MATLAB: Do I get the error ‘Subscript indices must either be real positive integers or logicals. ‘

MATLAB

Why do I get the following error message:
ERROR: Subscript indices must either be real positive integers or logicals.

Best Answer

This error occurs when you attempt to index into an array using indices that are not positive integers or logical values. Here are some tips for common situations that cause this error message:
1) Double check that your indices are positive integers. Indices in MATLAB cannot be 0, and by default, start with 1.
2) If you are using logical indexing to index into an array, be sure that your index array is of type 'logical', and not a 'double' array of 1s and 0s. You can convert a 'double' array to a logical array before attempting to use logical indexing. For example:
A = [1 2 3 4; 5 6 7 8];
ind_double = [0 1 0 1; 0 1 0 1];
ind_logical = logical(ind_double);
A(ind_logical)
For an index array 'ind', you can check its data type using the 'whos' function:
whos ind
3) If you use floating-point arithmetic to compute an index array, then the array values may not be exact integers. The 'round' function is handy when you know you have an index value that is nearly the integer index that you want. For example,
A = [1 2 3 4; 5 6 7 8];
ind_float = 2.00001;
ind_int = round(ind_float);
A(ind_float)
Below is a way to check if an index array 'ind'' contains exact integer values. This command returns a 'logical' array, where 1 indicates the index value is an exact integer, and 0 indicates it is not.
ind == round(ind)
4) If you assign a variable to the same name as a built-in function in MATLAB, then you will overwrite that function and encounter the error when you attempt to call it. For example,
max = rand(5); 
A = rand(5); 
max(A)
In this event, rename your variable and clear the old one to proceed:
B = max; 
clear max max(A)
For more information on indexing in MATLAB, see the following documentation page:
https://www.mathworks.com/help/matlab/math/array-indexing.html
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