CSC 161 Grinnell College Fall, 2011 Imperative Problem Solving and Data Structures

# 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
*/

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

int quarts;                           /* declarations */
double liters;                        /* double means real */

int main ()                           /* beginning of main program */
{ 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.

This document is available on the World Wide Web as

```http://www.cs.grinnell.edu/~walker/courses/161.fa11/modules/module-getting-started/quarts-annotated.shtml
```

 created created 18 July 2011 last revised 18 July 2011 For more information, please contact Henry M. Walker at walker@cs.grinnell.edu.