Fahrenheit to Celsius table temperature converter in C

Question

I am brand new to programming in C. I am following along with the book "The C Programming Language, Second Edition." I am making my own modifications to the example code presented in the book as I go along. Anyway I am posting my code here for review because I want to make sure I am off to a good start. There a few things in particular that I want to make sure that I am doing right as to not develop any future bad habits:

1. I would like to make sure I am adhering to the C standards. Based on my research I found that the current and most recent standard for C is C17 or ISO/IEC 9899:2018, please correct me if I am wrong. As a result I have been compiling the code with the following parameters: gcc -Wall -pedantic-errors -std=c17 source.c Is this a good practice?

2. I know I am a beginner but I would like to make sure that I am doing even the simplest things in a clean and efficient way.

So if I can get a review on this code sample, that would be great.

/* This program uses the formula C = (5/9)(F-32) to print a table of Farenheit
 * temperatures and their Centigrade or Celsius equivalents. Adapted from:
 * Kernighan, B. W., & Ritchie, D. M. (1988). The C programming language. */

#include <stdio.h>

/* Print Fahrenheit-Celsius table for Fahrenheit = 0, 20, ..., 300. */

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

    lower = 0;   /* Lower limit of temperature table. */
    upper = 300; /* Upper limit. */
    step = 20;   /* Step size. */

    printf("Fahrenheit to Celsius\n"); /* Table header */
    printf("%3s %6s\n", "F", "C");     /* Temperature label */

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

Answer 1

• It's great that you're using C17.
• Your gcc mostly looks sane, though I would add -ggdb and start learning about gdb.
• Since you are using above-C99, I would rewrite

    float fahr, celsius;
    int lower, upper, step;

    lower = 0;   /* Lower limit of temperature table. */
    upper = 300; /* Upper limit. */
    step = 20;   /* Step size. */

as

    const int lower = 0,   // Lower limit of temperature table.
              upper = 300, // Upper limit.
              step = 20;   // Step size.

and move fahr to a for index:

for (double fahr = lower; fahr <= upper; fahr += step)

Also, due to the semantics of float promotion,

(5.0/9.0) * (fahr-32.0)

can just be

(fahr - 32)*5/9

This:

printf("Fahrenheit to Celsius\n"); /* Table header */
printf("%3s %6s\n", "F", "C");     /* Temperature label */

can use implicit string concatenation and character fields:

printf(
    "Fahrenheit to Celsius\n"  // Table header
    "%3c %6c\n",               // Temperature label 
    'F', 'C'
);

Answer 2

I want to make sure I am off to a good start.

float vs double

Save float for cases when serious reduced memory needed or when performance has compelled the use of narrow floating point, else use double as the go-to floating point type.

Units

300 what?
Physical units deserve clarity, perhaps a comment, in code. Be clear or oops!.

// upper = 300; /* Upper limit. */
upper = 300 /* F */; /* Upper limit. */

---

make sure I am adhering to the C standards.

Warnings

Using a modern compiler with many, if not all, warnings enabled is a good thing. Maybe add -Wetra.

---

make sure that I am doing even the simplest things in a clean and efficient way.

Overall

Code looks very clean and mostly efficient.

Unnecessary FP mixed operations

Notice by using float objects, yet double constants, code is like below incurring up and down FP conversions.

celsius = (float)((5.0/9.0) * ((double)fahr-32.0));
printf("%3.0f %6.1f\n", (double)fahr, (double)celsius);

If wanting to stay with float, use float constants

celsius = (5.0f/9.0f) * (fahr-32.0f);

---

Idea: abstract formatting

#define F_WIDTH1 3
#define C_WIDTH2 6

printf("%*s %*s\n", F_WIDTH, "F", C_WIDTH, "C");
...
    printf("%*.0f %*.1f\n", F_WIDTH, fahr, C_WIDTH, celsius);