Logical Indexing With LinSpace Issues

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Lucas Graham
Lucas Graham 2023-3-8
评论： Walter Roberson 2023-3-9
Ran in:
Logical indexing is missing an equivalence in a linspace array. I have absolutely no idea why or how to fix this. I'm running MATALB R2022b on Windows 11.
Any ideas or help or explanation would be appreciated.
Here is how you can recreate my issue:
X = linspace(0.2,3,29); %Creates an array going from 0.2 -> 3 via steps of 0.1. So 1.3 is in it.
disp(X(12))
1.3000
flag = (X == 1.3);
any(flag) %Yet for some reason 1.3 isn't detected.
ans = logical
0
%Just to be absolutely sure I'm not mad
disp(X(12))
1.3000
X(12) == 1.3
ans = logical
0
% Yet other non-integers still work!
flag = (X == 1.1);
any(flag)
ans = logical
1
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采纳的回答

Star Strider
Star Strider 2023-3-8
Ran in:
format long
X = linspace(0.2,3,29) %Creates an array going from 0.2 -> 3 via steps of 0.1. So 1.3 is in it.
X = 1×29
0.200000000000000 0.300000000000000 0.400000000000000 0.500000000000000 0.600000000000000 0.700000000000000 0.800000000000000 0.900000000000000 1.000000000000000 1.100000000000000 1.200000000000000 1.300000000000000 1.400000000000000 1.500000000000000 1.600000000000000 1.700000000000000 1.800000000000000 1.900000000000000 2.000000000000000 2.100000000000000 2.200000000000000 2.300000000000000 2.400000000000000 2.500000000000000 2.600000000000000 2.700000000000000 2.800000000000000 2.900000000000000 3.000000000000000
disp(X(12))
1.300000000000000
flag = (X == 1.3);
any(flag) %Yet for some reason 1.3 isn't detected.
ans = logical
0
Check_Equality = 1.3 - X(12)
Check_Equality =
2.220446049250313e-16
%Just to be absolutely sure I'm not mad
disp(X(12))
1.300000000000000
X(12) == 1.3
ans = logical
0
% Yet other non-integers still work!
flag = (X == 1.1);
any(flag)
ans = logical
1
The‘Check_Equality’ assignment demonstrates that the two numbers are actually not equal.
You’re not mad! You just haven’t been introduced to the subtle mysteries of floating-point numeric repreesentation.
See Floating-Point Numbers and Accuracy of Floating-Point Data for an extensive discussion.
.

更多回答（1 个）

Chris
Chris 2023-3-8
编辑：Chris 2023-3-8
Ran in:
This looks like a problem of computer precision. Double-precision floating point numbers in matlab (and floating-point numbers in general) a̶r̶e̶ ̶n̶o̶t̶ ̶e̶x̶a̶c̶t̶ (edit: are a finite set of discrete values, so they are usually unlikely to exactly match the value we see displayed)
X = linspace(0.2,3,29);
Y = 0.2:0.1:3;
% X(11)==1.2 because the result of the linspace division
% is the same as the double representation for 1.2
sprintf('%.60f\n%.60f',X(11),Y(11))
ans =
'1.199999999999999955591079014993738383054733276367187500000000 1.199999999999999955591079014993738383054733276367187500000000'
% X(12) doesn't equal the double for 1.3.
sprintf('%.60f\n%.60f',X(12),Y(12))
ans =
'1.299999999999999822364316059974953532218933105468750000000000 1.300000000000000044408920985006261616945266723632812500000000'
You can use other comparison operators to test rough equivalence, e.g., (X(12)-1.3) < 1e-14
Here's a bit more information, if you're interested.
https://www.geeksforgeeks.org/ieee-standard-754-floating-point-numbers/
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Lucas Graham
Lucas Graham 2023-3-8
编辑：Lucas Graham 2023-3-8
My intuition is that using MATLAB's colon operator to build arrays is less likely to run into float point errors because it probably uses addition instead of division, is that correct?
For example, using like you did 0.2:0.1:3 vs. linspace(0.2,3,29).
Walter Roberson
Walter Roberson 2023-3-9
No, the opposite. Each time you add two floating point numbers, the round-off error can increase (unless the sequence of operations has been carefully chosen.) The error for 0.1, 0.1+0.1, 0.1+0.1+0.1 is greater than for (1:3)/10

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