String comparison - Mini Project: Build & Apply

String comparison

📖 Scenario: You are creating a simple program to compare two words typed by a user. This is like checking if two names or passwords are exactly the same.

🎯 Goal: Build a program that reads two strings and tells if they are the same or different using string comparison.

📋 What You'll Learn

Use char arrays to store the two strings

Use strcmp function to compare the strings

Print "Strings are equal" if they match exactly

Print "Strings are different" if they do not match

💡 Why This Matters

🌍 Real World

String comparison is used in many programs to check passwords, search for words, or match user input exactly.

💼 Career

Understanding string comparison is important for software development, especially in input validation, data processing, and user authentication.

1

Create two string variables

Create two char arrays called str1 and str2 each with size 20.

C

#include 
#include 

int main() {
    char str1[20];
    char str2[20];
    // Your code here
    return 0;
}

Hint

Use char str1[20]; and char str2[20]; to create the string variables.

2

Read two strings from the user

Use scanf to read input into str1 and str2.

C

#include 
#include 

int main() {
    char str1[20];
    char str2[20];
    scanf("%19s", str1);
    scanf("%19s", str2);
    // Your code here
    return 0;
}

Hint

Use scanf("%19s", str1); and scanf("%19s", str2); to read the strings safely.

3

Compare the two strings

Use strcmp(str1, str2) to compare the strings and store the result in an int variable called result.

C

#include 
#include 

int main() {
    char str1[20];
    char str2[20];
    scanf("%19s", str1);
    scanf("%19s", str2);
    int result = strcmp(str1, str2);
    // Your code here
    return 0;
}

Hint

Use int result = strcmp(str1, str2); to compare the strings.

4

Print if strings are equal or different

Use an if statement to check if result is 0. If yes, print "Strings are equal". Otherwise, print "Strings are different".

C

#include 
#include 

int main() {
    char str1[20];
    char str2[20];
    scanf("%19s", str1);
    scanf("%19s", str2);
    int result = strcmp(str1, str2);
    if (result == 0) {
        printf("Strings are equal\n");
    } else {
        printf("Strings are different\n");
    }
    // Your code here
    return 0;
}

Hint

Use if (result == 0) to check equality and printf to show the messages.

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