For each row, get column names where data is equal to a certain value

Question

I have a data frame (df) with 7 rows and 4 columns (named c1, c2, c3, c4):

c1  c2  c3  c4
Yes No  Yes No    
Yes Yes No  No    
No  Yes No  No    
Yes No  No  No    
Yes No  Yes No    
Yes No  No  No    
No  No  Yes No

I want to add a 5th column to the data frame named Expected Result if the values on columns 1 to 4 are equal to "Yes". For example, on row 1, I have "Yes" parameters in Column 1 and Column 3. To populate Expected Result column, I would concatenate and add Column1 name and Column 2 name to the result.

Here is the full results expected:

c1, c3    
c1, c2    
c2    
c1    
c1, c3    
c1    
c3

I have the following line of code but something is not quite right:

df$Expected_Result <- colnames(df)[apply(df,1,which(LETTERS="Unfit"))]

Answer 1

An option using data.table

library(data.table)
setDT(df)[, rownum:=1:.N,]
df$Expected_result <- melt(df, "rownum")[, 
                         toString(variable[value=="Yes"]), rownum]$V1