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Lesson 5 — The sigmoid
Turning any number into a probability
A probability is a number from 0 (impossible) to 1 (certain). Models often produce a raw score that can be anything, like −7 or 12. The sigmoid function squashes any score into 0..1: big positive scores go near 1, big negative ones near 0, and 0 becomes exactly 0.5.
import math
1 / (1 + math.exp(-0)) # 0.5
math.exp(x) means e (about 2.718) to the power x.
Why it matters: sigmoid is how a model says "I am 88% sure this email is spam" instead of printing a meaningless score.
Want it another way?
Task
Make sigmoid(x) return the sigmoid of x: 1 divided by (1 + e to the power of minus x). The loop prints the result for -2, 0 and 2.
Run your code — when the output matches the task, you will see a green check. Then mark the lesson complete.
Test your knowledge
Test your knowledgeA quick 3-question quiz on this lesson — instant feedback.