How to change a variable name/object pointer in R within a for loop?

Question

For example, I have 9 conditions lets say and i want to store the seed number within a loop that goes into the results list that contains con1-con9. In my actual code the seed number will be changing, but just have this here for simplicity.

seed <- 500
for(x in 1:length(results)){

results$con1$seednum <- seed

}

I thought something like this, but does not seem to work.

eval(parse(text= paste0("results$con", x)))$seednum <- 500

How can i have this such that the con1 will change to con2, con3...etc through the for loop and be able to store that seed value in each results$con1 through results$con9? I assume it has something to do with eval and parse while using the x index, but I am not sure how it can be done.

Thank you.

This is likely an XY problem. Whenever you find yourself wondering how to loop over a collection of variables which differ in name only by a tacked-on integer, there is a vector or list waiting to be born. Why not have the conditions in 1 data structure rather than 9 variables? That way you wouldn't need hacks involving things like eval. — John Coleman
– John Coleman, Commented Oct 28, 2022 at 1:13
I am currently doing a simulation study that does 100 replications within each condition. So i am looping between each condition (stored as a vector so that i can do x from 1:9)), and then doing 1 for loop that does the replications. I need to store the replications/values before looping back to the conditions though because I only want to make 1 for loop that does the replications, but some of the variable names will need to change from results$con1... to results$con2 after doing the first 100 reps and so on. — Duba Minus
– Duba Minus, Commented Oct 28, 2022 at 1:21
I've never used code like that so I don't know how to debug it. — John Coleman
– John Coleman, Commented Oct 28, 2022 at 1:30

Roland · Accepted Answer · 2022-10-28 05:53:32Z

3

Study help("$"). You want to use [[ here. In general, $ is mainly intended for interactive use.

seed <- 500
for(x in 1:length(results)){

results[[paste0("con", x)]][["seednum"]] <- seed

}

Never use eval(parse()). That would lead to nightmare scenarios for anyone who has to debug your code. That includes future you. (Also, parsing is slow.)

answered Oct 28, 2022 at 5:53

Roland

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GuedesBF · Accepted Answer · 2022-10-28 01:35:20Z

1

First, please put the elements on a list.

my_list <- mget(ls(pattern='con\\d+')

Then use a loop function to append the seed.

library(purrr)

my_list %>%
    map(append, seed)

answered Oct 28, 2022 at 1:35

GuedesBF

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