quarts.c: A first program in C

As a first program in C, consider the following code that converts a number of quarts to liters. Commentary after the program explains each element of this program. A subsequent section explains how this program may be run.

/* A simple program to convert a number of quarts to liters
 * Version 1:  global variables only
 * Author: Henry Walker 
*/

#include <stdio.h>                    /* reference to standard I/O library */

int main ()                           /* beginning of main program */
  {
    int quarts;                       /* variable declarations */
    double liters;                    /* double = real */
    
    printf ("This program converts a number of quarts to liters\n");  
                                      /* write opening statement */

    quarts = 2;                       /* specify the number of quarts as 2 */

    liters = quarts / 1.056710;       /* arithmetic, assignment */

    printf ("%d quarts = %lf liters\n", quarts, liters);
                                      /* write text and new line */

    return 0;                         /* the program ran without errors */
  }

Commentary on the quarts.c program

• Comments in C begin anywhere with the symbols /*, continuing until the symbols */, on the same or later lines.

• C makes use of libraries for many common operations, including the stdio library for input (reading from the keyboard) and output (printing). The statement

  #include <stdio.h>

  instructs the machine how to use the stdio operations.

• Each C program contains a driver function/procedure, called main. Here, main uses no input parameters (hence the parentheses with nothing between them following main). In standard C, the main function/procedure returns an integer as an error code (hence the int before main). When the program runs normally, without error, the integer 0 is usually returned.

• Variables may be declared globally or at the start of a C function. Here, quarts and liters are declared globally as integer and real variables, respectively. The term double specifies a real number, stored using double precision. (The term float may be used for single precision, real numbers.)

• Braces { and } are used in C to mark the beginning and ending of blocks. In this case, the braces indicate the statements for the main procedure.

• Semicolons (;) are used to terminate every statement in C.

• The equal sign (=) is used for assignment. (We will see later that == is used for the comparison operator.)

• Arithmetic operations include +, –, *, and / for addition, subtraction, multiplication, and division. For integers, the division operation / yields the integer quotient, while the modulus operation % gives the remainder.

• printf is used for output. The first parameter is a string, which indicates how the output will be formatted.
  • When printing text only, as in the first output line in this program, the text of the string is enclosed in double quotes " ", and the characters are printed exactly as given.
  • Within a format string, some symbols are used for special symbols. For example,
    • \n stands for a new-line character,
    • \t stands for a tab character,
    • \" stands for a double quote character, and
    • \\ stands for the backslash character itself.
  • When printing the value of a variable, the format string for printf gives the type of data to be displayed:
    • "%d" stands for a (decimal) integer,
    • "%f" stands for a (floating point) real number,
    • "%lf" stands for a double precision (long) real number,
    • "%c" stands for a (single) character,
    • "%s" stands for a character string.

Developing and Running a C Program

To prepare a program in C, one uses a text editor, such as emacs or vi. Do not use a word processing package, such as Word, OpenOffice, or LibreOffice, because these packages insert extensive formatting and font information that will confuse the computer as it tries to follow the instructions in your program.

Once the program has been written with the editor and saved, the program must be translated from C to machine language. (In Scheme, such work is done behind the scenes, but in C the translation must be made explicit.) The translation is called compiling, and the utility used to compile a program is called a compiler. A common compiler for C is called gcc.

The following interaction in a terminal window shows a sample session, including writing, translating, and running quarts.c.

perlis$ emacs quarts.c &
perlis$ gcc -o quarts quarts.c
perlis$ ./quarts
This program converts a number of quarts to liters
2 quarts = 1.892667 liters

In this example, emacs quarts.c & starts the emacs editor to prepare a new program called quarts.c. The ampersand (&) allows editing to take place while the terminal window can still be used for other activities.

To compile the program, the line gcc -o quarts quarts.c uses the gcc with the program we have edited (quarts.c). The directive -o quarts indicates we want the compiler to place our translated program in an output file called quarts.
Warning: Be sure the name of the output file (quarts) is different from the name of your original program (quarts.c). If you use the same name for both, your original program will be overwritten and lost!

You run the translated program by typing ./quarts . As with any command in a terminal window, the prefix ./ indicates that this program may be found in your current directory.