populating a dataframe with for loop in R

Question

I realise that there are similar questions to this one, but none of them have provided with me a solution to my problem.

I have made a function, which takes a year as argument and outputs a vectors with 8 elements:

> my_function(2004)
[1]  0  0  0  0 20 89  1  2

> my_function(2006)
[1]   0   0   0   0  83 205   0   1

I have attempted to create a for-loop, which would take all the output vectors for each year and convert them to columns in a dataframe:

ma <- matrix(ncol = 2, nrow = 8)
df <- data.frame(ma)
colnames(df)<-c(2004,2006)

for (i in c(2004,2006)){
df$`i` <- my_function(i)
}

I was expecting the output to look like this:

> df
  2004 2006
1    0    0
2    0    0
3    0    0
4    0    0
5   20   83
6   89  205
7    1    0
8    2    1

But instead the for loop just creates a new column called i and populates it with the last iteration:

 > df
  2004 2006   i
1   NA   NA   0
2   NA   NA   0
3   NA   NA   0
4   NA   NA   0
5   NA   NA  83
6   NA   NA 205
7   NA   NA   0
8   NA   NA   1

What I am doing wrong?

Best, Rikki

You can't use $ that way, it uses non-standard evaluation. Use [[ instead, so you can use df[[i]], with the note that you should then loop over c("2004", "2006"), otherwise your data.frame will end up with 2006 columns (but you might want to avoid using only numbers as column names anyway). — Axeman
– Axeman, Commented Oct 19, 2022 at 21:22
If you are looking for a non-loop solution too, try this: as.data.frame(lapply(c(2004, 2006), my_function)) — B. Christian Kamgang
– B. Christian Kamgang, Commented Oct 19, 2022 at 21:26
@Axeman. I works! Thanks a lot. However, I am very confused now, as I thought the double brackets were only used for accessing objects in lists? Secondly, I had no idea the a function takes both strings and numerals as arguments! — Rikki Franklin Frederiksen
– Rikki Franklin Frederiksen, Commented Oct 19, 2022 at 21:38
@B.ChristianKamgang. That is a beautiful simple solution! Thanks a lot. — Rikki Franklin Frederiksen
– Rikki Franklin Frederiksen, Commented Oct 19, 2022 at 21:44
Could someone please post the info from these comments as an answer? — Ben Bolker
– Ben Bolker, Commented Oct 19, 2022 at 21:47

Axeman · Accepted Answer · 2022-10-20 17:55:40Z

1

You can't use $ that way, it uses non-standard evaluation. Use [[ instead, so you can use df[[i]], with the note that you should then loop over c("2004", "2006"), otherwise your data.frame will end up with 2006 columns (but you might want to avoid using only numbers as column names anyway).

ma <- matrix(ncol = 2, nrow = 8)
df <- data.frame(ma)
colnames(df) <- c("2004", "2006")

for (i in c("2004", "2006")) {
  df[[i]] <- my_function(i)
}

– Axeman

edited Oct 20, 2022 at 17:55

Axeman

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answered Oct 19, 2022 at 21:57

Rikki Franklin Frederiksen

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2 Comments

Ben Bolker Over a year ago

Nice. You should probably also post the code that actually works as part of your answer.

Rikki Franklin Frederiksen Over a year ago

@BenBolker. Very good point. I will edit the answer.

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