cs50 Problem Set 1: Greedy. float 0.01 problem

Question

sorry for the long title I'm currently doing cs50's problem set 1's Greedy and I've run into an issue. It seems that every cent works except for one's that need to use .01 cent. I've changed my code to display what the command gets when I give it 0.01 cents. Here's the code:

#include <cs50.h>
#include <math.h>
#include <stdio.h>

int main (void)
{   
int changeamount = 0;
printf("Amount to be converted to change: ");
float change = GetFloat();
float roundf(float change);
for (changeamount = changeamount - 1 + 1; change >= .25; changeamount++)
{
    change = change - .25;
}
for (changeamount = changeamount - 1 + 1; change >= .10; changeamount++)
{
    change = change - .10;
}
for (changeamount = changeamount - 1 + 1; change >= .05; changeamount++)
{
    change = change - .05;

}
for (changeamount = changeamount - 1 + 1; change >= .01; changeamount++)
{
    change = change - .01;
}
change = change + changeamount;
printf("%.50f\n", change);

}

It only doesn't work whenever I input an amount of money which would take the 1 cent to show (like: 1.01 or 0.41). Here's what it tells me when I input 0.01:

0.00999999977648258209228515625000000000000000000000

I've tried rounding it using:

float round(float change)

I've also tried converting it to an int and then rounding it:

int changeint = change * 100 / 100

Both of which didn't end up helping. So any tips/hints at what's wrong with the code would be great! (I don't want to get a direct answer as that could be considered cheating)

EDIT: Here's the updated code:

#include <cs50.h>
#include <math.h>
#include <stdio.h>

int main (void)
{   
int changetwentyfive = 0;
int changeten = 0;
int changefive = 0;
int changeone = 0;
int changeamount = 0;
printf("Amount to be converted to change: ");
float changefloat = GetFloat();
if (changefloat < 0)
{
do
{
printf("Amount to be converted to change: ");
changefloat = GetFloat();
}
while(changefloat < 0);
}
int change = changefloat * 100;
for (changeamount = changeamount - 1 + 1; change >= 25; changeamount++)
{
    change = change - 25;
    changetwentyfive++;
}
for (changeamount = changeamount - 1 + 1; change >= 10; changeamount++)
{
    change = change - 10;
    changeten++;
}
for (changeamount = changeamount - 1 + 1; change >= 5; changeamount++)
{
    change = change - 5;
    changefive++;

}
for (changeamount = changeamount - 1 + 1; change >= 1; changeamount++)
{
    change = change - 1;
    changeone++;
}
printf("%d\n", changeamount);
printf("25: %d, 10: %d, 5: %d, 1: %d", changetwentyfive, changeten,     changefive, changeone);

}

Hope you can help me.

Answer 1

Most of the problem is that no matter what you do, you keep putting the result back into change, a float. Floats have this nasty little problem that they don't store fractional parts of a number, the decimal part, correctly. In fact, your question demonstrates it well. If you store 0.01 in a float, it actually gets stored as 0.00999999977648258209228515625. This happens because everything stored in a computer is stored in binary. Most fractional parts of a whole number simply cannot be completely accurately stored in a computer because of the conversion from decimal to binary. In fact, this is the whole point of this lesson!

That's why it is necessary to convert to something that is in whole numbers and can be stored as an integer. (That's a really big hint.) Also remember, when converting from a float to an int, the decimal portion gets truncated at the decimal point, not rounded. That means that if you have a float of 2.999 and store it in an int, the number that is stored is 2, not 3.

This should be enough to get you going. If this answers your question, please click on the check mark to accept. Let's keep up on forum maintenance. ;-)

Answer 2

When 4.2 is stored as float, it is stored as 4.199999....Doing changefloat=changefloat*100 causes the value stored in changefloat as 419.999....Now do changefloat=round(changefloat). This changes value of changefloat to 420.0000001...Storing this value in an integer variable by typecasting like: int change=(int)changefloat ; makes the value of change to be 420.

Answer 3

The test case having 4.2 as amount is stored as 4.19.Try this : changefloat = round(changefloat); int change = changefloat * 100; This will change the amount from 4.19 to 4.2.This change will surely help you pass the test case.If this answers your question, please click on the check mark to accept.