String handling using library functions - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the strlen() function in C?

The strlen() function returns the length of a string, not counting the null character \0 at the end.

beginner

How does strcpy() function work in C?

strcpy() copies the content of one string (source) into another string (destination), including the null terminator.

beginner

What does strcmp() return when two strings are equal?

strcmp() returns 0 if the two strings are exactly the same.

intermediate

Explain the difference between strcat() and strncat().

strcat() appends one string to another until it hits the null character. strncat() appends up to a specified number of characters, helping to avoid buffer overflow.

beginner

Why is it important to include string.h when using string library functions?

Including string.h provides the declarations of string handling functions so the compiler knows how to use them correctly.

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

A strcpy()

B strcmp()

C strlen()

D strcat()

strlen() returns the number of characters in a string excluding the null terminator.

What does strcpy(dest, src) do?

A Returns length of src

B Compares dest and src strings

C Concatenates src to dest

D Copies src string into dest string

strcpy() copies the source string into the destination string.

If strcmp(s1, s2) returns a negative value, what does it mean?

A s1 is lexicographically less than s2

B s1 is equal to s2

C s1 is lexicographically greater than s2

D There is an error

A negative return means s1 comes before s2 in dictionary order.

Which function safely appends a limited number of characters from one string to another?

A strncat()

B strcpy()

C strlen()

D strcmp()

strncat() appends up to a specified number of characters, preventing buffer overflow.

What header file must be included to use string handling functions like strlen() and strcpy()?

A stdio.h

B string.h

C math.h

D stdlib.h

The string.h header declares all standard string handling functions.

Describe how to copy one string into another using C library functions. What should you be careful about?

Think about how strcpy works and what can go wrong if the destination is too small.

Explain how to compare two strings in C and interpret the result of the comparison.

Consider what strcmp returns when strings are equal or different.

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