Strings

In this lesson, you will learn how to use variables that store text. As humans, text is the primary medium through which we interact with computer programs. Learning basic text storage and manipulation is crucial to making functional applications that people can and want to use.

Motivation

The world runs on text. Variables that can store text turn out to quite useful because of this. Consider a program which reads the user's name, then greets them with the number of characters in their name.

Program that loads a name

#include <iostream>
#include <string>

int main()
{
	std::string name;
	std::cout << "Enter your name: ";
	std::cin >> name;
	std::cout << "Welcome, " << name << "!\n";
	std::cout << "Your name has " << name.size() << " characters.\n";
}

A type for storing text

In previous lessons, we talked about different types of variables, though we focused on those that store numbers. We mentioned the char type, which stores a single character.

In programming, text is a series of characters ⁠— known as a string. C++ provides us with the type std::string to store and manipulate strings.

Header file

You must include the string header to use std::string in your code, in the same way as we did earlier with iostream:

#include <string>

The std::string type is used just like any other. Here, we make two variables — one of type int, and another of type std::string:

int number;
std::string text;

Initialization

Assigning the initial value is the same as for variables of other types, but note that text is specified between double quotes, not single quotes:

Initializing and outputting a string variable

std::string welcome = "Hello, World!";
std::cout << welcome;

If no value is given, the string will be empty. The text is length 0, and printing it to the console will show nothing.

String operations

User interaction

We can display the strings using std::cout and load them using std::cin.

A welcome message

#include <iostream>
#include <string>

int main()
{
	std::string name;
	std::cout << "Enter your name: ";
	std::cin >> name;
	std::cout << "Welcome, " << name << "!\n";
}

Note that std::cin takes the input text until it encounters the first whitespace character, such as a space, tab, or newline. Because of this, if we enter our first and last name, only the first name will go into the name variable. To get the whole line of text, you can use std::getline:

Reading a whole line of text

std::string full_name;
std::getline(std::cin, full_name);

std::getline is a function that takes two arguments - an input stream to read the line from, and an std::string variable to store the line in. This example shows how you can use this function to read a line from std::cin. So, this function is "called" by putting the name of the function std::getline, then parentheses (...), and inside the parentheses the arguments we want to provide std::cin, full_name.

Combining Strings

The + and += operators can be used to concatenate and append strings together. Consider the following abridged example with an input of Dwayne Johnson:

Concatenation and Appending

std::string first_name;
std::string last_name;

std::cin >> first_name;
std::cin >> last_name;

// Concatenation
std::string coolName = first_name + " \"The Rock\" " + last_name;
std::cout << "Your cool name is:\n" << coolName << "\n\n";

// Appending
coolName += " Mk II";
std::cout << "Your new name is:\n" << coolName;

Result

Your cool name is:
Dwayne "The Rock" Johnson

Your new name is:
Dwayne "The Rock" Johnson Mk II

Notice how coolName is the result of concatenating three strings - first_name, "The Rock", and last_name. Then, see how we modify coolName by appending " Mk II" to it.

Reading the length

To check how many characters are in a string we use the so-called method size. We'll talk more about functions and methods later in the course, as they are a very important concept.

Let's see how to use the size method in practice. Consider the following example with an input of Johnny:

Program that counts the size of a name

#include <iostream>
#include <string>

int main()
{
	std::string name;
	std::cout << "Enter your name: ";
	std::cin >> name;

	std::cout << "\nWelcome, " << name << "!\n";
	std::cout << "Your name has " << name.size() << " characters.\n";
}

Result

Welcome, Johnny!
Your name has 6 characters.

We call the size method by doing name.size(). The . indicates that we are accessing something on our name variable. size is the name of the method we are accessing. The () calls the size method. This gives back to us the total number of characters in the string. These terms aren't crucial to remember now, but you will see them later in the Functions and Methods lessons; just remember how to use it.

Just to reiterate, we specify the variable (name), then we write size after the period and then empty round brackets:

Calling the size method

name.size()

Conversions between numbers and strings

Numbers and strings cannot be freely converted to each other without a specialized function that does it for us. This is because number types and string types in C++ are disinctly different, and not generally compatible. While it may be annoying, this is actually very important for making bug-free code.

53
"53"

The first value is the arithmetic number 53. It represents a value that we can perform mathematical operations on. The second value is text consisting of the characters '5' and '3'. We can't perform math on strings; rather, the string operations we can perform are concatenations.

These are the incorrect ways of trying to convert between strings and numbers, all causing compilation errors:

❌ Incorrect

❌ Assigning a number variable into a string

int number = 53;
std::string str	= number; // Error

❌ Incorrect

❌ Assigning an integer literal into a string

std::string str	= 53; // Error

❌ Incorrect

❌ Assigning an string into a number

std::string str	= "53";
int number = str; // Error

❌ Incorrect

❌ Assigning an string literal into a number

int number = "53"; // Error

To convert from a number type (like int or float) to string, we need to use the std::to_string function.

✔ Converting a number to a text

int number = 53;
std::string number_in_str	= std::to_string(number);

In the same way, if we have a number stored inside a string, we can convert it to an int with std::stoi (string to int) and to float type with std::stof (string to float):

🔵 std::stoi

Convert std::string to int

std::string number_in_str = "53";
int number = std::stoi(number_in_str);

🟣 std::stof

Convert std::string to float

std::string number_in_str = "53.5"; // do not put `f` at the end in the text
float number = std::stof(number_in_str);

Summary

In this lesson, we learned about the std::string type, which is a type that represents a sequence of characters.

tip

Head over to sublessons to find out more about strings:

• 💡 Examples
• 🐞 Common problems
• 📚 Exercises