Iterations

Loops that iterate over a sequence of values are expressed as for loops in Q#. A for loop in Q# doesn't break based on a condition but instead corresponds to an iteration, or what is often expressed as foreach or iter in other languages. There are currently two data types in Q# that support iteration: arrays and ranges.

The expression consists of the keyword for, followed by a symbol or symbol tuple, the keyword in, an expression of array or Range type, and a statement block.

The statement block (the body of the loop) is run repeatedly, with one or more loop variables bound to each value in the range or array. The same deconstruction rules apply to the defined loop variables as to any other variable assignment, such as bindings in let, mutable, use, and borrow statements. The loop variables themselves are immutably bound, can't be reassigned within the body of the loop, and go out of scope when the loop terminates. The expression over which the loop iterates is evaluated before entering the loop and doesn't change while the loop is running.

This scenario is illustrated in the following example. Suppose qubits is a value of type Qubit[], then

for qubit in qubits {
    H(qubit);
}

mutable results : (Int, Result)[] = [];
for index in 0 .. Length(qubits) - 1 {
    results += [(index, M(qubits[index]))];
}

mutable accumulated = 0;
for (index, measured) in results {
    if measured == One {
        accumulated += 1 <<< index;
    }
}

Target-specific restrictions

Because there are no break or continue primitives in Q#, the length of the loop is known as soon as the iteration value is known. As such, for loops can be run on all quantum hardware.