Mojo quickstart

Verify your installation and learn basic Mojo syntax.

Prerequisites: Install Mojo

Setup

Get your command line ready for a new Mojo project.

uv
pixi

uv init temperature-analyzer && cd temperature-analyzer
uv venv && source .venv/bin/activate
uv pip install mojo \
  --extra-index-url https://modular.gateway.scarf.sh/simple/

pixi init temperature-analyzer \
  -c https://conda.modular.com/max-nightly/ -c conda-forge
cd temperature-analyzer
pixi add mojo
pixi shell

Create analyzer.mojo. Use your favorite IDE or editor for edits.

Hello Mojo

Add this to analyzer.mojo:

fn main():
    print("Temperature Analyzer")

Run it:

mojo analyzer.mojo

Insights:

If you see "Temperature Analyzer", your setup works.
All Mojo executables use main() as their entry point.

Variables and data

Update your file to add temperature data:

fn main():
    print("Temperature Analyzer")

    # At compile time, the square brackets tell Mojo the list type
    temps: List[Float64] = [20.5, 22.3, 19.8, 25.1]

    print("Recorded", len(temps), "temperatures")

Loops

Print each temperature. Add to main() under the print statement:

fn main():
    # ... existing code ...

    for index in range(len(temps)):  # The range is [0, len(temps))
        print("  Day {}: {}°C".format(index + 1, temps[index]))

Insights:

The day is offset by 1 because the range uses zero-based indexing.

Functions

Add this function above main() to calculate the average temperature:

fn calculate_average(temps: List[Float64]) -> Float64:
    total: Float64 = 0.0
    for index in range(len(temps)):
        total += temps[index]
    return total / len(temps)

fn main():
    # ... existing code ...

Add to the end of main() to call the function:

    avg = calculate_average(temps)
    print("Average: {}°C".format(avg))

Conditionals

Classify the average temperature. Add to the end of main():

    if avg > 25.0:
        print("Status: Hot week")
    elif avg > 20.0:
        print("Status: Comfortable week")
    else:
        print("Status: Cool week")

Raise errors

Handle empty data by updating calculate_average. Now the function can raise an error.

# Square brackets tell Mojo the list type at compile time
fn calculate_average(temps: List[Float64]) raises -> Float64:
    if len(temps) == 0:  # Empty list of temperatures
        raise Error("No temperature data")

    total: Float64 = 0
    for index in range(len(temps)):
        total += temps[index]
    return total / len(temps)

Handle errors

In main(), wrap your code in try-except for error handling:

    try:
        avg = calculate_average(temps)
        print("Average: {}°C".format(avg))

        if avg > 25:
            print("Status: Hot week")
        elif avg > 20:
            print("Status: Comfortable week")
        else:
            print("Status: Cool week")
    except e:
        print("Error:", e)

To test the error, replace temps with List[Float64](). Confirm that your app errors with "No temperature data".

Python integration

Add statistics with Python's numpy. First, install it:

uv
pixi

uv pip install numpy

pixi add numpy

Then, add the following imports at the top of your file:

from python import Python, PythonObject

Now, calculate Python stats at the end of the try block in main():

        np = Python.import_module("numpy")
        pytemps: PythonObject = [20.5, 22.3, 19.8, 25.1]
        print("Temperature standard deviation:", np.std(pytemps))

Final code

Your complete analyzer.mojo:

from python import Python, PythonObject

fn calculate_average(temps: List[Float64]) raises -> Float64:
    if len(temps) == 0.0:
        raise Error("No temperature data")

    total: Float64 = 0.0
    for index in range(len(temps)):
        total += temps[index]
    return total / len(temps)

fn main():
    print("Temperature Analyzer")
    temps: List[Float64] = [20.5, 22.3, 19.8, 25.1]
    print("Recorded", len(temps), "temperatures")

    for index in range(len(temps)):
        print("  Day {}: {}°C".format(index + 1, temps[index]))

    try:
        avg = calculate_average(temps)
        print("Average: {}°C".format(avg))

        if avg > 25.0:
            print("Status: Hot week")
        elif avg > 20.0:
            print("Status: Comfortable week")
        else:
            print("Status: Cool week")

        np = Python.import_module("numpy")
        pytemps: PythonObject = [20.5, 22.3, 19.8, 25.1]
        print("Temperature standard deviation:", np.std(pytemps))
        _ = pytemps^ # Allow the Python object to deallocate
    except e:
        print("Error:", e)

What you touched

Mojo variables, lists, loops, functions, conditionals, error handling, and Python integration, all in one working program.

Build something bigger: Tutorial: Game of Life Language guide: Mojo Manual API docs: Standard Library

Setup​

Hello Mojo​

Variables and data​

Loops​

Functions​

Conditionals​

Raise errors​

Handle errors​

Python integration​

Final code​

What you touched​