String handling using library functions

Introduction

We use library functions to easily work with text (strings) without writing long code. They help us find length, copy, join, or compare strings quickly.

When you want to find how many characters are in a word or sentence.

When you need to copy one string into another.

When you want to join two strings together.

When you want to check if two strings are the same.

When you want to find a part of a string inside another string.

Syntax

C

#include <string.h>

size_t strlen(const char *str);
char *strcpy(char *dest, const char *src);
char *strcat(char *dest, const char *src);
int strcmp(const char *str1, const char *str2);
char *strstr(const char *haystack, const char *needle);

Always include <string.h> to use these functions.

Strings in C are arrays of characters ending with a special \0 character.

Examples

Finds the length of the string "hello" which is 5.

C

#include <string.h>

size_t length = strlen("hello");

Copies the string "world" into the array dest.

C

char dest[20];
strcpy(dest, "world");

Joins "friend!" to the end of "Hello, ".

C

char str1[20] = "Hello, ";
strcat(str1, "friend!");

Compares two strings; returns 0 if they are the same.

C

int result = strcmp("apple", "apple");

Sample Program

This program shows how to find string length, copy strings, join strings, compare them, and find a substring using library functions.

C

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

int main() {
    char str1[50] = "Hello";
    char str2[] = " World";
    char str3[50];

    // Find length
    printf("Length of str1: %zu\n", strlen(str1));

    // Copy str1 to str3
    strcpy(str3, str1);
    printf("After copy, str3: %s\n", str3);

    // Concatenate str2 to str1
    strcat(str1, str2);
    printf("After concatenation, str1: %s\n", str1);

    // Compare str1 and str3
    int cmp = strcmp(str1, str3);
    if (cmp == 0) {
        printf("str1 and str3 are the same\n");
    } else {
        printf("str1 and str3 are different\n");
    }

    // Find substring
    char *pos = strstr(str1, "World");
    if (pos != NULL) {
        printf("Found 'World' in str1 at position: %ld\n", (long)(pos - str1));
    } else {
        printf("'World' not found in str1\n");
    }

    return 0;
}

Important Notes

Make sure destination arrays are large enough to hold the result to avoid errors.

String functions expect strings to end with \0, so always use proper string initialization.

Comparing strings with strcmp returns 0 if equal, negative if first is smaller, positive if first is larger.

Summary

Library functions make string tasks easy and fast.

Remember to include <string.h> and use proper string arrays.

Common functions: strlen, strcpy, strcat, strcmp, strstr.

Practice

1. Which library function in C is used to find the length of a string (excluding the null character)?

easy

A. strcpy()

B. strcat()

C. strcmp()

D. strlen()

Solution

1. Step 1: Understand the purpose of strlen()

   The function strlen() counts characters in a string until it reaches the null terminator '\0', giving the string length.

2. Step 2: Compare with other functions

   strcpy() copies strings, strcmp() compares strings, strcat() concatenates strings, so they do not measure length.

3. Final Answer:

   strlen() -> Option D

4. Quick Check:

   Length function = strlen() [OK]

Hint: Length means count chars before '\0', use strlen() [OK]

Common Mistakes:

• Confusing strcpy() with strlen()
• Using strcmp() to get length
• Forgetting null terminator in length count

2. Which of the following is the correct syntax to copy string src into dest using a standard library function?

easy

A. strcpy(src, dest);

B. strcpy(dest, src);

C. strcpy(dest);

D. strcpy(src);

Solution

1. Step 1: Recall strcpy() function signature

   strcpy() takes two arguments: destination string first, then source string to copy from.

2. Step 2: Check each option

   Only strcpy(dest, src); matches the correct order and argument count.

3. Final Answer:

   strcpy(dest, src); -> Option B

4. Quick Check:

   Copy syntax = strcpy(dest, src) [OK]

Hint: Destination first, source second in strcpy() [OK]

Common Mistakes:

• Swapping source and destination arguments
• Using only one argument
• Missing semicolon or parentheses

3. What will be the output of the following C code?

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

int main() {
    char str1[20] = "Hello";
    char str2[] = "World";
    strcat(str1, str2);
    printf("%s", str1);
    return 0;
}

medium

A. HelloWorld

B. Hello World

C. WorldHello

D. Hello

Solution

1. Step 1: Understand strcat() behavior

   strcat() appends the second string to the end of the first string without adding spaces.

2. Step 2: Analyze the code output

   str1 starts as "Hello", str2 is "World". After strcat(str1, str2), str1 becomes "HelloWorld".

3. Final Answer:

   HelloWorld -> Option A

4. Quick Check:

   Concatenate without space = HelloWorld [OK]

Hint: strcat joins strings directly, no spaces added [OK]

Common Mistakes:

• Expecting a space between strings
• Confusing strcat() with strcpy()
• Not allocating enough space in destination

4. Identify the error in the following code snippet:

#include <string.h>

int main() {
    char s1[5] = "Data";
    char s2[] = "Science";
    strcat(s1, s2);
    return 0;
}

medium

A. Missing #include <stdio.h>

B. strcat() syntax is incorrect

C. s1 array size is too small for concatenation

D. s2 should be declared as char *s2

Solution

1. Step 1: Check array sizes for strcat()

   s1 has size 5, holding "Data" plus '\0'. Concatenating "Science" (7 chars) will overflow s1.

2. Step 2: Verify other options

   strcat() syntax is correct, #include <stdio.h> is not mandatory here, and s2 declaration is valid as array.

3. Final Answer:

   s1 array size is too small for concatenation -> Option C

4. Quick Check:

   Buffer overflow risk = s1 too small [OK]

Hint: Destination array must fit combined strings [OK]

Common Mistakes:

• Ignoring buffer size for strcat()
• Assuming strcat() adds space automatically
• Confusing array and pointer declarations

5. Given two strings char s1[] = "apple"; and char s2[] = "apricot";, which library function call correctly checks if s1 comes before s2 in dictionary order?

hard

A. strcmp(s1, s2) < 0

B. strcmp(s1, s2) > 0

C. strcmp(s2, s1) < 0

D. strcmp(s2, s1) > 0

Solution

1. Step 1: Understand strcmp() return values

   strcmp() returns a negative value if first string is lexicographically less than second, zero if equal, positive if greater.

2. Step 2: Apply to given strings

   "apple" comes before "apricot", so strcmp(s1, s2) returns negative, meaning strcmp(s1, s2) < 0 is true.

3. Final Answer:

   strcmp(s1, s2) < 0 -> Option A

4. Quick Check:

   Lex order check = strcmp(s1, s2) < 0 [OK]

Hint: strcmp() negative means first string is before second [OK]

Common Mistakes:

• Reversing argument order in strcmp()
• Checking for > 0 instead of < 0
• Assuming strcmp() returns boolean