How can I use a loop index to reference variable names and create new variable names?

Question

Suppose I have 4 variables in R that belong to dataset data1 and are named x1, x2, y1, y2, and I want to use a loop to create the following variables zi = xi*yi for i=1,2 that will also belong to data1. These variables are defined as follows:

I was previously coding it as

data1$x1<-c(1,2,3)
data1$x2<-c(2,3,4)
data1$y1<-c(3,4,5)
data1$y2<-c(4,5,6)

I was previously coding the process as

data1$z1<-data1$x1*data1$y1
data1$z2<-data1$x2*data1$y2

This might seem practical for creating only two new variables, but I want to learn how to use a loop for situations where there are many more variables to create with this pattern.

I have tried searching online without success because I don't really know what to search. I know for the creating new variables I can use the paste0 and assign functions, but I am not sure how to reference existing variables using a loop index.

Any help is much appreciated!

G. Grothendieck · Accepted Answer · 2021-01-27 19:11:49Z

2

Have revised based on revision to question. Assuming data1 is the data frame shown in the Note at the end we get xnames (a character vector of the x column names), ynames and znames. Then multiply data1[xnames] and data1[ynames], set the names of the result to znames and cbind the original data with that.

x_names <- grep("x", names(data1), value = TRUE)
y_names <- grep("y", names(data1), value = TRUE)
z_names <- sub("x", "z", x_names)

cbind(data1, setNames(data1[xnames] * data1[ynames], znames))

giving:

  x1 x2 y1 y2 z1 z2
1  1  2  3  4  3  8
2  2  3  4  5  8 15
3  3  4  5  6 15 24

Note

Input in reproducible form:

data1 <- list()
data1$x1<-c(1,2,3)
data1$x2<-c(2,3,4)
data1$y1<-c(3,4,5)
data1$y2<-c(4,5,6)
data1 <- as.data.frame(data1)

edited Jan 27, 2021 at 19:11

answered Jan 27, 2021 at 15:20

G. Grothendieck

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2 Comments

dlnB Over a year ago

Sorry for my lack of clarity. The way I was previously coding it was as follows: data1$z1<-data1$x1*data1$y1 data1$z2<-data1$x2*data1$y2 and so on. I want to do it with a loop instead.

G. Grothendieck Over a year ago

Generally in R the whole object approach is preferred to loops.

user10917479 · Accepted Answer · 2021-01-27 15:43:04Z

1

Here is what I think you want in a simple for loop. The trick is to use the [[ notation instead of the $ to select your variables.

data1 <- data.frame(x1 = 1:4,
                    x2 = 11:14,
                    x3 = 21:24,
                    y1 = 101:104,
                    y2 = 111:114,
                    y3 = 121:124)

for (index in 1:3) {
  
  data1[[paste0("z", index)]] <- data1[[paste0("x", index)]] * data1[[paste0("y", index)]]
  
}

# > data1
#   x1 x2 x3  y1  y2  y3  z1   z2   z3
# 1  1 11 21 101 111 121 101 1221 2541
# 2  2 12 22 102 112 122 204 1344 2684
# 3  3 13 23 103 113 123 309 1469 2829
# 4  4 14 24 104 114 124 416 1596 2976

answered Jan 27, 2021 at 15:43

user10917479

Comments

ThomasIsCoding · Accepted Answer · 2021-01-27 15:15:19Z

0

Try the code below

x <- mget(ls(pattern = "^x"))
y <- mget(ls(pattern = "^y"))
list2env(setNames(Map("*",x,y),paste0("z",seq_along(x))),envir = .GlobalEnv)

answered Jan 27, 2021 at 15:15

ThomasIsCoding

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