Pointers and Strings

POINTERS AND STRINGS

In C, strings are treated as arrays of characters that are terminated with a binary zero character (written as '\0'). Consider, for example,

char str[10];

str[0] = 'H';

str[1] = 'i';

str [2] = '!':

str [3] = '\0';

C language provides two alternate ways of declaring and initializing a string. First, you may write:

char str[10] = {'H', 'i', '!', '\0');

But this also takes more typing than is convenient. So, C permits:

char str[10] = "Hi!";

When the double quotes are used, NULL character ('\0') is automatically appended to the end of the string.

When a string is declared like this, the compiler sets aside a contiguous block of memory 10 bytes long to hold characters and initializes its first four characters to Hi!\0.

Now, consider the following program that prints a text.

#include <stdio.h>

int main( )

{

char str[] = "Hello";

char *pstr;

pstr = str;

printf("\n The string is: ");

while (*pstr != '\0')

{

printf("%c", *pstr);

pstr++;

}

return 0;

}

Output

The string is: Hello

In this program, we declare a character pointer *pstr to show the string on the screen. We then point the pointer pstr at str. Then we print each character of the string in the while loop. Instead of using the while loop, we could have straightaway used the function puts() as shown below.

puts (pstr);

The function prototype for puts () is as follows:

int puts (const char *s);

Here the const modifier is used to assure the user that the function will not modify the contents pointed to by the source pointer. The address of the string is passed to the function as an argument.

The parameter passed to puts () is a pointer which is the address to which it points to, or, simply, an address. Thus writing puts (str); means passing the address of str[0].

Similarly, when we write puts (pstr); we are passing the same address, because we have written pstr str;

Now consider the code which displays a string using pointers.

#include <stdio.h>

int main(void)

{

char *str ="Welcome to the world of programming";

char *pstr; // pointer to character

pstr = str;

while (*pstr! = '\0')

{

printf("%c", *pstr);

pstr++;

}

return 0;

}

Welcome to the world of programming

In the above code, pstr is assigned the address of the string, str. The pointer pstr can then be used to point to successive characters until the NULL character is reached. Consider another program which reads a string and then scans each character to count the number of upper and lower case characters entered.

#include <stdio.h>

int main()

{

char str[100], *pstr;

int upper = 0, lower = 0;

printf("\n Enter the string: ");

gets (str);

pstr = str;

while (*pstr != '\0')

{

if (*pstr >= 'A' && *pstr <= 'Z')

upper++;

else if (*pstr >= 'a' && *pstr <= 'z')

lower++;

pstr++;

}

printf("\n Total number of upper case characters = %d", upper);

printf("\n Total number of lower case characters = %d", lower);

return 0;

}

Output

Enter the string: How are you

Total number of upper case characters = 1

Total number of lower case characters = 8

22. Write a program to read and print a text. Also count the number of characters, words, and lines in the text.

#include <stdio.h>

int main()

{

char str[100], *pstr;

int chars = 1, lines = 1, words = 1;

printf("\n Enter the string: ");

pstr = str;

while(*pstr != '\0')

{

if (*pstr = = '\n')

lines++;

if (*pstr = =' '&& *(pstr+1)!= ' ')

words++;

chars++;

pstr++;

}

printf("\n Enter the string: ");

puts (str);

printf("\n Number of characters = %d", chars);

printf("\n Number of lines = %d", lines);

printf("\n Number of words = %d", words);

return 0;

}

Output

Enter the string: How are you

Number of characters = 11

Number of lines = 1

Number of words = 3

23. Write a program to copy a character array in another character array.

#include <stdio.h>

#include <conio.h>

int main()

{

char str[100], copy_str [100];

char *pstr, *pcopy_str;

int i = 0;

clrscr();

pstr = str;

pcopy_str = copy_str;

printf("\n Enter the string: ");

gets (str);

while (*pstr != '\0')

{

*pcopy_str =*pstr;

pstr++, pcopy_str++;

}

*pcopy_str ='\0';

printf("\n The copied text is: ");

pcopy_str = copy_str;

while (*pcopy_str != '\0')

{

printf("%c", *pcopy_str);

pcopy_str++;

}

getch();

return 0;

}

Output

Enter the string: C Programming

The copied text is: C Programming

24. Write a program to copy n characters of a character array from the m^th position in another character array.

#include <stdio.h>

int main()

