String comparison

Introduction

We compare strings to check if two words or sentences are the same or to find which one comes first in order.

Checking if a username matches a stored name.

Sorting a list of words alphabetically.

Finding if a password entered matches the saved password.

Comparing two text inputs to see if they are equal.

Syntax

C

#include <string.h>

int strcmp(const char *str1, const char *str2);

strcmp returns 0 if the strings are exactly the same.

If the first string is 'less' than the second, strcmp returns a negative number; if 'greater', a positive number.

Examples

This compares two identical strings and result will be 0.

C

#include <string.h>

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

Since "apple" comes before "banana", result will be negative.

C

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

Since "banana" comes after "apple", result will be positive.

C

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

Sample Program

This program compares two pairs of strings and prints the results and their order.

C

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

int main() {
    char str1[] = "hello";
    char str2[] = "hello";
    char str3[] = "world";

    int res1 = strcmp(str1, str2);
    int res2 = strcmp(str1, str3);

    printf("Compare str1 and str2: %d\n", res1);
    printf("Compare str1 and str3: %d\n", res2);

    if (res1 == 0) {
        printf("str1 and str2 are the same.\n");
    } else {
        printf("str1 and str2 are different.\n");
    }

    if (res2 < 0) {
        printf("str1 comes before str3.\n");
    } else if (res2 > 0) {
        printf("str1 comes after str3.\n");
    } else {
        printf("str1 and str3 are the same.\n");
    }

    return 0;
}

Important Notes

Always include to use strcmp.

strcmp compares strings letter by letter using ASCII values.

Do not use == to compare strings in C; it compares addresses, not content.

Summary

Use strcmp to compare two strings in C.

strcmp returns 0 if strings are equal, negative if first is less, positive if first is greater.

Always include and do not use == for string content comparison.

Practice

1. Which function is used in C to compare the contents of two strings?

easy

A. strcpy

B. strlen

C. strcmp

D. strcat

Solution

1. Step 1: Identify string comparison functions

   Among the options, only strcmp is designed to compare two strings character by character.

2. Step 2: Understand other functions

   strcpy copies strings, strlen measures length, strcat concatenates strings; none compare contents.

3. Final Answer:

   strcmp -> Option C

4. Quick Check:

   strcmp compares strings [OK]

Hint: Use strcmp to compare strings, not == operator [OK]

Common Mistakes:

• Using == to compare strings
• Confusing strcpy with strcmp
• Assuming strlen compares strings

2. Which header file must be included to use the strcmp function in C?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall standard headers for string functions

   String functions like strcmp are declared in the <string.h> header file.

2. Step 2: Verify other headers

   <stdio.h> is for input/output, <stdlib.h> for general utilities, <ctype.h> for character handling, none provide strcmp.

3. Final Answer:

   <string.h> -> Option D

4. Quick Check:

   strcmp needs <string.h> [OK]

Hint: Include for strcmp and other string functions [OK]

Common Mistakes:

• Forgetting to include
• Including only
• Using wrong header like

3. What is the output of the following C code?

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

int main() {
    char a[] = "apple";
    char b[] = "apricot";
    int result = strcmp(a, b);
    if (result < 0) {
        printf("a is less than b\n");
    } else if (result > 0) {
        printf("a is greater than b\n");
    } else {
        printf("a and b are equal\n");
    }
    return 0;
}

medium

A. a is less than b

B. a is greater than b

C. a and b are equal

D. Compilation error

Solution

1. Step 1: Understand strcmp result for "apple" and "apricot"

   strcmp compares strings lexicographically; "apple" is less than "apricot" because 'l' (in apple) comes before 'r' (in apricot).

2. Step 2: Check the if conditions

   Since result is negative, the program prints "a is less than b".

3. Final Answer:

   a is less than b -> Option A

4. Quick Check:

   strcmp("apple", "apricot") < 0 [OK]

Hint: strcmp returns negative if first string is lexically smaller [OK]

Common Mistakes:

• Assuming strcmp returns boolean
• Confusing lex order of 'l' and 'r'
• Expecting equality for different strings

4. Identify the error in this code snippet comparing two strings:

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

int main() {
    char str1[] = "hello";
    char str2[] = "hello";
    if (str1 == str2) {
        printf("Strings are equal\n");
    } else {
        printf("Strings are not equal\n");
    }
    return 0;
}

medium

A. Using '==' to compare strings instead of strcmp

B. Missing #include header

C. Incorrect printf format specifier

D. Strings are not initialized properly

Solution

1. Step 1: Understand string comparison in C

   Using '==' compares addresses, not string contents, so it does not check if strings are equal.

2. Step 2: Correct method to compare strings

   Use strcmp(str1, str2) == 0 to check if strings have the same content.

3. Final Answer:

   Using '==' to compare strings instead of strcmp -> Option A

4. Quick Check:

   Use strcmp for string content comparison [OK]

Hint: Never use '==' for strings; use strcmp instead [OK]

Common Mistakes:

• Using '==' operator for string comparison
• Ignoring string content vs pointer comparison
• Not including when using strcmp

5. You want to check if two strings str1 and str2 are equal ignoring case differences in C. Which approach correctly achieves this?

hard

A. Use strcmp(str1, str2) == 0

B. Use strcasecmp(str1, str2) == 0

C. Use strcmp(tolower(str1), tolower(str2)) == 0

D. Use strcmp(str1, str2) != 0

Solution

1. Step 1: Understand case-insensitive comparison

   strcmp compares case-sensitively, so it fails if cases differ. strcasecmp compares ignoring case.

2. Step 2: Evaluate other options

   Use strcmp(str1, str2) == 0 is case-sensitive; Use strcmp(tolower(str1), tolower(str2)) == 0 is invalid because tolower expects a char, not a string; Use strcmp(str1, str2) != 0 checks inequality, not equality.

3. Final Answer:

   Use strcasecmp(str1, str2) == 0 -> Option B

4. Quick Check:

   strcasecmp compares strings ignoring case [OK]

Hint: Use strcasecmp for case-insensitive string comparison [OK]

Common Mistakes:

• Using strcmp for case-insensitive check
• Applying tolower to whole strings incorrectly
• Confusing equality and inequality checks