How can I grab the value of i for which out(i) is equal to s(2)?

Question

Preyanka Dey 2020-9-4

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/588742-how-can-i-grab-the-value-of-i-for-which-out-i-is-equal-to-s-2

评论： Star Strider 2020-9-4

Hi everyone,

I am trying to grab the value of i for which out(i) is equal to s(2). The segment is marked below by '% facing problem here'. Correct value of d is the answer. Can anyone please help me to figure that out? thanks a lot.

function main
n = 6; 
long_min = 1.;
lat_min  = 1.;
w = 2.;
h = 1.;
bound.xmin = long_min;
bound.xmax = long_min + w;
bound.ymin = lat_min;
bound.ymax = lat_min + h;
% generating the sample points 
long  = long_min + w * rand(1,n);
lat   = lat_min  + h * rand(1,n); 
%structure arrays
pts = struct('num',{},'x',{},'y',{});
for i=1:n
    pts(i).num=i;
    pts(i).x=long(i);
    pts(i).y=lat(i);
end
a = 5;
    
for i = 1:n
    out(i) = near_pt(pts(i).x, pts(i).y, pts(a).x, pts(a).y)  
end
% facing problem here
s = sort(out(:));
if (out(i)== s(2))
    d = [i]; % return the value of i for which out(i)== s(2)
end;
 disp(d);
end
function out = near_pt(p, q, r, s)
    out = sqrt((r - p)^2+(s - q)^2);    
end

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Answer 1

Star Strider 2020-9-4

1
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https://ww2.mathworks.cn/matlabcentral/answers/588742-how-can-i-grab-the-value-of-i-for-which-out-i-is-equal-to-s-2#answer_489520

编辑：Star Strider 2020-9-4

在 MATLAB Online 中打开

What you want to do is not obvious.

If you want to know the index of the second value of ‘s’ (the second lowest value of ‘out’ with ‘out’ sorted ascending), that is striaghtforward:

[s,idx] = sort(out(:))

since ‘s’ will be the sorted values of ’out’ and ‘idx’ will be their original locations in the ‘out’ vector.

In one run of your code:

s =
     0.0000e+000
     1.2888e+000
     1.5399e+000
     1.6480e+000
     1.8415e+000
     1.8834e+000
idx =
     5
     4
     2
     6
     1
     3

so the second value of ‘s’ was originally ‘out(4)’.

EDIT —

d = idx(2)

Is that the result you want?

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Preyanka Dey 2020-9-4

编辑：Preyanka Dey 2020-9-4

@Star Strider Thanks a lot for the solution. Yes I want idx(2).

Star Strider 2020-9-4

As always, my pleasure!

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Answer 2

David Hill 2020-9-4

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https://ww2.mathworks.cn/matlabcentral/answers/588742-how-can-i-grab-the-value-of-i-for-which-out-i-is-equal-to-s-2#answer_489529

在 MATLAB Online 中打开

function main
n = 6; 
long_min = 1.;
lat_min  = 1.;
w = 2.;
h = 1.;
bound.xmin = long_min;
bound.xmax = long_min + w;
bound.ymin = lat_min;
bound.ymax = lat_min + h;
% generating the sample points 
long  = long_min + w * rand(1,n);
lat   = lat_min  + h * rand(1,n); 
%structure arrays
pts = struct('num',{},'x',{},'y',{});
for i=1:n
    pts(i).num=i;
    pts(i).x=long(i);
    pts(i).y=lat(i);
end
a = 5;
    
for i = 1:n
    out(i) = near_pt(pts(i).x, pts(i).y, pts(a).x, pts(a).y); 
end
% facing problem here
s = sort(out(:));
d=find(out==s(2));
disp(d);
end
function out = near_pt(p, q, r, s)
    out = sqrt((r - p)^2+(s - q)^2);    
end