how to perform plot with if?

I have two data, X and R, and I want to make it understand after the following condition is executed, for example, if the first data of X and the first data of R or the 100th data of X and the 100th data of R, if it has a value, it should perform the drawing.

for i=1:1452
    if (x(i)>-20) && (x(i)<100) && (r(i)>-20) && (r(i)<100)
        x(i)=x(i)
        r(i)=r(i)
    else 
        x(i)=nan
        r(i)=nan
    end
end

[Dyuman Joshi]

Store the indices which satisfy the condition, and plot the corresponding values.

%logical indexing to find the indices
idx=(x>-20&x<100)&(r>-20&r<100);
%plotting
plot(x(idx),r(idx))