Problem using spline in MATLAB

I'm new to programming and I'm trying to make a code where I get a c-value after entering my v-value in the command window.

function c = consumption(v)

load roadster.mat;

x=2:200;

cc=spline(speed_kmph,consumption_Whpkm,x);

c=????

    if v<2
        error ('Too high');    
    elseif v>200
        error ('Too low');
    end
end

I want to get a consumption from a specific speed when I write, for example, consumption(80) in the command window.