Why does this function fail when a non-function version of the same code works?

3

This works:

dogs = {{"chow", "medium", "brown"}};
dogs = Append[dogs, {"poodle", "small", "white"}];
dogs//TableForm  
  chow   medium brown  
  poodle small  white

but this doesn't:

add[name_, fields_] := (name = Append[name, fields])  
add[dogs, {"pug", "small", "tan"}]
Set::shape: Lists {{chow,medium,brown},{poodle,small,white}} and {{chow,medium,brown},{poodle,small,white},{pug,small,tan}} are not the same shape. >>  

The problem is probably very obvious, but I don't see it.

George Wolfe

Posted 2012-10-26T02:37:13.707

Reputation: 5 088

Answers

7

You can't assign to an argument in a function (ie pass by reference) without using HoldFirst so you could fix your code like so:

SetAttributes[add, HoldFirst]
add[name_, fields_] := (name = Append[name, fields])

Gabriel

Posted 2012-10-26T02:37:13.707

Reputation: 2 165

Thanks. HoldFirst is still a little confusing for me. I wouldn't have figured this out. I should probably search on HoldFirst in SE and get a better handle on it. – George Wolfe – 2012-10-26T02:52:27.133

Yeah I hear you ... took me a long time to even start to wrap my head around this. I found this guide invaluable for these kind of issues http://www.mathprogramming-intro.org/

– Gabriel – 2012-10-26T02:55:23.200

1Smiles, is it just me that finds this scary? Isn't this exactly the type of bug propagating side effect that functional programming seeks to eliminate ? :) – image_doctor – 2012-10-26T11:01:14.953

1@image_doctor Leonid Shifrin comments (somewhere) that Mathematica really isn't a functional programming language, but is instead a replacement rules language that looks a lot like functional language. There seem to be a lot of examples of using HoldFirst to do this. – George Wolfe – 2012-10-26T13:34:47.520

1

You might also look at this question: Pass function or formula as function parameter . The accepted answer helped me a lot with a similar problem. The basic issue is that "name" is evaluated when it is passed in.

rogerl

Posted 2012-10-26T02:37:13.707

Reputation: 3 915

My search fould 101 HoldFirst questions. I was just wondering where to start. Thanks. – George Wolfe – 2012-10-26T03:04:20.520