I am trying to build an expression by first writing it out as a string and then using ToExpression to convert it to an expression. However, I cannot figure out how to make expression recognize x as an argument. Here is my code building the expression:

functionbuilder := Module[{}, 
   functionstring = "B1";
   nosines = Input["How many sine curves?"];
   For[n = 2, n <= 3*nosines - 1, n = n + 3, 
      string=StringJoin["+B", ToString[n], "*Sin[B", ToString[n + 1],"*x+B",ToString[n+2],"]"];
      functionstring = StringJoin[functionstring, string]

This returns exactly the expression I want. For example, when nosines=3, the output is:

B1 + B2 Sin[B4 + B3 x] + B5 Sin[B7 + B6 x] + B8 Sin[B10 + B9 x]

All I am looking for is how define this expression as a function with x as an argument. I have tried:


But this does not work. What syntax do I need to define a new function f[x] from my expression? To be clear, I want to be able to evaluate the function f[2.] and get:

B1 + B2 Sin[2. B3 + B4] + B5 Sin[2. B6 + B7] + B8 Sin[B10 + 2. B9]

1 Answer 1


You can use Symbol[] to sidestep all that rigamarole with ToExpression[]:

With[{n = 3}, 
     B1 + Sum[Symbol["B" <> IntegerString[k - 1]]
              Sin[Symbol["B" <> IntegerString[k]] x +
                  Symbol["B" <> IntegerString[k + 1]]], {k, 3, 3 n, 3}]]
   B1 + B2 Sin[B4 + B3 x] + B5 Sin[B7 + B6 x] + B8 Sin[B10 + B9 x]

However, I see no justification for needing to generate so many symbols. One can just input a list of parameters if an undetermined number of them is needed, and just let the listability of the arithmetic operators and elementary functions take care of it:

1 + {2, 5, 8}.Sin[{3, 6, 9} x + {4, 7, 10}]
   1 + 2 Sin[4 + 3 x] + 5 Sin[7 + 6 x] + 8 Sin[10 + 9 x]
  • $\begingroup$ Thank you! That should work great for me. Just FYI, I need all of those parameters to remain unknowns so that I can take partial derivatives with respect to each of them. $\endgroup$
    – ahle6481
    Commented Feb 23, 2016 at 18:07
  • $\begingroup$ I see. Is this for a nonlinear fit, or something else? $\endgroup$ Commented Feb 23, 2016 at 18:13
  • $\begingroup$ Yes, I am going to use this to build a Jacobian matrix for least-squares fitting. $\endgroup$
    – ahle6481
    Commented Feb 23, 2016 at 18:15
  • $\begingroup$ You do know that there is the built-in function FindFit[], no? $\endgroup$ Commented Feb 23, 2016 at 18:17
  • $\begingroup$ Yes I am familiar with that. I am just writing my own fitting function to recreate a very specific algorithm that another person wrote so that I can verify their results. $\endgroup$
    – ahle6481
    Commented Feb 23, 2016 at 18:21

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