issue with the conversion from float to int. When you try to convert from dollars to cents, you do so with this line of code:

int cents = (int)(n * 100);

However, this line of code, for $4.20, is returning a cent value of 419. This is a problem with rounding and floats, as 4.2 * 100 is returning 419.99999999 instead of 420.0000000, and integer casting truncates instead of rounding. This problem also occurs with $4.18, and probably other values as well.

To prevent this, add 0.5 before the cast, like so:

int cents = (int)(n * 100 + 0.5);

This will ensure that the rounding takes place in the proper direction, as you are never off by more that a trivial float error.

Using the math.h library, you could also use the roundf() function, which will work in the case of negative numbers, just in case.

int cents = (int)(roundf(n*100));