NAs introduced by coercion warning?

In my experience, it is generally due to something like this example:

 as.numeric(c('1','b','2'))

[1]  1 NA  2
Warning message:
NAs introduced by coercion

There may be other operations that generate that error, I can't say 
for sure. But the above example illustrates what I look for when I 
get that particular message.

-Don

I'm running cluster analysis on a data frame but when I calculate 
the distance I get this warning "NAs introduced by coercion".  What 
does this mean?

 			  		 
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--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
925-423-1062