Common string operations - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of string length calculation

What is the output of this C code snippet?

C

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

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

A 8

B 11

C 10

D 9

💡 Hint

Remember that strlen counts characters before the null terminator.

The string "Hello, C!" has 9 characters including the comma and space but excluding the null terminator.

❓ Predict Output

intermediate

Result of string concatenation with strcat

What will be printed by this C program?

C

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

int main() {
    char s1[20] = "Code";
    char s2[] = "Master";
    strcat(s1, s2);
    printf("%s", s1);
    return 0;
}

A Code Master

B MasterCode

C Code

D CodeMaster

💡 Hint

strcat appends the second string to the first one.

strcat adds the contents of s2 to the end of s1, resulting in "CodeMaster".

❓ Predict Output

advanced

Output of string comparison

What is the output of this C code?

C

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

int main() {
    char a[] = "apple";
    char b[] = "apricot";
    int result = strcmp(a, b);
    if (result < 0) {
        printf("A");
    } else if (result > 0) {
        printf("B");
    } else {
        printf("C");
    }
    return 0;
}

A A

B B

C Compilation error

D C

💡 Hint

strcmp returns a negative value if the first string is lexicographically less.

"apple" is lexicographically less than "apricot", so strcmp returns a negative number and prints "A".

❓ Predict Output

advanced

Behavior of strcpy with overlapping strings

What happens when this C code runs?

C

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

int main() {
    char str[] = "OverlapTest";
    strcpy(str + 3, str);
    printf("%s", str);
    return 0;
}

A OverlapTestOverlapTest

B Undefined behavior (may print corrupted string)

C OverTest

D Compilation error

💡 Hint

strcpy does not support overlapping source and destination.

Using strcpy with overlapping memory causes undefined behavior, so output may be corrupted or unpredictable.

🧠 Conceptual

expert

Correct use of snprintf to avoid buffer overflow

Which option correctly uses snprintf to safely copy a string into a fixed-size buffer without overflow?

A snprintf(buffer, sizeof(buffer), "%s", source);

B snprintf(buffer, strlen(source), "%s", source);

C snprintf(buffer, sizeof(source), "%s", source);

D snprintf(buffer, sizeof(buffer) - 1, "%s", source);

💡 Hint

Use the buffer size, not the source length, to limit snprintf.

Option A uses the buffer size to limit snprintf, preventing overflow and ensuring null termination.

Practice

1. Which C function is used to find the length of a string?

easy

A. strlen

B. strcpy

C. strcmp

D. strcat

Solution

1. Step 1: Understand string length concept

   The length of a string is the number of characters before the null terminator.

2. Step 2: Identify the correct function

   strlen returns the length of a string, while strcpy copies, strcmp compares, and strcat concatenates strings.

3. Final Answer:

   strlen -> Option A

4. Quick Check:

   Length function = strlen [OK]

Hint: Remember: length = strlen, copy = strcpy [OK]

Common Mistakes:

• Confusing strcpy with strlen
• Using strcmp to get length
• Thinking strcat returns length

2. Which of the following is the correct syntax to copy string src into dest in C?

easy

A. strcat(dest, src);

B. strcpy(src, dest);

C. strlen(dest, src);

D. strcpy(dest, src);

Solution

1. Step 1: Understand strcpy parameters

   strcpy copies the source string into the destination string. The first argument is destination, second is source.

2. Step 2: Check each option

   strcpy(dest, src); uses correct order (destination first, source second). strcpy(src, dest); reverses parameters. strcat(dest, src); concatenates strings. strlen(dest, src); returns length, not copies.

3. Final Answer:

   strcpy(dest, src); -> Option D

4. Quick Check:

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

Hint: Copy: destination first, source second in strcpy [OK]

Common Mistakes:

• Swapping source and destination arguments
• Using strcat instead of strcpy for copying
• Using strlen for copying strings

3. What is the output of this C code?

char str1[20] = "Hello, ";
char str2[] = "World!";
strcat(str1, str2);
printf("%s", str1);

medium

A. Hello,

B. World!Hello,

C. Hello, World!

D. Compilation error

Solution

1. Step 1: Understand strcat behavior

   strcat appends the second string to the end of the first string.

2. Step 2: Apply strcat to given strings

   Appending "World!" to "Hello, " results in "Hello, World!" stored in str1.

3. Final Answer:

   Hello, World! -> Option C

4. Quick Check:

   Concatenate str1 + str2 = Hello, World! [OK]

Hint: strcat adds second string to end of first [OK]

Common Mistakes:

• Expecting strcat to replace string
• Confusing order of concatenation
• Assuming strcat returns new string

4. Identify the error in this code snippet:

char s1[5] = "Hello";
char s2[10];
strcpy(s2, s1);
printf("%s", s2);

medium

A. s2 array size is too small for copy

B. s1 array size is too small for "Hello"

C. strcpy parameters are reversed

D. Missing null terminator in s2

Solution

1. Step 1: Check array size for s1

   String "Hello" has 5 characters plus 1 for '\0' terminator, so s1 needs size at least 6.

2. Step 2: Analyze s1 declaration

   s1 is declared with size 5, which is too small to hold "Hello" plus null terminator, causing overflow.

3. Final Answer:

   s1 array size is too small for "Hello" -> Option B

4. Quick Check:

   String size must include '\0' [OK]

Hint: String size must fit characters + '\0' terminator [OK]

Common Mistakes:

• Ignoring null terminator size
• Assuming strcpy reverses parameters
• Thinking s2 size is too small here

5. Given the code below, what will be printed?

char text[] = "Programming in C";
char *ptr = strstr(text, "in");
printf("%s", ptr);

hard

A. in C

B. Programming in C

C. in

D. NULL

Solution

1. Step 1: Understand strstr function

   strstr returns pointer to first occurrence of substring or NULL if not found.

2. Step 2: Find substring "in" in text

   "in" first appears inside "Programming in C" starting at "in C" substring.

3. Step 3: Print from pointer

   Printing from pointer prints "in C" including the substring and rest of string.

4. Final Answer:

   in C -> Option A

5. Quick Check:

   strstr returns substring start pointer [OK]

Hint: strstr returns pointer to substring start [OK]

Common Mistakes:

• Expecting strstr to return only substring
• Confusing strstr with strcmp
• Assuming strstr returns NULL if substring found