how to loop a variable name within a list

Question

I am working with a list, I want to loop a variable several times but I can't.

This is the example:

a=data.frame(ID=1:5,WT=1:5)  
b=data.frame(ID=101:105,WT=101:105)
c=list(a,b)
names(c)=c("P01","P02")

Now I want to add a new column to each data frame

c$P01$seq=1:length(c$P01$ID)  
c$P02$seq=1:length(c$P02$ID)

The problem that I got is that my list has 29 data frames, not two like in this example. How can I automatise this process, so I can add a new column to each data frame?

I've tried few things with no success like creating a loop, but it doesn't not work:

unique=names(c)
for (i in 1:length(unique) {
   c$unique[i]$seq=1:length(c$unique[i]$seq)  
}

akrun · Accepted Answer · 2018-03-28 06:04:12Z

1

We can use seq_along with lapply

lapply(c, transform, Seq = seq_along(ID))

Or using tidyverse

library(tidyverse)
c %>% 
   map(~ .x %>% 
            mutate(Seq = row_number()))

answered Mar 28, 2018 at 6:04

akrun

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L Castro Over a year ago

Thank you so much. I've been stucked in this for a long time. Cheers

kath · Accepted Answer · 2018-03-28 06:57:32Z

0

The reason, why your for-loop is not working, is that you are using a character-value in combination with the $-operator. You can fix this by indexing the list with [[ instead.

df_names <- names(dfs)
for (i in seq_along(df_names)) {
  dfs[[df_names[i]]]$seq <- seq_along(dfs[[df_names[i]]]$ID)
}

Further you can use the names itselves in the for-loop:

for (x in df_names) {
  dfs[[x]]$seq <- seq_along(dfs[[x]]$ID)
}

Data
Note: As c and unique are two functions in base R I changed the variable names in my example.

a <- data.frame(ID = 1:5, WT = 1:5)
b <- data.frame(ID = 101:105, WT = 101:105)
dfs <- list(a, b)
names(dfs) <- c("P01", "P02")

answered Mar 28, 2018 at 6:57

kath

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1 Comment

L Castro Over a year ago

Thank you for your kind explanation about the loop. It works now.

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