error while coding without using conv

i have been trying to convolve x and h, using this code and getting the error "Attempted to access y(-10.01); index must be a positive integer or logical." I have to attain convolution without using conv or any predefined function.

clear all
clc
Fs = 100;
   dt = 1/Fs;
   StartTime = -5;
   StopTime = 5;
   t = StartTime:dt:StopTime;
x = heaviside(t);
% subplot (2,1,1), plot (t,x)
% axis ([-10 10 -3 3])
% grid on
% xlabel ('time');
% ylabel('unit step function');
a = heaviside (t-3);
b = heaviside (t+2);
h = a+b;
% subplot (2,1,2), plot (t,h)
% axis ([-10 10 -3 3])
% grid on
% xlabel ('time');
% ylabel('impulse');
for (i= -10:0.01:10)
y = 0;
for (j= -10:i)
if(i-j+0.01>0)
            y(j)=y(j-0.01)+x(j)*h(j-0.01);
        else
        end
    end
end
plot (t,y);
grid on;

Answer 1

[Walter Roberson]

You are attempting to index an array by value, such as -10. You need to index by a positive integer instead.

For example (but check the boundary conditions)

xvals = -10 : 0.01 : 10;
numx = length(xvals);

for xidx = 1 : numx

    y = zeros(1, xidx);

    for yidx = 2 : xidx
      if xidx + 1 > jidx
        y(yidx) = y(yidx-1) + xvals(xidx) * h(xvals(yidx - 1));
      end
    end
  end

but you really need to rethink the idea of always re-initializing y inside the first loop (the "for i" loop in your code), as otherwise the value of y at the end will be only whatever it got assigned on the very last pass, in which case you might as well only execute the last pass and skip the rest.