stoi and stod
- C++ course
- Beginner
- Lesson 89/116
- 5 min
Learn string-to-number conversion with stoi and stod.
stoi: string to integer
Sometimes you need to convert an entire string into a single number. The stoi function turns a string into an integer; it skips leading whitespace and stops at the first non-digit character.
string s = "42";
int n = stoi(s); // n = 42If the string does not contain a valid number, std::invalid_argument is thrown. For this reason it is often wise to wrap the stoi call in a try block; we will cover exceptions in more detail later.
stod: string to double
For decimal numbers, the stod function is used. It works the same way as stoi; only the return type is double, and the decimal part after the dot is supported.
string s = "3.14";
double pi = stod(s); // pi = 3.14stod also throws std::invalid_argument on invalid input. Scientific notation like "1.5e3" is also supported, so you can safely fold user-supplied text into numeric calculations.
Practice Convert two numbers from strings and multiply
Define the string variables string a = "6"; and string b = "7.5";. Convert a to an integer with stoi and b to a double with stod. Print the product of the two numbers with cout.
Starter code
#include <iostream>
#include <string>
using namespace std;
int main() {
string a = "6";
string b = "7.5";
// convert to numbers with stoi and stod, print their product
return 0;
}Sample solution
#include <iostream>
#include <string>
using namespace std;
int main() {
string a = "6";
string b = "7.5";
int ia = stoi(a);
double db = stod(b);
cout << ia * db << endl;
return 0;
}Expected output
45
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does stoi("42") return?
- "42"
- 42 Correct answer
- 42.0
- '4'
Explanation: stoi returns an int value; it is 42.
2 Fill in the blank: stod call
string s = "9.81";
double g = ____(s);string s = "9.81";
double g = stod(s);Explanation: stod converts decimal numbers.
3 Match each function with its return type
| Term | Meaning |
|---|---|
stoi |
int |
stod |
double |
to_string |
string |