I don't know how to avoid eval

Question

Wan Ji 2021-9-3

0
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https://ww2.mathworks.cn/matlabcentral/answers/1445804-i-don-t-know-how-to-avoid-eval

评论： Chunru 2021-9-4

Hi friends from community,

I am very confused on how to manage 'eval' function existing in my code, i need it to be removed from my code and run the function as fast as possible.

The subIndex function runs as to solve such a problem.

Given an integer N, I want the summation of these M non-negative integers be less or equal than N.

List all cases of

, such that

.

For example

N = 2, M = 2;

Then p becomes

p = [0,0; 1,0; 0,1; 2,0; 0,2; 1,1];

Following is the function with 'eval':

function p = subIndex(M,N)
s = (N+1)*ones(1,M);
r = (1:prod(s))';
ch = '[';
for i = 1:1:M
    ch = [ch,'f',num2str(i),','];
end
eval([ch(1:end-1),']=ind2sub(s,r);']);
p = eval([ch(1:end-1),']-1']);
q = sum(p,2);
[~,idx] = sort(q);
p = p(idx,:);
q = sum(p,2)<=N&sum(p,2)>=1;
p = p(q,:);
end

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Stephen23 2021-9-4

编辑：Stephen23 2021-9-4

@Chunru: this is easy to implement, if you really need this feature.

If you take a copy of the function you can see that the output uses VARARGOUT.

Chunru 2021-9-4

@Stephen Sure it is not difficult to implement it. But we have to maintain the code ourselves that is very much similar to a built-in code.

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Answer 1

Robert U 2021-9-3

2
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Hi Wan Ji,

you can use cell-arrays. You don't need to assign variables with different names to multiple outputs.

function p = subIndex(M,N)
% Given an integer N, I want the summation of these M non-negative integers be less or equal than N. 
% List all cases of p1,p2,...pM, such that sum(p1...pM) <= N.
    s = (N+1)*ones(1,M);
    r = (1:prod(s))';
       
    f = cell(1,M);
    [f{:}] = ind2sub(s,r);
    f = [f{:}];
    
    p = f-1;
        
    q = sum(p,2);
    [~,idx] = sort(q);
    p = p(idx,:);
    q = sum(p,2)<=N&sum(p,2)>=1;
    p = p(q,:);
end

Kind regards,

Robert

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Stephen23 2021-9-4

More information on how this works:

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

https://www.mathworks.com/matlabcentral/answers/320713-how-to-operate-on-comma-separated-lists

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Answer 2

Walter Roberson 2021-9-3

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[f{1:M-1}] = ind2sub(s, r);
p = cell2mat(f)-1;

However, it looks to me as if you are doing "integer partitions" of M. People have posted functions for that, including https://www.mathworks.com/matlabcentral/answers/226437-obtain-all-integer-partitions-for-a-given-integer#answer_497158

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Robert U 2021-9-3

Hi Walter,

I am not convinced that "M-1" is correct. For the case M = 2 you would get only one cell with the x values of ind2sub. You would miss the second column of p if I am not mistaken.

Kind regards,

Robert

Walter Roberson 2021-9-3

在 MATLAB Online 中打开

Ah yes I was confused about what the -1 was doing in the original code. I see now that it was skipping a final comma. eval() is such a mess to use!

Also... strjoin() would have avoided having to remove the comma. And the loop could have been avoided with

strjoin("f"+(1:M), ',')

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Answer 3

Chunru 2021-9-3

1
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N = 2, M = 2;
%p = [0,0; 1,0; 0,1; 2,0; 0,2; 1,1];
p = subIndex(M,N)
p = subIndex1(M,N)
function p = subIndex(M,N)
s = (N+1)*ones(1,M);
r = (1:prod(s))';
ch = '[';
for i = 1:1:M
    ch = [ch,'f',num2str(i),','];
end
eval([ch(1:end-1),']=ind2sub(s,r);']);
p = eval([ch(1:end-1),']-1']);
q = sum(p,2);
[~,idx] = sort(q);
p = p(idx,:);
q = sum(p,2)<=N&sum(p,2)>=1;
p = p(q,:);
end
function p = subIndex1(M,N)
% Consider the M numbers are index pf M-D matrix, each dim has N+1 elements
s = (N+1)*ones(1,M);
k = cumprod(s);
ii =  (0:k(end)-1)';            % linear index (0 based)
idx = zeros(length(ii), M);     % sub []xM
% linear index i--> sub idx of size Mx1  (0 based)
for j=M:-1:2
    ir = rem(ii, k(j-1));
    idx(:, j) = (ii - ir) /k(j-1);
    ii = ir;
end
idx(:, 1) = ii;
p = idx(sum(idx,2)<=N, :);       
end