Simple logical statement

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Hi I have a question about logical statements.

I have a data set which includes some positive and some negative observations. I need to transform this data set is such a way that all observations that are positive become zero and those that are negative stay the same. What I have is the following:

SP_prices = xlsread('SP500_plusyear.xls', 'b2:b2780');

SP_returns = log(SP_prices(2:end)) - log(SP_prices(1:end-1));

SP_returns_extra = SP_returns(2527:end);

if (SP_returns_extra > 0)

{then SP_returns_extra = 0}

But this doesn't work, so I was wondering how I can change what I have to make it work. Thanks.