Matlab Code: Eval and Sub2ind
2 views (last 30 days)
Show older comments
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
ndim=2
idnames =',1,20'
ANS = eval(['sub2ind(ngrid.*ones(1,ndim)' idnames ');'])
0 Comments
Accepted Answer
Dyuman Joshi
on 9 Apr 2023
Edited: Dyuman Joshi
on 9 Apr 2023
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)
%You want to find the linear index of (2,3)
%Syntax is sub2ind(sizeofthematrix,rowindex,columnindex)
ind=sub2ind(size(y),2,3)
%Verification
out(2,3)
Now
['sub2ind(ngrid.*ones(1,ndim)' idnames ');']
%updates to
['sub2ind(ngrid.*ones(1,ndim),1,20);']
%As [] is a concatenation operator
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.
0 Comments
More Answers (2)
Walter Roberson
on 10 Apr 2023
Alternate code avoiding eval():
ngrid=20;
ndim=2;
idnames = {1,20};
ANS = sub2ind(ngrid.*ones(1,ndim), idnames{:})
0 Comments
See Also
Categories
Find more on Matrices and Arrays in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!