The given code throws the error "Array indices must be positive integers or logical values.". The code in the if-else is written for Heaviside as an array rather than a function making (y(1) - a) an index value. Indices for an array can only be integers while the given index value is a fractional value.
This can be resolved by creating a user-defined function as follows:
function y = Heaviside(x)
if x < 0
y = 0;
y = 1;
MATLAB also has a pre-defined function heaviside which can be directly used.
Furthermore, the plot command will also throw an error as the property LineWidth expects positive numerical values. The code can be rectified by removing the property LineWidth or specifying a numeric value for it as follows:
plot(t,y(1), 'b.',t,y(2), 'r.', 'LineWidth', 5);
For further information about code for solving ODEs using Runge-Kutta 4th Order Metod and Euler's Method, you can refer to the following resources: