Why does value change when converting floating point value to int [duplicate] - c

Question

This question already has answers here:
Is floating point math broken?
(31 answers)
Closed 4 years ago.
I am declaring the amount as a float. When I multiply the value by 100 and plug it into an "int" variable the value changes to 419... why is this happening?
#include <stdio.h>
#include <cs50.h>
#include <math.h>
float amount;
int cents, coins, quarters, dimes, nickels, pennies, Q, D, N, P;
int main(void){
do
{
amount = get_float("How much we talkin?\n");
}
while (amount < 0);
printf("cents = %.55f\n", amount);
cents = amount * 100;
printf("cents = %i\n", cents);
quarters = cents % 25;
Q = cents / 25;
printf("quarters = %i\n", quarters);
dimes = quarters % 10;
D = quarters / 10;
printf("dimes = %i\n", dimes);
nickels = dimes % 5;
N = dimes / 5;
printf("nickels = %i\n", nickels);
pennies = nickels % 1;
P = nickels / 1;
printf("pennies = %i\n", pennies);
coins = Q+D+N+P;
printf("%i\n", coins);
}
~/workspace/pset1/cash/ $ ./cash
How much we talkin?
4.2
cents = 4.1999998092651367187500000000000000000000000000000000000
cents = 419
quarters = 19
dimes = 9
nickels = 4
pennies = 0
22
... that link helped. Thank you!

Answer 1

Thank you, I resolved this issue by taking the input from the user as a float, multiplying that value by 100, then rounding to the nearest integer. The code below works, not the cleanest solution in the world.
#include <stdio.h>
#include <cs50.h>
#include <math.h>
float amount;
int cents, coins, quarters, dimes, nickels, pennies, Q, D, N, P;
int main(void){
do
{
amount = get_float("How much we talkin?\n");
}
while (amount < 0);
// Print float input to see value is not precise
printf("amount = %.55f\n", amount);
// Multiply amount by 100, then round to nearest
cents = roundf(amount * 100);
printf("cents = %i\n", cents);
// *Quarters*
quarters = cents % 25;
Q = cents / 25;
printf("quarters = %i\n", Q);
// *Dimes*
dimes = quarters % 10;
D = quarters / 10;
printf("dimes = %i\n", D);
// *Nickels*
nickels = dimes % 5;
N = dimes / 5;
printf("nickels = %i\n", N);
// *Pennies*
pennies = nickels % 1;
P = nickels / 1;
printf("pennies = %i\n", P);
// Add up all the coins
coins = Q+D+N+P;
printf("%i\n", coins);
}
~/workspace/pset1/cash/ $ ./cash
How much we talkin?
4.2
amount = 4.1999998092651367187500000000000000000000000000000000000
cents = 420
quarters = 16
dimes = 2
nickels = 0
pennies = 0
18

Answer 2

To understand this issue, you need to understand how floating point numbers are stored in memory, in their binary form.
You also need to understand that C's type conversion TRUNCATES a float when converting to an integer.
The number 4.2 is stored as:
1 x 4
0 x 2
0 x 1
0 x 0.5
0 x 0.25
1 x 0.125
1 x 0.0625
0 x 0.03125
0 x 0.015625
1 x 0.0078125
1 x 0.00390625
etc
Which sums to 4.199 (and eventually approximates to 4.199999 - but NEVER 4.2)
Multiplying by 100 gives you 419.999(etc) - and truncating this gives you 419 not 420.
The solution (as you have found) is to make sure you ROUND rather than TRUNC