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MathGroup Archive 1993

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Parsing a string within a block using ToExpression

  • To: mathgroup at yoda.physics.unc.edu
  • Subject: Parsing a string within a block using ToExpression
  • From: Joseph G. McWilliams <mcwilljg at euler.sfasu.edu>
  • Date: Thu, 9 Dec 93 15:54:21 -0600

Using the Input[] function, one can enter and parse a function of x  
by sequential application of ToString[] and ToExpression[]; e.g.,  if  
fx is assigned the value x^2 + Cos[x] via Input[], one can convert  
this to a legitimate delayed evaluation expression by

	 fxStrng = ToString[fx]
	 f[x_] = ToExpression[fxStrng]
	 

I have used this method for function input in many programs that  
prompt the user through use of  Input[].  However there is a  
breakdown in the parsing done by ToExpression[], when these two lines  
are used within a block structure when writing a function.   


If, in the function below, fncxy is passed the value x^2 + Cos[x^3],  
f[x,y] is assigned the expression x + Cos[x], although the string,  
fxy, is "x^2 + Cos[x^3]".  


someFunction[fncxy_] := 

	Block[{}, fxy = ToString[fncxy]; 

		     f[x_,y_] = ToExpression[fxy]
	     ]
		
The issue can be forced by passing fncxy a string, as in  
someFunction["x^2 + Cos[x^3]",....], but this is to be used by my  
students and there is a need to keep the input as simple and straight  
forward as possible.

My questions are:
	(1) Why am I getting different behavior within and without a  
block structure?
	(2) Is there perhaps a simpler, more consistent method of  
accomplishing the input of a function of x?
	
Any suggestions would be greatly appreciated.

---
Joseph McWilliams
Mathematics, Stephen F. Austin St.U.
Nacogdoches, TX
mcwilljg at euler.sfasu.edu







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