Help with if statements

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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

Accepted Answer

Walter Roberson
Walter Roberson on 18 May 2018
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.
Walter Roberson
Walter Roberson on 18 May 2018
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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