Fahrenheit to Celsius table

K&R Exercise 1-3: print a Fahrenheit–Celsius conversion table, then put a heading above the columns so a reader can tell which number is which.

The conversion

The two common temperature scales are related by:

℃ = (5 / 9) (℉ − 32)

Walk Fahrenheit from 0 to 300 in steps of 20, print the Celsius value beside each, and label the columns. Without a heading the output is just two numbers side by side — fine while you wrote it, opaque if you come back later.

Use floating-point constants

In C, 5 / 9 is integer division, so it becomes 0 and every Celsius value prints as zero. Write 5.0 / 9.0 (and 32.0) so the conversion actually runs.

Write the program

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

#include <stdio.h>

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

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

    printf("Fahrenheit\tCelsius\n");

    fahr = lower;
    while (fahr <= upper) {
        celsius = (5.0 / 9.0) * (fahr - 32.0);
        printf("%3.0f\t%6.1f\n", fahr, celsius);
        fahr = fahr + step;
    }

    return 0;
}

The heading is one extra printf before the loop. %3.0f prints Fahrenheit as a whole number; %6.1f gives Celsius one decimal place. The tabs keep the numbers under the words.

Compile and run

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

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

You should see:

Fahrenheit	Celsius
  0	 -17.8
 20	  -6.7
 40	   4.4
 60	  15.6
 80	  26.7
100	  37.8
120	  48.9
140	  60.0
160	  71.1
180	  82.2
200	  93.3
220	 104.4
240	 115.6
260	 126.7
280	 137.8
300	 148.9

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

Check the heading

The exercise is the heading, not the arithmetic. If the words sit above the columns and 0 °F still comes out around -17.8, you are done. Tweak the format widths if a column drifts.

What you should take away

A table without labels is half a program. printf can print words as well as numbers; put the heading on its own line before the loop. Floating-point constants keep 5 / 9 from collapsing to zero.

Next

→ Celsius to Fahrenheit table — Exercise 1-4