Programming Tutorials

Pointer Arrays and Pointers to Pointers in C

By: Tamil Selvan in C Tutorials on 2007-09-22  

Since pointers are variables themselves, they can be stored in arrays just as other variables can. Let us illustrate by writing a program that will sort a set of text lines into alphabetic order, a stripped-down version of the UNIX program sort.

Now we deal with lines of text, which are of different lengths, and which, unlike integers, can't be compared or moved in a single operation. We need a data representation that will cope efficiently and conveniently with variable-length text lines.

This is where the array of pointers enters. If the lines to be sorted are stored end-to-end in one long character array, then each line can be accessed by a pointer to its first character. The pointers themselves can bee stored in an array. Two lines can be compared by passing their pointers to strcmp. When two out-of-order lines have to be exchanged, the pointers in the pointer array are exchanged, not the text lines themselves.

This eliminates the twin problems of complicated storage management and high overhead that would go with moving the lines themselves.

The sorting process has three steps:

  read all the lines of input
  sort them
  print them in order

As usual, it's best to divide the program into functions that match this natural division, with the main routine controlling the other functions. Let us defer the sorting step for a moment, and concentrate on the data structure and the input and output.

The input routine has to collect and save the characters of each line, and build an array of pointers to the lines. It will also have to count the number of input lines, since that information is needed for sorting and printing. Since the input function can only cope with a finite number of input lines, it can return some illegal count like -1 if too much input is presented.

The output routine only has to print the lines in the order in which they appear in the array of pointers.

   #include <stdio.h>
   #include <string.h>

   #define MAXLINES 5000     /* max #lines to be sorted */

   char *lineptr[MAXLINES];  /* pointers to text lines */

   int readlines(char *lineptr[], int nlines);
   void writelines(char *lineptr[], int nlines);

   void qsort(char *lineptr[], int left, int right);

   /* sort input lines */
   main()
   {
       int nlines;     /* number of input lines read */

       if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
           qsort(lineptr, 0, nlines-1);
           writelines(lineptr, nlines);
           return 0;
       } else {
           printf("error: input too big to sort\n");
           return 1;
       }
   }

   #define MAXLEN 1000  /* max length of any input line */
   int getline(char *, int);
   char *alloc(int);

   /* readlines:  read input lines */
   int readlines(char *lineptr[], int maxlines)
   {
       int len, nlines;
       char *p, line[MAXLEN];

       nlines = 0;
       while ((len = getline(line, MAXLEN)) > 0)
           if (nlines >= maxlines || p = alloc(len) == NULL)
               return -1;
           else {
               line[len-1] = '\0';  /* delete newline */
               strcpy(p, line);
               lineptr[nlines++] = p;
           }
       return nlines;
   }

   /* writelines:  write output lines */
   void writelines(char *lineptr[], int nlines)
   {
       int i;

       for (i = 0; i < nlines; i++)
           printf("%s\n", lineptr[i]);
   }

The main new thing is the declaration for lineptr:

   char *lineptr[MAXLINES]

says that lineptr is an array of MAXLINES elements, each element of which is a pointer to a char. That is, lineptr[i] is a character pointer, and *lineptr[i] is the character it points to, the first character of the i-th saved text line.

Since lineptr is itself the name of an array, it can be treated as a pointer in the same manner as in our earlier examples, and writelines can be written instead as

   /* writelines:  write output lines */
   void writelines(char *lineptr[], int nlines)
   {
       while (nlines-- > 0)
           printf("%s\n", *lineptr++);
   }

Initially, *lineptr points to the first line; each element advances it to the next line pointer while nlines is counted down.

With input and output under control, we can proceed to sorting.

   /* qsort:  sort v[left]...v[right] into increasing order */
   void qsort(char *v[], int left, int right)
   {
       int i, last;
       void swap(char *v[], int i, int j);

       if (left >= right)  /* do nothing if array contains */
           return;         /* fewer than two elements */
       swap(v, left, (left + right)/2);
       last = left;
       for (i = left+1; i <= right; i++)
           if (strcmp(v[i], v[left]) < 0)
               swap(v, ++last, i);
       swap(v, left, last);
       qsort(v, left, last-1);
       qsort(v, last+1, right);
   }

Similarly, the swap routine needs only trivial changes:

   /* swap:  interchange v[i] and v[j] */
   void swap(char *v[], int i, int j)
   {
       char *temp;

       temp = v[i];
       v[i] = v[j];
       v[j] = temp;
   }

Since any individual element of v (alias lineptr) is a character pointer, temp must be also, so one can be copied to the other.