[R] Factor to variable

[bjlwilkinson at gmail.com]

I have a data frame called z that includes four columns:

Col1 Col2 Col3 Col4
Text1 Text2 X0.1 5
Text1 Text2 X0.2 10
Text1 Text2 X0.3 15
Text1 Text3 X0.1 5
Text1 Text3 X0.2 10
Text1 Text3 X0.3 15

I am trying to convert the 3rd column to the numeric value excluding the X so that I can produce a lattice plot. 

I have tried a number of approaches including z$Col3[z$Col3=="X0.1"]="0.1" as a first step but get an error: invalid factor level NAs generated. 


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