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C++ string

last modified March 19, 2021

C++ string tutorial shows how to work with strings in C++.

A string is a sequence of characters. C++ has the std::string type to represent strings.

The characters in a string literal must be enclosed between double quotation marks.

C++ string access characters

To access the characters of a string, we can use the [] operator or the at method. In addition, the front method accesses the first character and the back the last character.

access.cpp
#include <iostream>

using std::cout;
using std::endl;
using std::string;

int main() {

    string msg = "an old falcon";

    cout << msg.at(4) << endl;
    cout << msg[5] << endl;
    cout << msg.front() << endl;
    cout << msg.back() << endl;

    return 0;
}

In the example, we access characters of a string.

$ ./access 
l
d
a

C++ string concatenate

The + operator is used to concatenate strings.

concat.cpp
#include <iostream>

using std::string;
using std::cin;
using std::cout;
using std::endl;

int main() {

    string name, msg;

    cout << "Enter your name: ";
    getline(cin, name);

    msg = "Hello " + name + "!";

    cout << msg << endl;

    return 0;
}

With the getline method, we read the input from a user and concatenate it with other strings to form a message.

$ ./concat 
Enter your name: Jan
Hello Jan!  

C++ string to int

The stoi function converts a string to a signed integer.

str2int.cpp
#include <iostream>
#include <string>

using std::string;
using std::cout;
using std::endl;
using std::stoi;

int main () {

    string str1 = "12";
    string str2 = "18.97";
    string str3 = "4 foxes";

    int val1 = stoi(str1);
    int val2 = stoi(str2);
    int val3 = stoi(str3);

    cout << val1 << endl;
    cout << val2 << endl;
    cout << val3 << endl;

    return 0;
}

We convert three strings to integers.

$ ./str2int
12
18
4

C++ string modify

C++ has several methods to modify strings.

modify.cpp
#include <iostream>

using std::string;
using std::cout;
using std::endl;

int main() {

    string msg = "an old";

    msg.append(" falcon");

    cout << msg << endl;

    msg.push_back('.');

    cout << msg << endl;

    msg.pop_back();

    cout << msg << endl;

    msg.erase(0, 3);

    cout << msg << endl;

    msg.insert(4, "gray ");

    cout << msg << endl;

    msg.replace(9, 6, "eagle");

    cout << msg << endl;

    msg.clear();

    cout << msg.size() << endl;

    return 0;
}

In the example, the initial string is modified with append, push_back, pop_back, erase, insert, replace, and clear methods.

$ ./modify 
an old falcon
an old falcon.
an old falcon
old falcon
old gray falcon
old gray eagle
0

C++ string compare

Strings are compared with the compare method.

comparing.cpp
#include <iostream>

using std::string;
using std::cout;
using std::endl;

int main() {

    string word1 = "blue";
    string word2 = "blues";

    if (word1.compare(word2) == 0) {

        cout << "words are equal" << endl;
    } else {

        cout << "words are not equal" << endl;
    }

    if (word1.compare(0, 4, word2, 0, 4) == 0) {

        cout << "words are equal" << endl;
    } else {

        cout << "words are not equal" << endl;
    }

    return 0;
}

We compare two words. In the second case, we specify the range of characters to compare.

$ ./comparing 
words are not equal
words are equal

C++ substring

The substr function returns a substring.

substring.cpp
#include <iostream>
#include <string>

using std::string;
using std::cout;
using std::endl;

int main() {

    string word = "an old falcon";

    cout << word.substr(0, 2) << endl;
    cout << word.substr(3, 3) << endl;
    cout << word.substr(7, 6) << endl;

    return 0;
}

We get three substrings of the initial string.

$ ./substring 
an
old
falcon

C++ string loop

We can use while and for loops to go over a string.

looping.cpp
#include <iostream>

using std::cout;
using std::endl;
using std::string;

int main() {

    string msg = "an old falcon";

    int i = 0;

    while (i < msg.size()) {

        cout << msg[i] << " ";
        i++;
    }

    cout << endl;

    for (const auto &c : msg) {

        cout << c << " ";
    }

    cout << endl;

    for (auto it = msg.begin(); it != msg.end(); it++) {

        cout << *it << " ";
    }

    cout << endl;

    for (string::size_type i = 0; i < msg.size(); i++) {
        std::cout << msg.at(i) << " ";
    }

    cout << endl;

    return 0;
}

We loop over a string and print its characters. We use the classic while and for loops and the for-range loop.

$ ./looping 
a n   o l d   f a l c o n 
a n   o l d   f a l c o n 
a n   o l d   f a l c o n 
a n   o l d   f a l c o n 

C++ string find/rfind

The find searches the string for the first occurrence of the specified string while the rfind searches for the last occurrence.

finding.cpp
#include <iostream>

using std::string;
using std::cout;
using std::endl;

int main() {

    string text = "I saw a red fox yesterday; a red old fox.";

    int pos1 = text.find("fox");
    int pos2 = text.rfind("fox");
    int pos3 = text.find("fox", 15);

    cout << pos1 << endl;
    cout << pos2 << endl;
    cout << pos3 << endl;

    return 0;
}

We use the find and rfind methods to look for the "fox" string. The methods return the indexes of the character positions.

int pos3 = text.find("fox", 15);

The overloaded find method specifies the position where the search starts.

$ ./finding 
12
37
37

C++ read file

In the following example, we read a text file.

words.txt
wind
sky
blue
water
falcon
rock
wood
cup
cloud
war

We have a list of words in the file.

read_file.cpp
#include <iostream>
#include <fstream>
#include <string>

using std::string;
using std::cout;
using std::cerr;
using std::endl;
using std::getline;
using std::ifstream;

int main() {

    ifstream filename("words.txt");

    if (filename.is_open()) {

        string line;
        while (getline(filename, line)) {

            cout << line << endl;
        }

        filename.close();
    } else {
        cerr << "Unable to open file";
    }

    return 0;
}

We read a file line by line and print each line to the console.

$ ./read_file 
wind
sky
blue
water
falcon
rock
wood
cup
cloud
war

C++ string starts_with

The starts_with method checks if the string starts with the given prefix. The method was included in C++20.

starts_with.cpp
#include <iostream>
#include <fstream>

using std::string;
using std::cout;
using std::cerr;
using std::endl;
using std::getline;
using std::ifstream;

int main() {

    ifstream filename("words.txt");

    if (filename.is_open()) {

        string line;
        while (getline(filename, line)) {

            if (line.starts_with('w')) {
                cout << line << endl;
            }
        }

        filename.close();
    } else {
        cerr << "Unable to open file";
    }

    return 0;
}

We read the words from the words.txt file and print those that start with 'w'.

$ ./starts_with 
wind
water
wood
war

C++ string ends_with

The ends_with method checks if the string ends with the given suffix. The method was included in C++20.

ends_with.cpp
#include <iostream>
#include <sstream>

using std::string;
using std::cout;
using std::endl;
using std::getline;
using std::istringstream;

int main() {

    string words = "wind\nsky\blue\nwater\nfalcon\nrock\nwood\ncup\ncloud\nwar";
    istringstream data(words);

    for (string line; getline(data, line);) {

        if (line.ends_with('d')) {
            cout << line << endl;
        }
    }

    return 0;
}

We have a large string of words delimited by newline characters. We transform the string to a stream and read the words with getline. We print those that end with 'd'.

$ ./ends_with 
wind
wood
cloud

In this tutorial we have worked with strings in C++.