Index exceeds the number of array error

a = a certain 20x20 matrix;

[val,idx] = max(a(:));

[x, y] = find(a == val);

At the underlined part of the code, this displays the error "Index exceeds the number of array elements (1)". How to fix this?

[Chunru]

The result of 'find' may be empty so that x and y are empty. However idx is a positive number. This cause the problem.

[Chunru]

Then x is a scaler and you cannot find x(idx) if idx is greater than 1.

[Stephen23]

"x = 11 and y = 21 though"

And why do you expect that IDX should be able to index into scalar numerics?

Answer 1

[Arif Hoq]

var = randi(100,20,20);
[val,idx] = max(var(:))

val = 100

idx = 15

[x, y] = find(var == val)

x = 7×1

15 5 15 14 16 7 14

y = 7×1

1 5 7 13 19 20 20

idx2=2;
x0 = x(idx2)

x0 = 5

y0 = y(idx2)

y0 = 5

here your idx=2(intentionally fixed) and your number of element of x and y is 7. so x0 and y0 can extract from index 2. now see the below example

%%

var = randi(100,20,20);
[val,idx] = max(var(:))

val = 100

idx = 18

[x, y] = find(var == val)

x = 5×1

18 16 7 4 10

y = 5×1

1 4 8 9 14

% idx2=2;
x0 = x(idx)

Index exceeds the number of array elements. Index must not exceed 5.

y0 = y(idx)

here your idx=18, but your number of element of x and y is 5.so it can not extract form index 18. therefore, you are getting error.

[Arif Hoq]

as Stephen said already you can not specify index in a sclar value. you need a range of element.

x=1:2:1550;
idx = 631;
x0=x(idx)

x0 = 1261

idx or index extract the value from the position 631. if you notice in x the 631th value is 1261.

[Stephen23]

"Stephen said already you can not specify index in a sclar value"

It is certainly possible to index into a scalar value:

A = 6;
A(1,1)

ans = 6

But just like any other indexing, those indices must be within the size of that (scalar) array.... which IDX is not.