Medium data structure and algorithm practice puzzle.
Given a list of predicted probabilities and the index of the true class, return the cross-entropy loss, rounded to 4 decimals. It's -log(p), where p is the probability given to the right answer. Nothing else matters. A probability of 0.5 costs about 0.69, but 0.01 costs 4.6, so confident wrong answers get punished hard.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Sample Smoothing Rig, a pipeline you can run and download. See every Software Engineer project.
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