How to put minus (-) before some specific columns in R : I have a data frame containing 2000 columns. Majority of the columns have "X111, X222 ,X123" , all of this X111, X222, X333 are all numeric variables, 
and I want to convert all the positive values to negative values 

Before:

¦ 1COL1 ¦ 2COL ¦ 3COL ¦ XCOL ¦ 4COL ¦ XXCOL ¦

   All (+)ve values.      

After:

¦ 1COL1 ¦ 2COL ¦ 3COL ¦ XCOL ¦ 4COL ¦ XXCOL ¦

    +      +      +       _       +     _