Compund Operators and Assignment Expressions

You’ve already used = to store a value in a variable. The following examples do more than just assign. They update a variable based on its current value:

score = score + 2;balance = balance + 200;countdown = countdown - 1;

C++ gives you a shorthand for exactly this pattern in the form of compound assignment operators.

Compound assignment operators

Each operator below performs the arithmetic operation and immediately stores the result back into the same variable:

#include <iostream>int main() {    int balance = 1000;    balance += 200;   // same as: balance = balance + 200    balance -= 50;    // same as: balance = balance - 50    balance *= 2;     // same as: balance = balance * 2    balance /= 4;     // same as: balance = balance / 4    balance %= 100;   // same as: balance = balance % 100    std::cout << "Final balance: " << balance << "\n";  // 75    return 0;}

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What types can you use these with?

All the compound assignment operators work on numeric types: int, double, float, and their variants. The %= (modulo assignment) is the only exception, as one might expect. It only works on integer types, not floating-point.

#include <iostream>int main() {    double temperature = 98.6;    temperature += 1.4;    std::cout << temperature << "\n";  // 100.0    int remainder = 17;    remainder %= 5;    std::cout << remainder << "\n";    // 2    // temperature %= 1.0;  // error: % doesn't work on doubles    return 0;}

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Chaining assignments

C++ also lets you assign the same value to multiple variables in one line:

#include <iostream>int main() {    int a, b, c;    a = b = c = 0;    std::cout << a << " " << b << " " << c << "\n";  // 0 0 0    return 0;}

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This works because in C++, assignment itself is an expression. In other words, once a variable is assigned a value, the entire expression takes on that value. So c = 0 evaluates to 0, then b = 0 evaluates to 0, then a = 0.

In particular, the evaluation follows how the chained assignment is grouped. The grouping starts from the right, so a = b = c = 0 is the same as a = (b = (c = 0)). That grouping is what we mean when we say the assignment operator is right-associative.

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Compund Operators and Assignment Expressions

You’ve already used = to store a value in a variable. The following examples do more than just assign. They update a variable based on its current value:

score = score + 2;balance = balance + 200;countdown = countdown - 1;

C++ gives you a shorthand for exactly this pattern in the form of compound assignment operators.

Compound assignment operators

Each operator below performs the arithmetic operation and immediately stores the result back into the same variable:

#include <iostream>int main() {    int balance = 1000;    balance += 200;   // same as: balance = balance + 200    balance -= 50;    // same as: balance = balance - 50    balance *= 2;     // same as: balance = balance * 2    balance /= 4;     // same as: balance = balance / 4    balance %= 100;   // same as: balance = balance % 100    std::cout << "Final balance: " << balance << "\n";  // 75    return 0;}

Terminal

What types can you use these with?

#include <iostream>int main() {    double temperature = 98.6;    temperature += 1.4;    std::cout << temperature << "\n";  // 100.0    int remainder = 17;    remainder %= 5;    std::cout << remainder << "\n";    // 2    // temperature %= 1.0;  // error: % doesn't work on doubles    return 0;}

Terminal

Chaining assignments

C++ also lets you assign the same value to multiple variables in one line:

#include <iostream>int main() {    int a, b, c;    a = b = c = 0;    std::cout << a << " " << b << " " << c << "\n";  // 0 0 0    return 0;}

Terminal