Why is Null printing?

Question

I am making a function that will evaluate several other functions, using the same inputs, in my program and give me the results. I am using Print to do this (see Picture.) The problem is that one of them is printing Null along with the answer. Is there a way to get this Null to not print out? Maybe another way to write this function of functions?

Poly = {w1, w2, w3, w4, d} \[Function] Module[{},

Print[FFFF[w1, w2, w3, w4, d]];
Print[FFC2[w1, w2, w3, w4, d]]];

And it prints out:

Here is the function that is giving the null...

FFC2 = {w1, w2, w3, w4, d} \[Function] 
Module[{weights, allsol, dup, sol, rsol},
weights = {w1, w2, w3, w4};
allsol = DeleteCases[DeleteDuplicates[Flatten[Table[
     If[
      l != i != j != 
        k &&
              {d/weights[[l]], 
         d/weights[[i]]} \[Element] 
        Integers &&

       Chain2[weights[[j]], weights[[k]], d] != {},

      Flatten[{d/weights[[l]], d/weights[[i]],    
        Take[Chain2[weights[[j]], weights[[k]], d], 2], 
        weights[[l]],
        weights[[i]], 
        Drop[Chain2[weights[[j]], weights[[k]], d], 2]}, 1],
                         {}] , {l, 4}, {i, 4}, {j, 4}, {k, 4}], 
    3]], {}]; 
dup = DeleteCases[
  Flatten[Table[
    If[i < j && 
      Sort[Take[Drop[allsol[[i]], 4], 2]] == 
       Sort[Take[Drop[allsol[[j]], 4], 2]], allsol[[i]], {}], {i, 
     Length[allsol]}, {j, Length[allsol]}], 1], {}];
sol = Complement[allsol, dup];
If[sol == {{}} \[Or] sol == {}, "No Possible FFC2", 
 For[k = 1, k < Length[sol] + 1, k++,

  Print[StringTemplate[
   "FFC2: w^``+x^``+y^``z+z^`` with weights: {``,``,``,``}"][
  Sequence @@ Take[sol, {k}][[1]]]]]]];

And it prints out: The reason Null is printing is because the answer that is given by the function in the second picture is a result of Print, but I don't know how to have the answer print without it.

Answer 1

Here are two similar functions:

f1[x_] := Module[{t = x^2},
    Print[t]
]

f2[x_] := Module[{t = x^2},
    t
]

The first function prints something, and returns the output of a Print statement, which is Null. The second function returns t. The second function is the usual way of defining a function using Module in Mathematica. The very last statement is the return value (note: it is possible to wrap the last statement in Return, but this is discouraged because it is not always clear what data structure a Return statement is returning from). Let's compare f1 and f2:

In[36]:= f1[2] // InputForm

During evaluation of In[36]:= 4

Out[36]//InputForm=
Null

Notice how 4 is printed in a Print cell, and the output is simply Null (the InputForm is needed because in StandardForm, Null outputs are suppressed). Now, compare this with:

In[37]:= f2[2]

Out[37]= 4

The above explains what you're seeing. The return value of FFC2 is Null, as the last statement of FFC2 is a Print statement. Printing the output of FFC2 thus prints Null. It is similar to:

In[41]:= Module[{},
    Print[f1[2]]
  ] // InputForm

During evaluation of In[41]:= 4

During evaluation of In[41]:= Null

Out[41]//InputForm=
Null

I would discourage the usage of Print, unless you are using it for diagnostic purposes.