C++ Convert Strings and Numbers

Convert Strings and Numbers

In the Numbers and Strings chapter, you learned that "10" + "20" gives "1020", because the values are text, not numbers. You also saw that you cannot add a number to a string.

To calculate with a number that is stored as text, or to add a number to a text, you must convert it first. Use these functions from the <string> library:

• stoi() converts a string to an int
• stod() converts a string to a double
• to_string() converts a number to a string

Convert a String to a Number

Use stoi() (string to int) to convert text to a whole number, so you can calculate with it:

string x = "10";
string y = "20";

cout << x + y << "\n";              // 1020
cout << stoi(x) + stoi(y) << "\n";  // 30

Use stod() (string to double) for numbers with decimals:

string price = "9.99";
double myPrice = stod(price);

cout << myPrice * 2;  // 19.98

Convert a Number to a String

Use to_string() to convert a number to a string. This lets you add a number to a text:

int score = 100;
string text = "Score: " + to_string(score);

cout << text;  // Score: 100