Reading input and formatting output

Reading with std::cin

std::cin is the "character input" stream: what the user types in the terminal. The >> operator reads from it into a variable. It skips spaces and newlines first, then reads one value that matches the variable's type.

#include <iostream>

int main() {
    int age;
    double height;
    std::cout << "Age and height: ";
    std::cin >> age >> height;
    std::cout << "You are " << age << " and " << height << " m tall\n";
}

Checking that input worked

If the user types "abc" where a number is expected, the read fails: the variable gets 0 (since C++11) and cin goes into a fail state, after which every further read does nothing. You can test the stream like a bool.

int qty;
if (std::cin >> qty) {
    std::cout << "OK: " << qty << '\n';
} else {
    std::cout << "Please type a number\n";
}

Whole lines: std::getline

>> stops at the first space, so reading the name "Asha Verma" with >> gives only "Asha". Use std::getline(std::cin, text) to read a full line into a std::string.

#include <iostream>
#include <string>

int main() {
    int roll;
    std::string fullName;
    std::cin >> roll;
    std::cin.ignore();                 // throw away the '\n' left after the number
    std::getline(std::cin, fullName);
    std::cout << roll << ": " << fullName << '\n';
}

Formatting numbers with <iomanip>

#include <iostream>
#include <iomanip>

int main() {
    double price = 1234.5;
    std::cout << std::fixed << std::setprecision(2) << price << '\n';   // 1234.50
    std::cout << std::setw(8) << 42 << "|\n";                           // "      42|"
    std::cout << std::boolalpha << true << '\n';                        // true
}

Note that std::fixed and std::setprecision are "sticky": they stay in effect for all later output on that stream. setw applies only to the next value.

C++20: std::format

#include <format>
#include <iostream>

int main() {
    double fare = 1450.5;
    std::cout << std::format("Fare: Rs {:.2f} for {} seats\n", fare, 3);
}

std::format is like Python's f-strings: a format string with {} placeholders. It needs a compiler with C++20 library support (GCC 13+, recent Clang with libc++, or Visual Studio 2019 16.10+). If <format> is missing on your compiler, use <iomanip>. C++23 adds std::print, which prints directly.

Fast input for big data

std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);