Question about finding the number of times an element appears in an array

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Lewis 2021-11-4

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评论： Lewis 2021-11-4

Can someone explain why/how this code works in creating an array B which corresponds to the number of each integer from 1-9 in array A?

c=[1:9]
B=sum(A(:)==c, 1)

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Lewis 2021-11-4

E.g. for A=[1 2 4] it will create B = [1 1 0 1]

Rik 2021-11-4

Combining unique with histc (or histcounts) might be either safer, or more performant for larger arrays than this setup.

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Answer 1

Chris 2021-11-4

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https://ww2.mathworks.cn/matlabcentral/answers/1579184-question-about-finding-the-number-of-times-an-element-appears-in-an-array#answer_823868

编辑：Chris 2021-11-4

在 MATLAB Online 中打开

A = randi(9,2)
A = 2×2
     8     8
     5     9

A(:) formats A in a column vector.

A(:)
ans = 4×1
     8
     5
     8
     9

A(:)==c compares the column to each element of c (possible because the dimensions of a and c are orthogonal), returning an nx9 matrix with ones where the comparison is true.

A(:)==1:9
ans = 4×9 logical array
   0   0   0   0   0   0   0   1   0
   0   0   0   0   1   0   0   0   0
   0   0   0   0   0   0   0   1   0
   0   0   0   0   0   0   0   0   1

Summing across dimension 1 (the default):

B=sum(A(:)==1:9)
B = 1×9
     0     0     0     0     1     0     0     2     1

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Lewis 2021-11-4

Thank you !

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