Coding Grandmaster · project 43 of 46 · premium, free for now

A single neuron

Write sigmoid(z) and neuron(inputs, weights, bias) which returns the sigmoid of the weighted sum plus the bias. Print the neuron's output, rounded to 4 decimal places, for each input pair.

Learn

An artificial neuron adds up its weighted inputs plus a bias, then squashes the total with an activation function. The sigmoid, 1 ÷ (1 + e^−z), squashes any number into the range 0 to 1, so the output can be read as a confidence. Big positive totals give nearly 1, big negative ones nearly 0, and a total of 0 gives exactly 0.5.

Your task

Write sigmoid(z) and neuron(inputs, weights, bias) which returns the sigmoid of the weighted sum plus the bias. Print the neuron's output, rounded to 4 decimal places, for each input pair.

Stretch challenge

Show that sigmoid(-z) equals 1 - sigmoid(z) for several z.

← A perceptron learns AND A two-layer network →

Python

Press Run to see what your program prints.

Keep going in Coding Grandmaster

A two-layer network

Write forward(x1, x2) for a network with two hidden neurons and one output neuron, using the given weights, and print the output rounded to 3 decimal places for the four input pairs.

Start →45

Teaching a neuron OR

Train a neuron on the OR table, starting from weights 0.1, -0.1 and bias 0, for 1,000 epochs with learning rate lr. Print the four outputs rounded to 2 decimal places.

Start →46

Why XOR needs a hidden layer

Write step(z) and xor_net(a, b) using two hidden neurons, h1 = step(a + b - 0.5) and h2 = step(a + b - 1.5), and the output step(h1 - h2 - 0.5). Print the XOR table, one line per row, like 0 1 -> 1.

Start →

Practise the idea

🔌 Play Logic Gate Challenge

Predict the output of AND, OR, XOR, NAND and NOR gates from their inputs.

Play now

FAQ

A single neuron: common questions

I am stuck. What can I do?

Press Hint for a nudge, read the message next to each goal, and compare your code with the lesson text. If you still need it, Show answer reveals a model solution. Try to type it yourself rather than copy it.

Does my work get saved?

Your ticks and XP are saved in this browser, and in your account if you log in. The code you type is not stored on a server.

Start a conversation

Enjoying coding? Meet a teacher.

Tell us the learner's age, country and what they would like to build, and we will suggest a class. A parent or guardian can make the enquiry.

Tell us the learner’s age or grade and goals
We’ll suggest a suitable class or learning path
We’ll confirm format, timing and fees with a parent or guardian

Play it, don't just read it

Write some code right now

Free games that need no sign-up. Pick one and play for five minutes.

All free games →

Pick a game

Today's maths challengeFive questions · the same for everyone today · keep your streak goingPlay