MATLAB Indexing type dependence (bug?)

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Ben
Ben 2025-2-20
移动：Stephen23 2025-2-21
Please see this strange example, where the datatype (single vs double) of the starting index affects how many data points are returned:
y = rand(67347456+50,1);
y = single(y);
N = single(50);
size(y(N+1:end))
MATLAB returns: >> 67347454 1 (incorrect, missing 2 elements)
size([N+1:length(y)])
MATLAB returns: >> 1 67347456 (which is correct)
and
size(y(double(N)+1:end))
MATLAB returns >> 67347456 1 (also correct)
Other datatypes like uint32 also work fine.
Please help me understand: If N is only 50, what does it matter to the indexing whether it is a single or a double? Also, aren't these ending index values well within the presicion of single?

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采纳的回答

Stephen23
Stephen23 2025-2-20
移动：Stephen23 2025-2-21
Ran in:
"what does it matter to the indexing whether it is a single or a double?"
Because single precision cannot represent all of those indices. The largest index is:
y = rand(67347456+50,1);
x = numel(y)
x = 67347506
which cannot be represented using SINGLE. The closest SINGLE value is:
single(x)
ans = single 67347504
"...aren't these ending index values well within the presicion of single?"
Nope, those indices are well above FLINTMAX:
flintmax('single')
ans = single 16777216
"MATLAB Indexing type dependence (bug?)"
I do not see anything that indicates a bug.

更多回答（1 个）

Walter Roberson
Walter Roberson 2025-2-20
Ran in:
eps(single(67347456+50))
ans = single 8
single precision numbers are 8 apart by the time of 67347456

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