String handling using library functions - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of string concatenation with strcat

What is the output of this C program?

C

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

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

A Compilation error

B Hello

C Hello World

D WorldHello

💡 Hint

Remember that strcat appends the second string to the first one.

The function strcat appends str2 to the end of str1. Since str1 has enough space, the result is "Hello World".

❓ Predict Output

intermediate

Result of strlen on a string

What is the output of this C program?

C

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

int main() {
    char str[] = "C programming";
    printf("%zu", strlen(str));
    return 0;
}

A 12

B 14

C Compilation error

D 13

💡 Hint

strlen counts characters before the null terminator.

The string "C programming" has 13 characters including the space. strlen returns 13.

❓ Predict Output

advanced

Output of strcmp comparing two strings

What is the output of this C program?

C

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

int main() {
    char s1[] = "apple";
    char s2[] = "apples";
    int result = strcmp(s1, s2);
    printf("%d", result);
    return 0;
}

A 0

B A negative number

C A positive number

D Compilation error

💡 Hint

strcmp returns negative if first string is less than second.

Since "apple" is shorter and lexicographically less than "apples", strcmp returns a negative value.

❓ Predict Output

advanced

Behavior of strcpy with overlapping strings

What is the output of this C program?

C

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

int main() {
    char str[] = "overlap";
    strcpy(str + 2, str);
    printf("%s", str);
    return 0;
}

A Undefined behavior (may crash or print garbage)

B ovoverlap

C laplap

D overlap

💡 Hint

strcpy does not support overlapping source and destination.

Using strcpy with overlapping source and destination causes undefined behavior. The output may vary or crash.

🧠 Conceptual

expert

Which function safely copies strings with buffer size limit?

Which of the following C functions is designed to copy strings safely by limiting the number of characters copied to avoid buffer overflow?

A strncpy

B strcpy

C strcat

D strcmp

💡 Hint

One function lets you specify the maximum number of characters to copy.

strncpy copies up to a specified number of characters, helping prevent buffer overflow. strcpy does not limit copying.

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