C Language Strings Explained

1. What Is a String

In C, strings are stored as character arrays and terminated by a null character \0. The C standard library provides various functions for operating on strings. Strings are declared as follows:

char str1[] = "Hello, World!";
char str2[20] = "Hello, C!";
char str3[20]; // Only declared, not yet assigned

2. String Input and Output

In C, you can use printf and scanf functions to output and input strings.

#include <stdio.h>

int main() {
    char str[100];
    printf("Enter a string: ");
    scanf("%s", str);  // Note: scanf can only read strings without spaces
    printf("You entered: %s\n", str);
    return 0;
}

Reading Strings with Spaces

If you need to read strings containing spaces, you can use the gets function or the fgets function.

#include <stdio.h>

int main() {
    char str[100];
    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);  // fgets can read strings containing spaces
    printf("You entered: %s\n", str);
    return 0;
}

3. String Length

Use the strlen function to get the length of a string (excluding \0).

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

int main() {
    char str[] = "Hello, World!";
    int length = strlen(str);
    printf("Length of the string: %d\n", length);
    return 0;
}

4. String Copy

Use the strcpy function to copy one string to another.

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

int main() {
    char src[] = "Hello, World!";
    char dest[50];
    strcpy(dest, src);
    printf("Destination string: %s\n", dest);
    return 0;
}

5. String Concatenation

Use the strcat function to concatenate two strings together.

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

int main() {
    char str1[50] = "Hello, ";
    char str2[] = "World!";
    strcat(str1, str2);
    printf("Concatenated string: %s\n", str1);
    return 0;
}

6. String Comparison

Use the strcmp function to compare two strings. If equal, it returns 0; if the first is greater than the second, it returns a positive value; if the first is less than the second, it returns a negative value.

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

int main() {
    char str1[] = "Hello";
    char str2[] = "World";
    int result = strcmp(str1, str2);
    if (result == 0) {
        printf("Strings are equal.\n");
    } else if (result > 0) {
        printf("str1 is greater than str2.\n");
    } else {
        printf("str1 is less than str2.\n");
    }
    return 0;
}

7. Substring Search

Use the strstr function to search for one string within another.

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

int main() {
    char str[] = "Hello, World!";
    char substr[] = "World";
    char *position = strstr(str, substr);
    if (position != NULL) {
        printf("Substring found at position: %ld\n", position - str);
    } else {
        printf("Substring not found.\n");
    }
    return 0;
}

8. String Conversion

C provides some functions to convert strings to other data types, such as atoi (convert string to integer), atof (convert string to floating-point number), etc.

#include <stdio.h>
#include <stdlib.h>

int main() {
    char str[] = "12345";
    int num = atoi(str);
    printf("Integer: %d\n", num);
    return 0;
}

9. Common String Functions Summary

Function	Description
strlen	Calculate the length of a string
strcpy	Copy a string
strncpy	Copy the first n characters of a string
strcat	Concatenate strings
strncat	Concatenate the first n characters of a string
strcmp	Compare strings
strncmp	Compare the first n characters of a string
strstr	Search for a substring within a string
strchr	Search for a character within a string
strrchr	Search for a character within a string from the end
strtok	Split a string
atoi	Convert a string to an integer
atof	Convert a string to a floating-point number
sprintf	Format a string and store it in another string
sscanf	Read formatted input from a string into variables
fgets	Read a line from a file or standard input into a string
fputs	Write a string to a file or standard output

10. Example Program

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

int main() {
    // 1. String input and output
    char str1[100];
    printf("Enter a string: ");
    fgets(str1, sizeof(str1), stdin);
    printf("You entered: %s\n", str1);

    // 2. String length
    printf("Length of the string: %lu\n", strlen(str1));

    // 3. String copy
    char str2[100];
    strcpy(str2, str1);
    printf("Copied string: %s\n", str2);

    // 4. String concatenation
    char str3[100] = "Hello, ";
    strcat(str3, str2);
    printf("Concatenated string: %s\n", str3);

    // 5. String comparison
    char str4[] = "Hello";
    int cmpResult = strcmp(str4, "Hello");
    if (cmpResult == 0) {
        printf("Strings are equal.\n");
    } else {
        printf("Strings are not equal.\n");
    }

    // 6. Substring search
    char *pos = strstr(str3, "World");
    if (pos) {
        printf("Substring found at position: %ld\n", pos - str3);
    } else {
        printf("Substring not found.\n");
    }

    // 7. String conversion
    char numStr[] = "12345";
    int num = atoi(numStr);
    printf("Converted number: %d\n", num);

    return 0;
}