---
name: Q#
contributors:
    - ["Vincent van Wingerden", "https://github.com/vivanwin"]
    - ["Mariia Mykhailova", "https://github.com/tcNickolas"]
    - ["Andrew Ryan Davis", "https://github.com/AndrewDavis1191"]
    - ["Alex Hansen", "https://github.com/sezna"]
filename: LearnQSharp.qs
---

Q# is a high-level domain-specific language which enables developers to write quantum algorithms. Q# programs can be executed on a quantum simulator running on a classical computer and (in future) on quantum computers.

```c#
// Single-line comments start with //


/////////////////////////////////////
// 1. Quantum data types and operators

// The most important part of quantum programs is qubits.
// In Q# type Qubit represents the qubits which can be used.
// This will allocate an array of two new qubits as the variable qs.
operation QuantumDataTypes() : Unit {
    use qs = Qubit[2];

    // The qubits have internal state that you cannot access to read or modify directly.
    // You can inspect the current state of your quantum program
    // if you're running it on a classical simulator.
    // Note that this will not work on actual quantum hardware!
    Std.Diagnostics.DumpMachine();

    // If you want to change the state of a qubit
    // you have to do this by applying quantum gates to the qubit.
    H(qs[0]);   // This changes the state of the first qubit
    // from |0⟩ (the initial state of allocated qubits)
    // to (|0⟩ + |1⟩) / sqrt(2).
    // qs[1] = |1⟩; - this does NOT work, you have to manipulate a qubit by using gates.

    // You can apply multi-qubit gates to several qubits.
    CNOT(qs[0], qs[1]);

    // You can also apply a controlled version of a gate:
    // a gate that is applied if all control qubits are in |1⟩ state.
    // The first argument is an array of control qubits,
    // the second argument is the target qubit.
    Controlled Y([qs[0]], qs[1]);

    // If you want to apply an anti-controlled gate
    // (a gate that is applied if all control qubits are in |0⟩ state),
    // you can use a library function.
    ApplyControlledOnInt(0, X, [qs[0]], qs[1]);

    // To read the information from the quantum system, you use measurements.
    // Measurements return a value of Result data type: Zero or One.
    // You can print measurement results as a classical value.
    Message($"Measured {M(qs[0])}, {M(qs[1])}");
}


/////////////////////////////////////
// 2. Classical data types and operators

function ClassicalDataTypes() : Unit {
    // Numbers in Q# can be stored in Int, BigInt or Double.
    let i = 1;            // This defines an Int variable i equal to 1
    let bi = 1L;          // This defines a BigInt variable bi equal to 1
    let d = 1.0;          // This defines a Double variable d equal to 1

    // Arithmetic is done as expected, as long as the types are the same
    let n = 2 * 10;                // = 20
    // Q# does not have implicit type cast,
    // so to perform arithmetic on values of different types,
    // you need to cast type explicitly
    let nd = Std.Convert.IntAsDouble(2) * 1.0; // = 20.0

    // Boolean type is called Bool
    let trueBool = true;
    let falseBool = false;

    // Logic operators work as expected
    let andBool = true and false;
    let orBool = true or false;
    let notBool = not false;

    // Strings
    let str = "Hello World!";

    // Equality is ==
    let x = 10 == 15; // is false

    // Range is a sequence of integers and can be defined like: start..step..stop
    let xi = 1..2..7; // Gives the sequence 1,3,5,7

    // Assigning new value to a variable:
    // by default all Q# variables are immutable;
    // if the variable was defined using let, you cannot reassign its value.

    // When you want to make a variable mutable, you have to declare it as such,
    // and use the set word to update value
    mutable xii = true;
    set xii = false;

    // You can create an array for any data type like this
    let xiii = [0.0, size = 10];

    // Getting an element from an array
    let xiv = xiii[8];

    // Assigning a new value to an array element
    mutable xv = [0.0, size = 10];
    set xv w/= 5 <- 1.0;
}


/////////////////////////////////////
// 3. Control flow

operation ControlFlow() : Unit {
    let a = 1;
    // If expressions support a true branch, elif, and else.
    if (a == 1) {
        // ...
    } elif (a == 2) {
        // ...
    } else {
        // ...
    }
    use qubits = Qubit[2];

    // For loops can be used to iterate over an array
    for qubit in qubits {
        X(qubit);
    }

    // Regular for loops can be used to iterate over a range of numbers
    for index in 0..Length(qubits) - 1 {
        X(qubits[index]);
    }

    // While loops are restricted for use in classical context only
    mutable index = 0;
    while (index < 10) {
        set index += 1;
    }

    let success_criteria = true;
    // Quantum equivalent of a while loop is a repeat-until-success loop.
    // Because of the probabilistic nature of quantum computing sometimes
    // you want to repeat a certain sequence of operations
    // until a specific condition is achieved; you can use this loop to express this.
    repeat {
        // Your operation here
    } until (success_criteria) // This could be a measurement to check if the state is reached
    fixup {
        // Resetting to the initial conditions, if required
    }
}

/////////////////////////////////////
// 4. Putting it all together

// Q# code is written in operations and functions
operation ApplyXGate(source : Qubit) : Unit {
    X(source);
}

// If the operation implements a unitary transformation, you can define
// adjoint and controlled variants of it.
// The easiest way to do that is to add "is Adj + Ctl" after Unit.
// This will tell the compiler to generate the variants automatically.
operation ApplyXGateCA(source : Qubit) : Unit is Adj + Ctl {
    X(source);
}

// Now you can call Adjoint ApplyXGateCA and Controlled ApplyXGateCA.


// To run Q# code, you can put @EntryPoint() before the operation you want to run first
operation XGateDemo() : Unit {
    use q = Qubit();
    ApplyXGate(q);
}

// Here is a simple example: a quantum random number generator.
// We will generate a classical array of random bits using quantum code.
// Callables (functions or operations) named `Main` are used as entry points.
operation Main() : Unit {
    mutable bits = [0, size = 5];                // Array we'll use to store bits
    use q  = Qubit();
    {
        // Allocate a qubit
        for i in 0..4 {
            // Generate each bit independently
            H(q);                             // Hadamard gate sets equal superposition
            let result = M(q);                // Measure qubit gets 0|1 with 50/50 prob
            let bit = result == Zero ? 0 | 1; // Convert measurement result to integer
            set bits w/= i <- bit;            // Write generated bit to an array
        }
    }
    Message($"{bits}");                       // Print the result
}
```


## Further Reading

The Quantum Katas ([repo](https://github.com/microsoft/qsharp/tree/main/katas) [hosted tutorials](https://quantum.microsoft.com/en-us/tools/quantum-katas) offer great self-paced tutorials and programming exercises to learn quantum computing and Q#. 

[Q# Documentation](https://docs.microsoft.com/quantum/) is official Q# documentation, including language reference and user guides.