Expression Result Unused Greedy Algorithm - c

Question

I have run this program and get the error expression result unused. I may be doing something simple wrong, but I have spent the day trying to figure it out to no avail. Any help you can provide is greatly appreciated.
#include <stdio.h>
#include <cs50.h>
int main()
{
int x, y = 0;
printf("Enter the amount of change ");
x = GetFloat() * 100;
while (x != 0)
{
if (x >= 25)
{
x - 25;
y = y + 1;
}
if (x >= 10 && x < 25)
{
x - 10;
} y = y + 1;
if (x >= 5 && x < 10)
{
x - 5;
} y = y + 1;
if (x >= 1 && x < 5)
{ x - 1;
y= y + 1;
}
}
printf("The number of coins neccessary is %d", y);
}

Answer 1

if (x >= 25)
{
x - 25; // This accomplishes nothing
y = y + 1;
}
if (x >= 10 && x < 25)
{
x - 10; // This accomplishes nothing
} y = y + 1;
if (x >= 5 && x < 10)
{
x - 5; // This accomplishes nothing
} y = y + 1;
if (x >= 1 && x < 5)
{
x - 1; // This accomplishes nothing
y= y + 1;
}
In each of those lines you're subtracting a number from x, but you're doing nothing with the result. If you're trying to update x with the result, you need to do just like you're doing with y, and put x = in front of the expression.
So if you want x to go down by 25, you should write:
x = x - 25;
Alternatively, you can write the shorthand:
x -= 25; // Note the equal sign

Answer 2

All the 4 statements x - 25, x- 10, x- 5, x - 1 will turn out to be useless unless you assign that value to x;
Because you are trying to subtract the value from x, but you are not assigning the new value to x.
Here is the solution of your problem:
#include <stdio.h>
#include <cs50.h>
int main()
{
int x, y = 0;
printf("Enter the amount of change ");
x = GetFloat() * 100;
while (x != 0)
{
if (x >= 25)
{
x = x - 25; //or x-=25;
y = y + 1;
}
if (x >= 10 && x < 25)
{
x = x - 10; //or x-=10;
y = y + 1;
}
if (x >= 5 && x < 10)
{
x = x - 5; //or x-=5;
y = y + 1;
}
if (x >= 1 && x < 5)
{
x = x - 1; //or x-=1; or x--; or --x; :)
y = y + 1;
}
}
printf("The number of coins neccessary is %d", y);
}

Answer 3

I would remain to be convinced about your loop structure. There's the division operator that can be used to good effect:
int total = 0;
int ncoins;
int amount = GetFloat() * 100;
assert(amount >= 0);
ncoins = amount / 25;
total += ncoins;
amount -= ncoins * 25;
assert(amount < 25);
ncoins = amount / 10;
total += ncoins;
amount -= ncoins * 10;
assert(amount < 10);
ncoins = amount / 5;
total += ncoins;
amount -= ncoins * 5;
assert(amount < 5);
total += amount;
That's written out longhand; you could devise a loop, too:
int values[] = { 25, 10, 5, 1 };
enum { N_VALUES = sizeof(values) / sizeof(values[0]) };
int total = 0;
int ncoins;
int amount = GetFloat() * 100;
assert(amount >= 0);
for (int i = 0; i < N_VALUES && amount > 0; i++)
{
ncoins = amount / values[i];
total += ncoins;
amount -= ncoins * values[i];
}
assert(amount == 0);