Medium data structure and algorithm practice puzzle.
Given predictions and actual targets, return the R-squared score, rounded to 3 decimals. Add up the squared errors of your predictions. Then add up the squared errors of just guessing the average every time. R-squared is 1 minus the first divided by the second. 1 is perfect. A model worse than guessing the average scores below 0.
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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