Strings

Objectives

This section brings together the use of two of C's fundamental data types, ponters and arrays, in the use of handling strings.

Having read this section you should be able to:

1. handle any string constant by storing it in an array.

Stringing Along

You might not think that you need a function to copy strings, but simple assignment between string variables doesn't work. For example:

char a[l0],b[10];
b = a;

does not appear to make a copy of the characters in a, but this is an illusion. What actually happens is that the pointer b is set to point to the same set of characters that a points to, i.e. a second copy of the string isn't created.

To do this you need strcopy(a,b) which really does make a copy of every character in a in the array b up to the first null character. In a similar fashion strcat(a,b) adds the characters in b to the end of the string stored in a. Finally there is the all-important strcmp(a,b) which compares the two strings character by character and returns true - that is 0 - if the results are equal.

Again notice that you can't compare strings using a==b because this just tests to see if the two pointers a and b are pointing to the same memory location. Of course if they are then the two strings are the same, but it is still possible for two strings to be the same even if they are stored at different locations.

You can see that you need to understand pointers to avoid making simple mistakes using strings. One last problem is how to initialise a character array to a string. You can't use:

a = "hello";

because a is a pointer and "hello" is a string constant. However, you can use:

strcopy(a,"hello")

because a string constant is passed in exactly the same way as a string variable, i.e. as a pointer. If you are worried where the string constant is stored, the answer is in a special area of memory along with all of the constants that the program uses. The main disadvantage of this method is that many compilers use an optimisation trick that results in only a single version of identical constants being stored. For example:

strcopy(b,"hello");

usually ends up with b pointing to the same string as a. In other words, this method isn't particularly safe!

A much better method is to use array initialisation. You can specify constants to be used to initialise any variable when it is declared. For example:

int a=10;

declares a to be an integer and initialises it to 10. You can initialise an array using a similar notation. For example:

int a[5] = {1,2,3,4,5};

declares an integer array and initialises it so that a[0]= 1, a[1] = 2 and so on. A character array can be initialised in the same way. For example:

char a[5]={'h','e','l','l','o'};

but a much better way is to write:

char a[6]="hello";

which also automatically stores a null character at the end of the string - hence a[6] and not a[5]. If you really want to be lazy you can use:

char a[] = "hello";

and let the compiler work out how many array elements are needed. Some compilers cannot cope with the idea of initialising a variable that doesn't exist for the entire life of the program. For those compilers to make initialisation work you need to add the keyword static to the front of the string declaration, therefore:

static char a[] = "hello";

As easy as... B or C?

A Sort Of Bubble Program

The main routine is sort which repeats the scan function on the array until the variable done is set to 0. The scan function simply scans down the array comparing elements that are next door to each other. If they are in the wrong order then function swap is called to swap them over.

Study this program carefully with particular attention to the way arrays, array elements and variables are passed. It is worth saying that in some cases there are better ways of achieving the same results. In particular, it would have been easier not to use the variable done, but to have returned the state as the result of the scan function.

#include <stdio.h>


void randdat(int a[] , int n);
void sort(int a[] , int n);
void scan(int a[] , int n , int *done);
void swap(int *a ,int *b);

main()
 {
   int i;
   int a[20];

   randdat(a , 20);
   sort(a , 20);

   for(i=0;i<20;++i) printf("%d\n" ,a[i]);
 }

void randdat(int a[1] , int n)
 {
  int i;
  for (i=0 ; i<n ; ++i)
   a[i] = rand()%n+1;
 }

void sort(int a[1] , int n)
 {
  int done;
  done = 1;
  while(done == 1) scan(a , n , &done);
 }

void scan(int a[1] , int n , int *done)
 {
  int i;
  *done=0;
  for(i=0 ; i<n-1 ; ++i)
   {
     if(a[i]<a[i+1])
      {
       swap(&a[i],&a[i+1]);
       *done=1;
      }
   }
 }

void swap(int *a ,int *b)
 {      
  int temp;
  temp = *a;
  *a   = *b;
  *b   = temp;
 }

[program]

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