Help with if statements

Question

Wendy Cameron 2018-5-18

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https://ww2.mathworks.cn/matlabcentral/answers/401500-help-with-if-statements

评论： Walter Roberson 2018-5-18

Crop factor.xls

If I have a number in one table column, how do I add a new column where the numbers in the second column depend on on the first column. For example, if the first column = 1, new column = 0, if first column = 4, new column = 0.2, if 9, new column = 0.5 etc. I don't want to replace the first column but want to add a second column that is based on the first. I feel I should be using if statements but am new to matlab and I just don't seem to be understanding how to use these yet.

I have attached a table to show how it should look. (The actual table has about 500,000 rows).

Much appreciated, Wendy

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Answer 1

Walter Roberson 2018-5-18

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https://ww2.mathworks.cn/matlabcentral/answers/401500-help-with-if-statements#answer_320894

在 MATLAB Online 中打开

num = xlsread('Crop factor.xls');
input_table = [1, 4, 9];
replacement_table = [0, 0.2, 0.5];
new_column = nan(size(num));
[tf, whichcase] = ismember(num, input_table);
new_column(tf) = replacement_table(whichcase(tf));

This will leave nan for any column that was not matched.

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Wendy Cameron 2018-5-18

...which wasn't what I originally asked for but the great thing about the forums is the interaction and ideas and learning. Thanks!!

Walter Roberson 2018-5-18

whichcase is just an arbitrary variable name; the key is that it is the second output of ismember()

Using logical indexing is fine for a small number of cases, but it gets out of hand quickly as you add more cases. With the ismember() version, you can expand to as many replacements as you need just by adding to input_table and replacement_table.

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