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Topic: Do we need a When function?
Replies: 9   Last Post: Mar 23, 2014 5:01 AM

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David Bailey

Posts: 714
Registered: 11/7/08
Re: Do we need a When function?
Posted: Mar 17, 2014 2:27 AM
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On 15/03/2014 07:51, Itai Seggev wrote:

> If[NumericQ[x], f[x]] (*are NumberQ, if want an actual number type*)

The difference, which at least Szabolcs appreciates, can be seen in this

In[3]:= expr = If[NumericQ[x], ToString[x]]

In[4]:= expr /. x -> 2.1

(* The output here is Null *)

In[7]:= SetAttributes[When, HoldAll];
When[cond_, val_] := val /; cond

In[9]:= expr = When[NumericQ[x], ToString[x]]

Out[9]= When[NumericQ[x], ToString[x]]

In[10]:= expr /. x -> 2.1

Out[10]= "2.1"

The effect of When is to delay its evaluation until something is true -
which can be quite useful!

David Bailey

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