Celsius to Fahrenheit table

K&R Exercise 1-4: print the temperature table the other way around — Celsius on the left, Fahrenheit on the right.

You already have Fahrenheit to Celsius. Same loop, same heading habit, inverted formula.

The formula

Fahrenheit is (9 / 5) × ℃ + 32. Walk Celsius from 0 to 300 in steps of 20, print Fahrenheit beside each value, and label the columns.

Use floating-point constants

In C, 9 / 5 is integer division, so it becomes 1. Then 100 Celsius prints as 132 instead of 212. Write 9.0 / 5.0 (and 32.0) so the conversion actually runs.

Write the program

Create a file called celsius-to-fahrenheit.c:

#include <stdio.h>

int main(void)
{
    float fahr, celsius;
    int lower, step, upper;

    lower = 0;
    step = 20;
    upper = 300;

    printf("Celsius\tFahrenheit\n");

    celsius = lower;
    while (celsius <= upper) {
        fahr = (9.0 / 5.0) * celsius + 32.0;
        printf("%7.0f\t%10.0f\n", celsius, fahr);
        celsius = celsius + step;
    }

    return 0;
}

The skeleton is the book’s table: lower, upper, step, a while. The conversion line is the only real change.

Compile and run

In Terminal, from the folder that holds celsius-to-fahrenheit.c:

gcc celsius-to-fahrenheit.c -o celsius-to-fahrenheit
./celsius-to-fahrenheit

You should see:

Celsius	Fahrenheit
      0	        32
     20	        68
     40	       104
     60	       140
     80	       176
    100	       212
    120	       248
    140	       284
    160	       320
    180	       356
    200	       392
    220	       428
    240	       464
    260	       500
    280	       536
    300	       572

If gcc is missing, finish Getting your macOS ready for C first.

Check the numbers

0 Celsius is freezing, so Fahrenheit should be 32. 100 Celsius is boiling, so Fahrenheit should be 212. If those two are wrong, you probably wrote 9 / 5 without the decimals.

What you should take away

Inverting a conversion is a formula change, not a new program. The loop, the heading, and the floating-point warning stay the same.

Next

→ Reverse temperature table — Exercise 1-5