R: for loop for subsetting data frame, conserving always first column

Question

i've a data frame with 253 columns, and 100000 + rows.

I want to make a subset of 2 columns, always keeping the first one, and summing 1 to take the next column from the right.

First loop: df[,c(1,2)]
Second loop: df[,c(1,3)]
....
Last loop: df[,c(1,253)]

I 've this code:

for(i in 2:length(colnames(df))){
    paste("a",i,sep ="") <- df[,c(1,i)]
    i += 1
}

But get this error:

> for(i in 2:length(colnames(df))){
+     paste("a",i,sep ="") <- df[,c(1,i)]
+     i += 1
Error: unexpected '=' in:
"    paste("a",i,sep ="") <- df[,c(1,i)]
    i +="
> }
Error: unexpected '}' in "}"

Note: I'm calling the new data frames a2, a3, ... Actually, i would like to name them, with the name of the corresponding colum. For example, TVs <- df[,c(1,2)] instead of: a2 <- df[,c(1,2)]

**UPDATE:

After changing i += 1 to i = i + 1, i get this error:

Error in paste("a", i, sep = "") <- df[, c(1, i)] : 
  target of assignment expands to non-language object

**UPDATE 2:

Based on comment i've used:

for(i in 2:length(colnames(df))){
    name <- paste("a",i,sep ="")
    name <- df[,c(1,i)]
    i = i + 1
}

But got just one data frame:

for(i in 2:length(colnames(df))){
    name <- paste("a",i,sep ="")
    assign(name) <- df[,c(1,i)]
    i = i + 1
}

But just got thi error:

Error in assign(name) <- df[, c(1, i)] : 
  could not find function "assign<-"

possible duplicate of how to assign values to dynamic names variables in R — Metrics
– Metrics, Commented Feb 20, 2015 at 22:09
do not do that; put it in a list: lapply(2:ncol(df), function(i) df[, c(1,i)]) — eddi
– eddi, Commented Feb 20, 2015 at 22:32
@eddi, may you elaborate on this, please? Make it a full answer, please. — Omar Gonzales
– Omar Gonzales, Commented Feb 20, 2015 at 22:36
this issue has been rehashed many times already, including in the above question linked by @Metrics and you'll find a ton of info if you just search a little — eddi
– eddi, Commented Feb 20, 2015 at 22:45

Metrics · Accepted Answer · 2015-02-21 02:03:58Z

-1

Based on @eddi answers, i arreived to this:

First answer:

for(i in 2:ncol(df)){
    assign(paste("a", i, sep=''), df[,c(1,i)])
}

But, as i wanted the column name as name for every new data frame, i've changed the code a bit:

for(i in 2:ncol(df)){
    assign(paste(as.character(colnames(df[i])), i, sep=''), df[,c(1,i)])
}

Thanks @eddi, . I didn't know about the assign function.

edited Feb 21, 2015 at 2:03

Metrics

15.5k7 gold badges56 silver badges83 bronze badges

answered Feb 20, 2015 at 22:54

Omar Gonzales

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7 Comments

eddi Over a year ago

uhmm no, this is not based in any way on what I wrote - do not use assign

eddi Over a year ago

require(fortunes); fortune(236)

Omar Gonzales Over a year ago

Yeah, i know. I'm not familiar with the apply functions. I'll make another answer, with the lapply function. Until know, when i use the lapply function, R start running, but don't make individual data frames. Any hint is welcome.

eddi Over a year ago

lapply makes a list (of individual data.frames if you use the code I suggested); that list will make your life much better than having individual data.frames scattered all over your global environment; please read the link I gave

Omar Gonzales Over a year ago

@eddi, Haha, i've seen the fortune(236). Ill read the link. Thanks.

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