Why is celsius = 5 * (fahr - 32) / 9 ?
There is a program to print Fahrenheit-Celsius table as below.
#include <stdio.h>
/* print Fahrenheit-Celsius table
for fahr = 0, 20, ..., 300 */
int main()
{
int fahr, celsius;
int lower, upper, step;
lower = 0; /* lower limit of temperature table */
upper = 300; /* upper limit */
step = 20; /* step size */
fahr = lower;
while (fahr <= upper) {
celsius = 5 * (fahr - 32) / 9;
printf("%d\t%d\n", fahr, celsius);
fahr = fahr + step;
}
}
The right part of the
figure is the output of this program. The Celsius temperature is computed and assigned to the variable celsius by
the statement
celsius = 5 * (fahr - 32) / 9;
The reason for multiplying by 5 and then dividing by 9 instead of just multiplying by 5/9 is that in C, as in many other languages, integer division truncates: any fractional part is discarded. Since 5 and 9 are integers, 5/9 would be truncated to zero and so all the Celsius temperatures would be reported as zero.
