Problem multiplying exponential function with negative value

I want to plot x(t) = t*(e−0.15t), − 20 ≤ t ≤ 20

I've tried:

t = -20:1:20;
x(t) = t.*(exp(-0.15*t));

Array indices must be positive integers or logical values.

plot(t,x(t))

Answer 1

[Voss]

This statement

x(t) = t.*(exp(-0.15*t));

says to set the value of x at indexes given by t to the expression on the right-hand side. Indexes must be positive integers (1, 2, 3, ...), but t has some negative values and a zero value in it, so that's why the error happens.

In fact the syntax for what you want to do is slightly simpler. You can just say:

t = -20:1:20;
% no need to specify indexes into x, just set the whole thing:
x = t.*(exp(-0.15*t));

Then to plot:

plot(t,x)

[Voss]

That makes t and x each vectors. Alternatively, you can make x a function of t if you want, and do the same thing:

x = @(t)t.*(exp(-0.15*t));

In this case the syntax x(t) evaluates the function x at the values of t given in t. Then your original syntax in the plot statement would be correct:

t = -20:1:20;
plot(t,x(t));

Answer 2

[Yongjian Feng]

Try this:

t = -20:1:20;
x_t = t.*(exp(-0.15*t));
plot(t, x_t)