Finding maximum value and its location in an array

I am working with a code in matlab that simulates a wave travelling in time. The following picture is my output at time zero.

I need to plot the amplitude of the wave as a function of time, since the wave gets smaller as it interacts with the ramp. To do this it's been suggested to do a loop that goes through the array and compares two values u_j+1 and u_j to check if u_j+1<u_j. This is a sample of my code, where u is a column vector.

for j = 2 : length(t)

diff_first=A*u1/(2*h);        
sum_w = B*u1;                   
diff_middle=2*diff_first.*sum_w;    
diff_third=A_third*u1/(2*h^3);

if j == 2
    u1 = u - dt*(diff_middle+diff_third); %Computing the first step
    figure(1); plot(x,u1,'r.'); 

else
    u2 = u - 2*dt*(diff_middle+diff_third); %Computing further steps
    figure(1); plot(x,u2,'r.'); 
    u = u1; u1 = u2;
end
hold on; axis([x(1) x(end) -1 Am]); hold off; 
end

Answer 1

[KSSV]

REad about the function max. If v is your array.

[val,idx] = max(v)