Indexed variable - Function that contains summation

Question

I have the following problem: I have a function that looks like $f(c_1 , c_2 ) = c_1 + c_2 + c_3$. The variables are $c_1$ and $c_2$, $c_3$ is a known constant. I would like to define a function in mathematica that resembles this $f$ by telling the compiler that $c_1$ and $c_2$ are my variables, but $c_3$ is not! My first approach looked like this:

Subscript[c,3] = 2
f[Subscript[c,1],Subscript[c,2]] := Sum[Subscript[c,i],{i,1,3}]

However, as a test, f[1,2] does not return the number 5 as it should.

I have read other posts regarding this topic but have not understood them, because I am a complete beginner. Would appreciate your help.

Answer 1

Input variables of functions in Mathematica should have _ at the end of their names. Using Subscript in Mathematica is a little tricky and sometimes leads to unexpected results. It's recommended to not use them. Also, you can't use _ in your variables, except in inputs of functions.

But for your problem, I will show some alternatives:

Solution 1

c3 = 2
f[c1_, c2_] := c1 + c2 + c3

f[1, 2]
(*Out: 5 *)

Solution 2

c3 = 2
f[c1_, c2_] := Sum[i, {i, {c1, c2, c3}}]

f[1, 2]
(*Out: 5 *)

Solution 3

If you'd defined your variables in c1, c2 format:

c3 = 2;
f[c1_, c2_] := Evaluate[Sum[Symbol["c" <> ToString[i]], {i, 1, 3}]]

f[1, 2]
(*Out: 5 *)

This solution will use Symbol to automatically access variables by building their names as string.

Answer 2

Clear["Global`*"]

I recommend that you avoid the use of subscripted variables and instead use indexed variables. You can display the indexed variables in any desired manner using Format.

Format[c[n_]] := Subscript[c, n]

n = 5;

var = Array[c, n]

To set variables c[3] through c[n] equal to a constant

Set[c[#], k] & /@ Range[3, n];

Then

var

To use the variables as arguments to a function

f @@ var

Total@var

Sum[c[m]/m!, {m, 1, n}]