A = rand(1,100);
B = rand(1,100);
MyBElements = B(cumsum(A)<10)
Above, MyBElements is all the elements in B until the cumulative sum of A reaches 10.
Plotting the cumulative sum of A, a threshold at 10, and the individual elements of A, B, and MyBElements can be done like this:
figure;
subplot(2,1,1)
plot(cumsum(A));
hold on
plot([1 100],[10 10],'r')
axis tight
title('cumulative sum of A')
subplot(2,1,2)
plot(1:100,A,'b+',1:100,B,'rx')
hold on
box off
plot(MyBElements,'ro')
legend('A','B','MyB')
