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Matlab Code: Eval and Sub2ind

조회 수: 2 (최근 30일)
Wei Wen
Wei Wen 2023년 4월 9일
답변: Image Analyst 2023년 4월 10일
Hi, I come through this code and i couldn't understand how it works.
It is combination of eval and sub2ind. Could anybody explain to me with this problem. How to get the final answer = 381?
Thanks in advance.
Problem
ngrid=20
ngrid = 20
ndim=2
ndim = 2
idnames =',1,20'
idnames = ',1,20'
ANS = eval(['sub2ind(ngrid.*ones(1,ndim)' idnames ');'])
ANS = 381

채택된 답변

Dyuman Joshi
Dyuman Joshi 2023년 4월 9일
편집: Dyuman Joshi 2023년 4월 9일
sub2ind() converts subscripts to linear indices.
Say for example, you have a mxn matrix, and you want to find the linear index of (i,j)th element (given i<=m, j<=n). (Linear indexing in MATLAB is done column wise.) sub2ind() will be useful for that
%Random values
m=randi(7);
n=randi(7);
y=rand(m,n);
%As stated above, an example of Linear indexing
out=reshape(1:m*n,m,n)
out = 6×7
1 7 13 19 25 31 37 2 8 14 20 26 32 38 3 9 15 21 27 33 39 4 10 16 22 28 34 40 5 11 17 23 29 35 41 6 12 18 24 30 36 42
%You want to find the linear index of (2,3)
%Syntax is sub2ind(sizeofthematrix,rowindex,columnindex)
ind=sub2ind(size(y),2,3)
ind = 14
%Verification
out(2,3)
ans = 14
Now
['sub2ind(ngrid.*ones(1,ndim)' idnames ');']
%updates to
['sub2ind(ngrid.*ones(1,ndim),1,20);']
%As [] is a concatenation operator
What eval() does is evaluates the string into it's equivalent mathematical form
so
x=3;y=2;z=1;
eval('x+y-z')
%is equivalent to
x+y-z
%In terms of the above example, it is equivalent to
sub2ind(ngrid.*ones(1,ndim),1,20)
%which is equal to
sub2ind(20.*[1 1],1,20)
%i.e. the linear index of (1,20) in a 20x20 matrix,
%and thus you get the value 381
Hope this is helpful.
FYI - Usage of eval is generally not recommended, refer to the Note on the documentation of eval().

추가 답변 (2개)

Walter Roberson
Walter Roberson 2023년 4월 10일
Alternate code avoiding eval():
ngrid=20;
ndim=2;
idnames = {1,20};
ANS = sub2ind(ngrid.*ones(1,ndim), idnames{:})
ANS = 381

Image Analyst
Image Analyst 2023년 4월 10일

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