Local variables whose definitions depend on each other [duplicate]

Question

Consider the following (pseudo-)function definition:

f[l_] := Let[{
          a = Length[l],
          b = First[l],
          c = b/a,
         }, c*10]

So for instance, f[{2,3,4,5}] should output 5.

Currently, I only know variants for Let that

• set a, b and c globally (on the first call of f) or
• don't allow the definition of c because a and b are not defined.

What is the most simple way to define local variables whose definitions may depend on each other (upwards)?

Answer 1

If you have Version 10 use the function Where in the GeneralUtilities package like this:

Needs["GeneralUtilities`"]

f[l_] := Where[a = Length[l], b = First[l], c = b/a, c*10]

Then:

f[{2, 3, 4, 5}]

5

Answer 2

Why not just this?

f[l_] := Block[{a,b,c},
          a = Length[l];
          b = First[l];
          c = b/a;
          c*10]

Then

f[{2, 3, 4, 5}]

5