Beginner R programmer here.

I have 73 variables, 70 with questions (named Q1:Q72) and a grouping variable ("Color") with values "red" or "blue". If the participant (1 participant/row) is in group "red" they have values between 5 and 76 in columns Q1:Q76. If they are in group "blue" they have values between 77 and 148 in columns. However, I want all answers to be between 1 and 72. That is, I want to subtract 76 from Q1:Q72 if the participant is in group "blue" and subtract 4 from Q1:Q72 if participant is in group "red".

So far, my solution has been to split the df into two new df ("dfBlue" and "dfRed"), then subtract 5 from "dfRed" and subtract 76 from "dfBlue" and finally merge the two new dataframes.

Can someone help me with a more elegant solution where no new dataframe is needed?

Thanks!!

df:

How I want it to be:

CodePudding user response：

A dplyr option:

library(dplyr)
df %>% mutate(across(starts_with("Q"), ~if_else(Color == "red", .x - 4, .x - 76)))
#  Color Q1 Q2 Q72
#1   red  1 42  28
#2  blue 31  9  18
#3  blue  7 69  52
#4   red 43 30  70

PS. Please note the typo in your expected output: row 3, Q72 should be 128 - 76 = 56 52.

Sample data

df <-read.table(text = "Color   Q1  Q2  Q72
red 5   46  32
blue    107 85  94
blue    83  145 128
red 47  34  74", header = T)