Question

I’m doing data analysis in Matlab, and I have two columns. I’m using find(column1>0) to find the positive values of the first column in a data set. Now, I want to plot (column1,column2), but it is of course not possible, as the size is not the same. The question:

How do I get the corresponding values in column2 for the positive values in column1? Like, if row 17 and row 42 have a positive value in column1, how do I find the value of row 17 and row 42 in column2?

Answer 1

The term for what you are doing is indexing. You can use find, which generates linear indices, but you shouldn’t in this case. Logical indexing is more appropriate.

index = column1 > 0; #% creates a logical index with true where the  
#% condition is satisfied and false otherwise.

values1 = column1(index);
values2 = column2(index);
#% values1 and values2 will be the same size, since they were indexed the same

plot(values1,values2); #% or however you want to do it.