Need help looping through concatenation in R

Question

Hi I'm trying to return strings that subset all the columns in iris dataset. The output I'm looking for is 'iris$Sepal.Length', 'iris$Sepal.Width', 'iris$Petal.Length', 'iris$Petal.Width', 'iris$Species'.

I tried the following code below, doing a for loop with paste0 function but nothing is returned.

for(i in colnames(iris)){ paste0('iris$',i , collapse ="") }

How can I solve this?

Can you explain a bit more what you're trying to do with the vector of strings? This might be an XY problem and there might be better ways to approach your problem. — Maurits Evers
– Maurits Evers, Commented Mar 20, 2019 at 2:40

Ronak Shah · Accepted Answer · 2019-03-20 02:41:05Z

paste0 is vectorized so you can directly do

paste0("iris$", names(iris)) #[1] "iris$Sepal.Length" "iris$Sepal.Width" "iris$Petal.Length" # "iris$Petal.Width" "iris$Species"

In for loop you need to explicitly tell R to do things

for(i in colnames(iris)){ print(paste0('iris$',i)) } #[1] "iris$Sepal.Length" #[1] "iris$Sepal.Width" #[1] "iris$Petal.Length" #[1] "iris$Petal.Width" #[1] "iris$Species"

Or you can store it in a character vector

string_name <- character(length = ncol(iris)) for(i in seq_along(names(iris))){ string_name[i] <- paste0('iris$',names(iris)[i]) } string_name #[1] "iris$Sepal.Length" "iris$Sepal.Width" #"iris$Petal.Length" "iris$Petal.Width" "iris$Species"

NM_ · Accepted Answer · 2019-03-20 02:51:53Z

1

Using your own logic, your code can be corrected as follows:

for( i in 1:length(colnames(iris)) ){ print( paste0('iris$', colnames(iris)[i] , collapse ="") ) } [1] "iris$Sepal.Length" [1] "iris$Sepal.Width" [1] "iris$Petal.Length" [1] "iris$Petal.Width" [1] "iris$Species"

However @ronak-shah 's solution is mode elegant. I just wanted to show the corrections that could be made to your particular loop.

edited Mar 20, 2019 at 2:51

answered Mar 20, 2019 at 2:40

NM_

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2 Comments

Maurits Evers Over a year ago

This is really not a good solution and should definitely not be the accepted answer; if you wanted to know how to correct OPs syntax take a look at @RonakShah's answer. However and more importantly, it would've been critical to point out that paste0 is already vectorised (again, see Ronak's post). SO is about a collection of best coding practises and solutions that (ideally) generalise and apply to other future questions.

NM_ Over a year ago

@MauritsEvers I agree that this is not an optimal solution. Based on the OP’s code, they appeared to be a beginner and the idea was to guide them through their logic. However in order to keep with best practices, let me know if you think I should delete this answer? Please advise.

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