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Unrolling a 20x20 grid of pixels into a 400 dimensional vector.

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Unlawful waffle
Unlawful waffle on 12 Jan 2021
Answered: Jan on 12 Jan 2021
During an exercise of a machine learning course, the task was to use logistic regression and neural networks to recognize handwirtten digits. For this, we are given a dataset containing 5000 training examples of handwritten digits where each example is a 20 x 20 pixel grayscale image of the digit. (I am not looking the answer on to how to do it)
When loading the data, it loads a 5000x400 matrix X where every row is a training example.
I would like to know how they have "unrolled" (as they call it) the 20x20 grid of pixels into a 400-dimensional vector and created this 5000x400 matrix X.
Thank you for yout help!

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Answers (1)

Jan
Jan on 12 Jan 2021
If the data are a collection of 500 matrices of the size 20x20:
data = rand(5000, 20, 20)
You can create the 2D matrix by:
data2D = reshape(data, 5000, 400)

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