Shortcuts, Conversions and cmath

Shortcut operators

Changing a variable using its own value is so common that C++ has shortcuts:

#include <iostream>
using namespace std;

int main() {
    int coins = 10;
    coins += 5;
    cout << coins << "\n";
    coins *= 2;
    cout << coins << "\n";
    coins++;
    cout << coins << "\n";
    coins--;
    coins--;
    cout << coins << "\n";
    return 0;
}

Output

15
30
31
29

Tip: Now you know where the name C++ comes from: C++ means “C, increased by one”.

Converting types: static_cast

Sometimes you have two ints but want a decimal answer, like an average. static_cast<double>(x) turns x into a double for that calculation:

#include <iostream>
using namespace std;

int main() {
    int total = 17;
    int count = 4;
    cout << total / count << "\n";                        // int division: 4
    cout << static_cast<double>(total) / count << "\n";   // 4.25

    double price = 9.99;
    int rounded = static_cast<int>(price);                // cuts off .99
    cout << rounded << "\n";
    return 0;
}

Output

4
4.25
9

Maths functions: <cmath>

Add #include <cmath> to get ready-made maths functions:

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    cout << sqrt(81) << "\n";       // square root
    cout << pow(2, 10) << "\n";     // 2 to the power of 10
    cout << abs(-7) << "\n";        // distance from zero
    cout << round(4.6) << "\n";     // nearest whole number
    cout << floor(4.6) << "\n";     // round down
    cout << ceil(4.1) << "\n";      // round up
    return 0;
}

Output

9
1024
7
5
4
5

Controlling decimal places

To always show, say, 2 decimal places (great for money), use fixed and setprecision from <iomanip>:

#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    double price = 7.5;
    cout << fixed << setprecision(2);
    cout << "Price: $" << price << "\n";
    cout << "Tax:   $" << price * 0.08 << "\n";
    return 0;
}

Output

Price: $7.50
Tax:   $0.60