06 - loop control

Continue statement

break jumps outside of the loop. continue jumps to the end of current iteration (step statement is still executed).

The example below prints combinations of numbers but only if they are different.

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#include <iostream>

int main()
{
	for (int j = 0; j < 4; ++j)
	{
		for (int i = 0; i < 4; ++i)
		{
			if (i == j)
				continue;

			std::cout << j << "-" << i << ", ";
		}
	}
}

The same behavior could be achieved with if statements alone, but continue is better because only this keyword is nested inside if, not the whole loop body. Less nesting generally means more readable code.

Exiting nested loops

break can be used to exit a loop (the most enclosing one). But how to exit nested loops? Some languages offer such feature by extending syntax of the break keyword, usually by allowing labels or numbers (how many scopes to exit). There is no such thing in C++.

There are multiple alternatives:

• Use goto. Heavily discouraged because it breaks structured programming model.

• throw an exception. Heavily discouraged because exceptions should be used only for error handling, never for control flow.

• Use a bool variable as a flag.

• Exit the entire function by a return statement.

Because of the mentioned rationale, only last 2 are used in practice.

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bool exit = false;
for (int y = 0; y < size_y; ++y)
{
	if (exit)
		break;

	for (int x = 0; x < size_x; ++x)
	{
		if (/* ... */)
		{
			exit = true;
			break;
		}
	}
}

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for (int y = 0; y < size_y; ++y)
{
	for (int x = 0; x < size_x; ++x)
	{
		if (/* ... */)
		{
			return /* ... */;
		}
	}
}