Matlab Code: Eval and Sub2ind

Wei Wen

2023 4 9

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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

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采纳的回答

Dyuman Joshi 2023-4-9

编辑：Dyuman Joshi 2023-4-9

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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

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