Producing incorrect results with abs command

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Cole
Cole 2022-9-4
评论： dpb 2022-9-4
Hey, I am trying to do some pretty basic math here. The equation Im using is which as a i understand would be written f8=abs(x.^x)-2.*x where x is a -10:10 matrix. However, when I run the code, the answers are incorrect. they should start at -20 but the code outputs 0E9. any ideas what I am missing?
  2 个评论
dpb
dpb 2022-9-4
Dunno. Attach actual case that you think fails, don't just try to describe it to us...
Cole
Cole 2022-9-4
This is my code output for a a -10:10 matrix.
using a calculator to check those outputs shows it is wrong. for example |-10^-10|-(2*-10) would equal 20.00000000001 not 0 like the output shows. I have to believe it is either the abs function or the subtraction because if I input the values manually, i get the correct answers

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采纳的回答

Walter Roberson
Walter Roberson 2022-9-4
Ran in:
You overlooked the "1.0e+09 *" part of the output. You are using format short, which displays only a limited number of decimal places for each item in order to create a compact display.
x = -10:10;
f8 = abs(x.^x) -2.*x;
f8
f8 = 1×21
1.0e+09 * 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 -0.0000 0 0.0000 0.0000 0.0000 0.0000 0.0008 0.0168 0.3874 10.0000
format long g
f8
f8 = 1×21
1.0e+00 * 20.0000000001 18.0000000025812 16.0000000596046 14.0000012142657 12.0000214334705 10.00032 8.00390625 6.03703703703704 4.25 3 1 -1 0 21 248 3115 46644 823529 16777200 387420471 9999999980
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Torsten
Torsten 2022-9-4
@dpb
f8 will be negative in a certain subset of the x-range ...

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更多回答（1 个）

dpb
dpb 2022-9-4
You didn't provide the input x array so we can (easily) duplicate, but now this one is easy... :)
You're confusing the output display with default format short compared to the actual result stored in memory.
NB: the "1.0e+09 *" at the beginning -- that's a common e-format multiplier applied on all results as printed. The result of the first location thus is reported as "0.000E9" which clearly doesn't have sufficient precision to show a value of 20-something on a 10E9 range.
Try looking at
f8(1)
all by itself and the mystery will go away.

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