Common string operations

Introduction

Strings are used to store text. Common operations help us read, change, and compare text easily.

When you want to find the length of a word or sentence.

When you need to join two pieces of text together.

When you want to check if two texts are the same.

When you want to copy text from one place to another.

When you want to find a smaller text inside a bigger text.

Syntax

C

strlen(string)       // Returns length of string
strcpy(dest, src)    // Copies src string to dest
strcat(dest, src)    // Adds src string at the end of dest
strcmp(str1, str2)   // Compares two strings
strstr(haystack, needle) // Finds needle in haystack

All strings in C end with a special character '\0' to mark the end.

Make sure destination arrays have enough space before copying or adding strings.

Examples

Finds the length of the string "Alice" which is 5.

C

char name[20] = "Alice";
int length = strlen(name);

Copies the string from name to copy.

C

char copy[20];
strcpy(copy, name);

Adds the name "Alice" to the greeting, making "Hello, Alice".

C

char greeting[30] = "Hello, ";
strcat(greeting, name);

Checks if the string name is exactly "Alice".

C

if (strcmp(name, "Alice") == 0) {
    // strings are equal
}

Sample Program

This program joins two words with a space, prints the combined string and its length, then checks if the word "World" is inside the combined string.

C

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

int main() {
    char first[20] = "Hello";
    char second[20] = "World";
    char combined[50];

    // Copy first into combined
    strcpy(combined, first);
    // Add a space
    strcat(combined, " ");
    // Add second into combined
    strcat(combined, second);

    printf("Combined string: %s\n", combined);
    printf("Length: %zu\n", strlen(combined));

    // Check if combined contains "World"
    if (strstr(combined, "World") != NULL) {
        printf("Found 'World' inside combined.\n");
    } else {
        printf("Did not find 'World'.\n");
    }

    return 0;
}

Important Notes

Always include <string.h> to use these string functions.

Be careful with buffer sizes to avoid errors or crashes.

Strings in C are arrays of characters ending with '\0'.

Summary

Use strlen to get string length.

Use strcpy to copy strings safely.

Use strcat to join strings.

Use strcmp to compare strings.

Use strstr to find a substring inside another string.

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