How to convert Charagter variables into numeric

I am importing a dataset in R where some of the variable are numerical and
some of them are character...but the problem is that R is treating 
numerical variables as character (I am using "is.character" to judge the
type). Now the question is how can I convert these character variables into
numeric and also let me know about the other conversion like "numeric to
categorical","numeric to continuous" etc.Thank you.

Arup

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