C++ String Class

In C++, the string class is used to represent a sequence of characters as an object of the class.

We can access the various string class functions by including the <string> header in our file.

#include <string>

String Class Functions

The common functions that are used with the string class are.

Example 1: Find a Substring in the Given String

The find() function searches the string for the first occurrence of the specified substring and returns the position of the first character of the first match.

The rfind() function searches the string for the last occurrence of the specified substring and returns the position of the first character of the last match.

For example,

#include <iostream>
#include <string>
using namespace std;

int main() {

    string str = "Hello world, wonderful world!";
    cout << "String: " << str << endl;
    
    // find the first occurrence
    size_t first_occurrence = str.find("world");
    
    // find the last occurrence
    size_t last_occurrence = str.rfind("world");
    
    if (first_occurrence != string::npos) {
        cout << "First occurrence: 'world' found at position: " << first_occurrence << endl;
        cout << "Last occurrence: 'world' found at position: " << last_occurrence << endl;
    }
    else {
        cout << "'world' not found" << endl;
    }

    return 0;
}

Output

String: Hello world, wonderful world!
First occurrence: 'world' found at position: 6
Last occurrence: 'world' found at position: 23

The line if (first_occurrence != string::npos) checks if the variable first_occurrence contains a valid position where the substring world was found within the string.

• If world is found in string, first_occurrence will be set to the position of its first occurrence.
• If world is not found, it will be set to a special constant value string::npos, which indicates that the substring was not found in the string.

Note: Both functions are case-sensitive. So, if our string was Hello World, wonderful world!, then the find() function would return 23 as the first occurrence of the substring.

Example 2: Append to a String

We can append a string to an existing string using the append() function. For example,

#include <iostream>
#include <string>
using namespace std;

int main() {

    string str = "Hello world,";
    cout << "Before: " << str << endl;

    //append the string
    str.append(" Have a good day!");

    cout << "After: " << str << endl;

    return 0;
}

Output

Before: Hello world,
After: Hello world, Have a good day!

Example 3: Insert a String at a Given Position

We can insert a string at any given position using the insert() function. For example,

#include <iostream>
#include <string>
using namespace std;

int main() {

    string str = "Hello world, world!";
    cout << "Before: " << str << endl;
    
    // insert "beautiful" at the 13th index
    str.insert(13, " beautiful");

    cout << "After: " << str << endl;

    return 0;
}

Output

Before: Hello world, world!
After: Hello world,  beautifulworld!

Here, we have inserted the substring beautiful at the 13th index of the string. Keep in mind that this doesn't add space at the end of the added string.

Example 4: Erase N Characters From the Given Position

#include <iostream>
#include <string>
using namespace std;

int main() {

    string str = "Hello world, beautiful world!";
    cout << "Before: " << str << endl;
    
    // erase five characters starting from the seventh index
    str.erase(7, 5);

    cout << "After: " << str << endl;

    return 0;
}

Output

Before: Hello world, beautiful world!
After: Hello w beautiful world!

Here, we have removed five characters starting from the seventh index using the erase() function.

Example 5: Replace N Characters Within A String

#include <iostream>
#include <string>
using namespace std;

int main() {

    string str = "Hello world, beautiful world!";
    cout << "Before: " << str << endl;
    
    // replace two characters with "Earth"
    // starting from the seventh index
    str.replace(6, 2, "Earth");

    cout << "After: " << str << endl;

    return 0;
}

Output

Before: Hello world, beautiful world!
After: Hello Earthrld, beautiful world!

Here, we have replaced the two characters (wo) with Earth. We can replace any length of string with any length of another string using the replace() function.

Example 6: Compare Strings Alphabetically

To get the lexicographic relations between strings, we use the compare() function.

The compare() function in the C++ string class returns an integer that indicates the lexicographical relationship between the compared strings. It returns:

• 0 - if the compared strings are equal.
• < 0 - if the calling string is lexicographically less than the compared string.
• > 0 - if the calling string is lexicographically greater than the compared string.

For example,

#include <iostream>
#include <string>
using namespace std;

int main() {

    string str1 = "Hello world";
    string str2 = "Hello world";
    string str3 = "Hello";
    string str4 = "Hello world, What's Up?";
    
    cout << "String 1: " << str1 << endl;
    cout << "String 2: " << str2 << endl;
    cout << "String 3: " << str3 << endl;
    cout << "String 4: " << str4 << endl;
    
    // compare the strings
    cout <<"Comparing String 1 and String 2: "  << str1.compare(str2) << " (Equal)" <<endl;
    cout <<"Comparing String 1 and String 3: " << str1.compare(str3) << " (String 1 is Longer)" << endl;
    cout <<"Comparing String 1 and String 4: " << str1.compare(str4) <<" (String 1 is Smaller)" << endl;

    return 0;
}

Output

String 1: Hello world
String 2: Hello world
String 3: Hello
String 4: Hello world, What's Up? 
Comparing String 1 and String 2: 0 (Equal)
Comparing String 1 and String 3: 6 (String 1 is Longer)
Comparing String 1 and String 4: -12 (String 1 is Smaller)

Here,

• str1.compare(str2) returns 0 because str1 and str2 are equal.
• str1.compare(str3) returns a positive number because str1 is lexicographically greater than str3 (it has more characters).
• str1.compare(str4) returns a negative number because str1 is lexicographically less than str4.