How can I get a character string in r to refer to a particular variable in a data table?

Question

I have a data frame with variables labeled var1-var38 and I am trying to multiply var1*var20-38 and add these new variables to the existing data frame. My issue is that I can create a string with var20, but it is not referring to var20 in the data frame. My code is as follows:

inter.data <- read.csv("interactions.csv", header = T, sep = ",")
a <- 20
while(a <= 38) {
 name <- paste("var", a, sep = "")
 inter.data[paste("var1*", name, sep="")] <- NA
 inter.data$var1*name <- (inter.data$var1)*(inter.data$name)
 a <- a+1
}

I have tried

inter.data <- read.csv("interactions.csv", header = T, sep = ",")
a <- 20
attach(inter.data)
while(a <= 38) {
 name <- paste("var", a, sep = "")
 inter.data[paste("var1*", name, sep="")] <- NA
 inter.data$var1*name <- (var1)*(name)
 a <- a+1
}

as well

Florian · Accepted Answer · 2017-08-01 15:19:24Z

2

I don't think you need a for loop for this. You can add the columns you are trying to create as follows. In this example, we have a dataframe with four columns, and we want to add the column var1*var3 and var1* var4

df = data.frame(var1=c(2,2,2),var2=c(1,2,3),var3=c(4,5,6),var4=c(7,8,9))

new_df <- df[,3:4] * df[,"var1"]
colnames(new_df) = paste0("var1*",colnames(new_df))
cbind(df,new_df)

output:

  var1 var2 var3 var4 var1*var3 var1*var4
1    2    1    4    7         8        14
2    2    2    5    8        10        16
3    2    3    6    9        12        18

Hope this helps!

answered Aug 1, 2017 at 15:19

Florian

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