Matlab Code: Eval and Sub2ind

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

Answer 1

[Dyuman Joshi]

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().

Answer 2

[Walter Roberson]

Alternate code avoiding eval():

ngrid=20;
ndim=2;
idnames = {1,20};

ANS = sub2ind(ngrid.*ones(1,ndim), idnames{:})

ANS = 381

Answer 3

[Image Analyst]