Matlab variable rounding issue

Question

Heather Purdie 2022-7-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1764760-matlab-variable-rounding-issue

评论： Heather Purdie 2022-7-22

I have a variable (B_WD) that is a 295x4 double, which I am plotting using quiver. It is wind direction vectors ploted over time along a line.

quiver (B_WD(:,1),B_WD(:,2),B_WD(:,3),B_WD(:,4), 'k');

Varibles before code runs.......(4 lines)

738226.7222 0.0000 -0.3000 -0.8000

738226.7292 0.0000 0.2000 -0.5000

738226.7361 0.0000 0.3000 -0.5000

738226.7431 0.0000 0.3000 -1.1000

The first column is the position on x-axis (I am plotting the arrows along a line -hence the zeros in column 2).

When I run the code the first column switches to less precision, rounding to 2dp and some arrows plot from the same start point

Variable after code runs......see rounding

738226.7200 0.0000 -0.3000 -0.8000

738226.7300 0.0000 0.2000 -0.5000

738226.7400 0.0000 0.3000 -0.5000

738226.7400 0.0000 0.3000 -1.1000

I have been reading the help and have tried changing the format of the variable to 'long', I have replaced numbers and re-saved, nothing seems to stop this.

Advice greatly appreciated

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Heather Purdie 2022-7-21

738226.7361 changes to 738226.7400 and 738226.7431 changes to 738226.7400 so they end up plotting at the same starting point

Bruno Luong 2022-7-21

编辑：Bruno Luong 2022-7-21

"When I run the code the first column switches to less precision"

You should tel us what is the code. If you don't change/assign the variable it should remain identical.

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Answer 1

Chunru 2022-7-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1764760-matlab-variable-rounding-issue#answer_1012155

编辑：Chunru 2022-7-21

在 MATLAB Online 中打开

B_WD = [

738226.7222 0.0000 -0.3000 -0.8000

738226.7292 0.0000 0.2000 -0.5000

738226.7361 0.0000 0.3000 -0.5000

738226.7431 0.0000 0.3000 -1.1000 ];

quiver (B_WD(:,1),B_WD(:,2),B_WD(:,3),B_WD(:,4), 'k');

% The first column looks like datenum

datestr(B_WD(:, 1))

ans = 4×20 char array

'11-Mar-2021 17:19:58' '11-Mar-2021 17:30:02' '11-Mar-2021 17:39:59' '11-Mar-2021 17:50:03'

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Heather Purdie 2022-7-22

I created this as a new variable in the work space, then copied and pasted 4 colums of data (numbers to 4dp) into the array and saved it. I am a relatively new Matlab user, as you can probably tell...

Heather Purdie 2022-7-22

Thanks to those who have tried to help - much appreciated. A colleague has helped me resolve this problem by using the 'digits' command and 'vpa' command to set the back-ground level of precision for the variables, which has in-turn dealt with the auto-rounding....

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