Define initial value of an function

the initial value of a solution v(x,t) of an equation is defined by: v(x,0)= v_0 \times Id_{[0,x_0]} where Id_{[0,x_0]} (x) is equal to 1 when x \in [0,x_0], else it’s equal to 0.
My question is: in FreeFem++, how we define Vold the initial value of v?

Thank’s in advance.

I am not sure if I have understood your question correctly, if you want to define the function Id_{[0,x_0]} (x), you can use
v(x,0)= v_0 * ( 0 < x < x_0 )