Processing strings using pointers

 

Processing Strings with Pointers in C

Using pointers to manipulate strings in C allows efficient traversal, copying, concatenation, and modification without relying on array indexing.

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1. Basics of Strings with Pointers

In C, a string is an array of characters terminated by a null character ('\0'). A pointer can be used to access each character sequentially.

#include <stdio.h>

int main() {
    char str[] = "Hello, World!";
    char *p = str;  // Pointer to the first character of the string

    // Print each character using the pointer
    while (*p != '\0') {
        printf("%c ", *p);  // Dereference to get character
        p++;                // Move to the next character
    }

    return 0;
}

Explanation:

• p starts at &str[0].

• *p gives the character at the current pointer location.

• p++ moves the pointer to the next character.

• This prints all characters in the string sequentially.

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2. Finding the Length of a String Using Pointers

We can find the length of a string without strlen() by iterating until the null character is reached.

#include <stdio.h>

int string_length(char *p) {
    int length = 0;
    while (*p != '\0') {
        length++;
        p++;  // Move to the next character
    }
    return length;
}

int main() {
    char str[] = "Pointer Example";
    printf("Length of '%s' is: %d\n", str, string_length(str));
    return 0;
}

Explanation:

• The pointer p iterates through the string.

• Each character increments the length counter until '\0' is reached.

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3. Copying a String Using Pointers

Pointers can be used to copy a string from one location to another.

#include <stdio.h>

void string_copy(char *dest, char *src) {
    while (*src != '\0') {
        *dest = *src;  // Copy character
        src++;         // Move to next character in source
        dest++;        // Move to next position in destination
    }
    *dest = '\0';      // Null-terminate destination
}

int main() {
    char src[] = "Copy me!";
    char dest[20];  // Ensure large enough to hold source
    string_copy(dest, src);
    printf("Copied string: %s\n", dest);
    return 0;
}

Explanation:

• Characters are copied one by one from src to dest.

• Both pointers advance after each copy.

• Null character '\0' is added at the end.

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4. Reversing a String Using Pointers

Two pointers can be used to reverse a string in place.

#include <stdio.h>

void string_reverse(char *str) {
    char *start = str;
    char *end = str;

    // Move 'end' to the last character
    while (*(end + 1) != '\0') {
        end++;
    }

    // Swap characters until pointers meet
    while (start < end) {
        char temp = *start;
        *start = *end;
        *end = temp;
        start++;
        end--;
    }
}

int main() {
    char str[] = "Reverse me!";
    string_reverse(str);
    printf("Reversed string: %s\n", str);
    return 0;
}

Explanation:

• start points to the first character, end to the last.

• Swap *start and *end, then move pointers toward each other.

• Repeat until start >= end.

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5. Concatenating Two Strings Using Pointers

To concatenate, move the dest pointer to the end of the first string and copy the second string there.

#include <stdio.h>

void string_concat(char *dest, char *src) {
    // Move dest to end of first string
    while (*dest != '\0') {
        dest++;
    }

    // Copy src to dest
    while (*src != '\0') {
        *dest = *src;
        dest++;
        src++;
    }

    *dest = '\0';  // Null-terminate concatenated string
}

int main() {
    char str1[50] = "Hello, ";
    char str2[] = "World!";

    string_concat(str1, str2);
    printf("Concatenated string: %s\n", str1);
    return 0;
}

Explanation:

• dest pointer moves to the null terminator of the first string.

• src is copied character by character to the end of dest.

• Null character is added at the end to terminate the concatenated string.