Programming Tutorials

Character Arrays in C

By: Manoj Kumar in C Tutorials on 2007-09-20  

The most common type of array in C is the array of characters. To illustrate the use of character arrays and functions to manipulate them, let's write a program that reads a set of text lines and prints the longest. The outline is simple enough:

   while (there's another line)
       if (it's longer than the previous longest)
           (save it)
           (save its length)
   print longest line

This outline makes it clear that the program divides naturally into pieces. One piece gets a new line, another saves it, and the rest controls the process.

Since things divide so nicely, it would be well to write them that way too. Accordingly, let us first write a separate function getline to fetch the next line of input. We will try to make the function useful in other contexts. At the minimum, getline has to return a signal about possible end of file; a more useful design would be to return the length of the line, or zero if end of file is encountered. Zero is an acceptable end-of-file return because it is never a valid line length. Every text line has at least one character; even a line containing only a newline has length 1.

When we find a line that is longer than the previous longest line, it must be saved somewhere. This suggests a second function, copy, to copy the new line to a safe place.

Finally, we need a main program to control getline and copy. Here is the result.

   #include <stdio.h>
   #define MAXLINE 1000   /* maximum input line length */

   int getline(char line[], int maxline);
   void copy(char to[], char from[]);

   /* print the longest input line */
   main()
   {
       int len;            /* current line length */
       int max;            /* maximum length seen so far */
       char line[MAXLINE];    /* current input line */
       char longest[MAXLINE]; /* longest line saved here */

       max = 0;
       while ((len = getline(line, MAXLINE)) > 0)
           if (len > max) {
               max = len;
               copy(longest, line);
           }
       if (max > 0)  /* there was a line */
           printf("%s", longest);
       return 0;
   }

   /* getline:  read a line into s, return length  */
   int getline(char s[],int lim)
   {
       int c, i;

       for (i=0; i < lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
           s[i] = c;
       if (c == '\n') {
           s[i] = c;
           ++i;
       }
       s[i] = '\0';
       return i;
   }

   /* copy:  copy 'from' into 'to'; assume to is big enough */
   void copy(char to[], char from[])
   {
       int i;

       i = 0;
       while ((to[i] = from[i]) != '\0')
           ++i;
   }

The functions getline and copy are declared at the beginning of the program, which we assume is contained in one file.

main and getline communicate through a pair of arguments and a returned value. In getline, the arguments are declared by the line

   int getline(char s[], int lim);

which specifies that the first argument, s, is an array, and the second, lim, is an integer. The purpose of supplying the size of an array in a declaration is to set aside storage. The length of an array s is not necessary in getline since its size is set in main. getline uses return to send a value back to the caller, just as the function power did. This line also declares that getline returns an int; since int is the default return type, it could be omitted.

Some functions return a useful value; others, like copy, are used only for their effect and return no value. The return type of copy is void, which states explicitly that no value is returned.

getline puts the character '\0' (the null character, whose value is zero) at the end of the array it is creating, to mark the end of the string of characters. This conversion is also used by the C language: when a string constant like

   "hello\n"

appears in a C program, it is stored as an array of characters containing the characters in the string and terminated with a '\0' to mark the end.

The %s format specification in printf expects the corresponding argument to be a string represented in this form. copy also relies on the fact that its input argument is terminated with a '\0', and copies this character into the output.

It is worth mentioning in passing that even a program as small as this one presents some sticky design problems. For example, what should main do if it encounters a line which is bigger than its limit? getline works safely, in that it stops collecting when the array is full, even if no newline has been seen. By testing the length and the last character returned, main can determine whether the line was too long, and then cope as it wishes. In the interests of brevity, we have ignored this issue.

There is no way for a user of getline to know in advance how long an input line might be, so getline checks for overflow. On the other hand, the user of copy already knows (or can find out) how big the strings are, so we have chosen not to add error checking to it.