{

char str[100], copy_str [100];

char *pstr, *pcopy_str;

int m, n, i = 0;

pstr = str;

pcopy_str = copy_ str;

printf("\n Enter the string: ");

gets (pstr);

printf("\n Enter the position from which to start: ");

scanf("%d", &m);

printf("\n Enter the number of characters to be copied: ");

scanf("%d", &n);

pstr = pstr + m;

i=0;

while(*pstr != '\0' && i <n)

{

*pcopy_str = *pstr;

pcopy_str++;

pstr++;

i++;

}

*pcopy_str = '\0';

printf("\n The copied text is: ");

puts (copy_str);

return 0;

}

Output

Enter the string: How are you

Enter the position from which to start: 2

Enter the number of characters to be copied: 5

The copied text is: w are

25. Write a program to copy the last n characters of a character array in another character array.

#include <stdio.h>

#include <string.h>

int main()

{

char str[100], copy_str [100];

char *pstr, *pcopy_str;

int i = 0, n;

pstr = str;

pcopy_str = copy_str;

printf("\n Enter the string: ");

gets (str);

printf("\n Enter the number of characters to be copied(from the end): ");

scanf("%d", &n);                                      

pstr = pstr + strlen(str) - n

while (*pstr != '\0')

{

*pcopy_str = *pstr;

pstr++; pcopy_str++;

}

*pcopy_str = '\0';

printf("\n The copied text is: ");

puts(copy_str);

return 0;

}

Output

Enter the string: Hi there

Enter the number of characters to be copied (from the end): 5

The copied text is: there

26. Write a program to read a text, delete all the semi- colons it has, and finally replace all '.' with a ','.

#include <stdio.h>

int main()

{

char str[100], copy_str [100];

char *pstr, *pcopy_str;

pstr = str;

pcopy_str = copy_str;

printf("\n Enter the string: ");

gets (str);

while(*pstr != '\0')

if (*pstr != ';')

{ } // do nothing

else if (*pstr =='.')

*pcopy_str = ','

else

*pcopy_str = *pstr;

pstr++; pcopy_str++;

}

*pcopy_str = '\0';

printf("\n The new text is: ");

pcopy_str = copy_str;

while (*pcopy_str != '\0')

{

printf("%c", *pcopy_str);

pcopy_str++;

}

return 0;

}

Output

Enter the string: Programming in C; is a book written by; Reema Thareja.

The new text is: Programming in C is a book written by Reema Thareja,

27. Write a program to reverse a string.

#include <stdio.h>

int main()

{

char str[100], copy_str [100];)

char *pstr, *pcopy_str;

pstr = str;

pcopy_str = copy_str;

printf("\n Enter * to end");

printf("\n Enter the string: ");

scanf("%c", pstr);

while (*pstr != '*')

{

pstr++;

scanf("%c", pstr);

}

*pstr = '\0';

pstr--;

while (pstr >= str)

{

*pcopy_str = *pstr;

pcopy_str++;

pstr--;

}

*pcopy_str = '\0';

printf("\n The new text is: ");

pcopy_str = copy_str;

while (*pcopy_str != '\0')

{

printf("%c", *pcopy_str);

pcopy_str++;

}

return 0;

}

Output

Enter to end

Enter the string: Learning C++

The new text is: ++ C gninraeL

28. Write a program to concatenate two strings.

#include <stdio.h>

int main()

{

char str1 [100], str2 [100], copy_str [200];

char *pstrl, *pstr2, *pcopy_str;

clrscr();

pstr1 = str1;

pstr2 = str2;

pcopy_str = copy_str;

printf("\n Enter the first string: ");

gets (strl);

printf("\n Enter the second string: ");

gets (str2);

while (*pstrl != '\0')

{

*pcopy_str = *pstr1;

pcopy_str++, pstr1++;

}

while (*pstr2 != '\0')

{

*pcopy_str = *pstr2;

pcopy_str++, pstr2++;

}

*pcopy_str = '\0';

printf("\n The new text is: ");

pcopy_str = copy_str;

while (*pcopy_str != '\0')

{

printf("%c", *pcopy_str);

pcopy_str++;

}

return 0;

}

Output

Enter the first string: Programming in C by

Enter the second string: Reema Thareja

The new text is: Programming in C by Reema Thareja