Dynamically init a struct with values calculated from other values in the struct?

[Robert M. Münch]

I need to dynamically initialize a fixed number of entries of the form: 
(start, length) and iterate over them.

Hence, I think a struct makes sense:

struct S {
   s1, l1
 , s2, l2
 , s3, l3
}

Now I need to initialize the values like this:

S myS = {s1:0, l1:10, s2:(2*s1), ...};

So I somehow would like to reference the prior values to initialize the 
other members. I haven't found a way to do it. Is this possible at all? 
Or do I just init the struct empty and then set the entries?

-- 
Robert M. Münch
http://www.saphirion.com
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