for loop in an array to substract the values of the array and find a specific value

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I have an array of 200 values. This is the operation I need to program:
The value in the position X minus the value in the position Y EQUALS 0.55. Therefore, I am using a for loop and If statement.
The problem is that I know neither value nor the position of each value. I guess both of them are in the first 50 values of the array. So, how can I code the operation below n times (k-1, k-2, k-3 ...) until I got this number, and disp the 'Here'?
I;
for k =length (I)
x=I(k)-I %Take last value and substract the first value of I, second, k times to the last value.
if x==0.55
disp 'Here'
else
disp 'Not here'
end
end

采纳的回答

Guillaume
Guillaume 2020-2-1
编辑:Guillaume 2020-2-1
If you are trying to find the indices X and Y for which I(X) - I(Y) is equal to a given values, this is easily done without a loop with:
[X, Y] = find(I - I.' == seachvalue); %I must be a vector
This will returrn all the XY pairs that match.
  4 个评论
Jose Rego Terol
Jose Rego Terol 2020-2-2
Ok, now I understand. that's right, I do not need a loop.
[X, Y] = find(abs(I - I.' - seachvalue) <= tol)
With tol is hard to find the value. Is there any way to round the result to the nearest value in the vector?
Thanks

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更多回答(2 个)

Subhadeep Koley
Subhadeep Koley 2020-2-1
Try this.
for k = 2:length(I)
x = I(k) - I(k-1);
if x == 0.55
disp('Here');
else
disp 'Not here'
end
end
  1 个评论
Jose Rego Terol
Jose Rego Terol 2020-2-1
Is not here, but let me ask you what is it doing?
k = 2:length(I)
From the second value to the last.
x = I(k) - I(k-1);
x= the second value - the first one, then the third value - the second one. Is that right?
What happen if the operation is,i.e., the value at the 40 % of the length of the array minus the 20 % of the length of the array? How can I code, try first the sequence you said, then take the 3rd value (k=3:length(I)). When finish, take the third value and substract the first value.
I think it is completely insane for Matlab.

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Image Analyst
Image Analyst 2020-2-1
编辑:Image Analyst 2020-2-1
First of all read the FAQ : Click here to learn why you shouldn't use == to compare floating point values. You should use ismembertol():
x = zeros(size(I));
for k = 1 : length(I)
% Take last value and substract the first value of I, second, k times to the last value.
% I have no idea what the above means but let's subtract I(1) and see
x(k) = I(k) - I(1);
if ismembertol(x(k), 0.55, 0.004)
fprintf(' ----> Found a match at index %d where I(%d) = %f and x = %f.\n', k, k, I(k), x);
break; % Let's quit when we find a match.
else
fprintf('Found no match at index %d where I(%d) = %f and x = %f.\n', k, k, I(k), x(k));
end
end
histogram(x);
  5 个评论
Jose Rego Terol
Jose Rego Terol 2020-2-1
What happen if I want to do this operation n times?
IDiff = I(1) - I;
IDiff = I(2) - I;
IDiff = I(3) - I;
IDiff = I(4) - I;
IDiff = I(5) - I;
IDiff = I(N) - I;
I need a loop for this reason.

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