Replace letters in matrix

Question

Ymkje Lize Neuteboom 2019-11-12

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/490632-replace-letters-in-matrix

评论： Walter Roberson 2019-11-12

I need to replace compass abbreviations with their corresponding direction in degrees. I've done the following, and it works. However, this seems like an unnecessary long piece of code. Is there a way to do this more concise?

for i = 1:size(Wave_direction,1)
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'NNE', '22.5');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'ENE', '67.5');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'ESE', '112,5');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'SSE', '157.5');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'SSW', '202.5');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'WSW', '247.5');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'WNW', '292.5');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'NNW', '337.5');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'NE', '45');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'SE', '135');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'SW', '225');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'NW', '315');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'N', '360');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'E', '90');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'S', '180');
    Wave_direction(i,2)=replace(Wave_direction(i,2), 'W', '270');
end

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Walter Roberson 2019-11-12

在 MATLAB Online 中打开

regexprep(Wave_direction(i,2), {'NNE', 'ENE', 'ESE'}, {'22.5', '67.5', '112.5'})

You should list the longer patterns first, like you do already.

Ymkje Lize Neuteboom 2019-11-12

Thanks! Works like a charm =)

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Answer 1

Guillaume 2019-11-12

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/490632-replace-letters-in-matrix#answer_401110

在 MATLAB Online 中打开

I would be wary of using regexprep as per Walter's code for that. It relies on the fact that replacements are attempted in the order they occur in the cell array. While this is most likely the case, the documentation does not specify it so you're relying on undefined behaviour. At some point this may change and the code will return incorrect results.

Guaranteed to work and just as simple:

patterns = {'NNE', 'ENE', 'ESE', 'SSE', ..etc};  %order does not matter
replacements = {'22.5', '67.5', '112.5', '157.5', ..etc};  %order has to match patterns obviously
[found, whichrep] = ismember(Wave_direction(:, 2), patterns);
Wave_direction(found, 2) = replacements(whichrep(found));

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Walter Roberson 2019-11-12

It is documented, so it can be counted on.

https://www.mathworks.com/help/matlab/ref/regexprep.html

When expression is a cell array or a string array, regexprep applies the first expression to str, and then applies each subsequent expression to the preceding result.

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