Problem storing array value in MATLAB

[geft]

I have the following function

function [W] = calcw(k1,k2,d,x)

if x<d
    W = k1*x;
elseif x>=d
    W = k1*x+2*k2*(x-d);
end;

save('varw.mat');

with the following script

k1 = 10e4;
k2 = 1.5e4;
d = 0.1;
x = 0:0.01:0.3;
    
weight = calcw(k1,k2,d,x);

plot(weight,x);

but the problem is that the W values refuse to be stored in the weight variable in the form of an array. The code works if x is a single value, but it doesn't since x is an array. Any help please?

 

[jhae2.718]

When you use a comparison on an array the way you currently have it, MATLAB will only evaluate the code under the condition if it is true for every element in the array.

[COLOR="Blue"]if [/COLOR]x<d
    W = k1*x; [COLOR="SeaGreen"]% This will only happen if *all* elements of x < d[/COLOR]
[COLOR="Blue"]elseif[/COLOR] x>=d
    W = k1*x+2*k2*(x-d); [COLOR="SeaGreen"]% This will only happen if all elements of x >= d[/COLOR]
[COLOR="Blue"]end[/COLOR]

I suspect you want to create a vector W that weights the individual elements of the input x vector. You'll need a for loop and to access the individual members of the x vector then.

 

[geft]

Do you mean using something like this?

for i = 1:length(x)
weight[i] = calcw(k1,k2,d,x[i]);
end;

I keep getting parse errors for the '=' and ')'.

 

[jhae2.718]

Your syntax is incorrect. In MATLAB, to call an element of a matrix/vector, use

[COLOR="SeaGreen"]% Vector (i.e. a 1 x n or n x 1 matrix)[/COLOR]
someElement = vector(i); [COLOR="SeaGreen"]% i is some integer 1< i < length(matrix)[/COLOR]
[COLOR="SeaGreen"]% Matrix[/COLOR]
someElement = matrix(i,j); [COLOR="SeaGreen"]% i,j are ints such that 1<i<number of rows and 1<j<number of columns[/COLOR]

I would vectorize the function instead, like this:

[COLOR="Blue"]function[/COLOR] [W] = calcw(k1,k2,d,x)
    W = zeros(1,length(x)); [COLOR="SeaGreen"]% initialize W[/COLOR]
    [COLOR="Blue"]for [/COLOR]i=1:length(x);
        [COLOR="Blue"]if[/COLOR] x(i)<d
            W(i) = k1*x(i);
        [COLOR="Blue"]elseif[/COLOR] x(i)>=d
            W(i) = k1*x(i)+2*k2*(x(i)-d);
        [COLOR="Blue"]end [/COLOR][COLOR="SeaGreen"]% end if[/COLOR]
    [COLOR="Blue"]end[/COLOR] [COLOR="SeaGreen"]% end for[/COLOR]
[COLOR="Blue"]end[/COLOR] [COLOR="SeaGreen"]% end function[/COLOR]

Then you can just call:

weight = calcw(k1,k2,d,x);

 

[geft]

It works! Many thanks :)