String comparison - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of strcmp with equal strings

What is the output of the following C code?

C

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

int main() {
    char a[] = "hello";
    char b[] = "hello";
    int result = strcmp(a, b);
    printf("%d", result);
    return 0;
}

A Undefined behavior

B 0

C -1

D 1

💡 Hint

strcmp returns 0 when strings are equal.

The strcmp function returns 0 when both strings are exactly the same.

❓ Predict Output

intermediate

Output of strcmp with first string less than second

What is the output of this C code snippet?

C

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

int main() {
    char a[] = "apple";
    char b[] = "banana";
    int result = strcmp(a, b);
    printf("%d", result);
    return 0;
}

A A negative number

B 0

C A positive number

D Compilation error

💡 Hint

strcmp returns negative if first string is less in lexicographic order.

Since "apple" is lexicographically less than "banana", strcmp returns a negative value.

❓ Predict Output

advanced

Output of strcmp with case difference

What will this C program print?

C

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

int main() {
    char a[] = "Hello";
    char b[] = "hello";
    int result = strcmp(a, b);
    printf("%d", result);
    return 0;
}

A A positive number

B 0

C Runtime error

D A negative number

💡 Hint

Uppercase letters have lower ASCII codes than lowercase letters.

Since 'H' (ASCII 72) is less than 'h' (ASCII 104), strcmp returns a negative number.

❓ Predict Output

advanced

Output of strcmp with empty string

What is the output of this C code?

C

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

int main() {
    char a[] = "";
    char b[] = "nonempty";
    int result = strcmp(a, b);
    printf("%d", result);
    return 0;
}

A A negative number

B 0

C A positive number

D Segmentation fault

💡 Hint

Empty string is less than any non-empty string.

strcmp returns a negative number because empty string is lexicographically less than "nonempty".

❓ Predict Output

expert

Value of variable after strcmp with partial match

What is the value of variable result after running this code?

C

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

int main() {
    char a[] = "test123";
    char b[] = "test124";
    int result = strcmp(a, b);
    printf("%d", result);
    return 0;
}

A 0

B 1

C -1

D Undefined behavior

💡 Hint

strcmp compares character by character until difference found.

The strings differ at the last character: '3' (ASCII 51) vs '4' (ASCII 52), so strcmp returns a negative number (-1).

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