[R] Loop over columns of dataframe and change values condtionally

[PIKAL Petr]

Hi

you could operate with whole data frame (sometimes)
head(iris)
  Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1          5.1         3.5          1.4         0.2  setosa
2          4.9         3.0          1.4         0.2  setosa
3          4.7         3.2          1.3         0.2  setosa
4          4.6         3.1          1.5         0.2  setosa
5          5.0         3.6          1.4         0.2  setosa
6          5.4         3.9          1.7         0.4  setosa

chenge all

> head(iris[,1:4]+10) 
  Sepal.Length Sepal.Width Petal.Length Petal.Width
1         15.1        13.5         11.4        10.2
2         14.9        13.0         11.4        10.2
3         14.7        13.2         11.3        10.2
4         14.6        13.1         11.5        10.2
5         15.0        13.6         11.4        10.2
6         15.4        13.9         11.7        10.4

change only some
> iris[,1:4][iris[,1:4]<2] <- iris[,1:4][iris[,1:4]<2]+10
> head(iris)
  Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1          5.1         3.5         11.4        10.2  setosa
2          4.9         3.0         11.4        10.2  setosa
3          4.7         3.2         11.3        10.2  setosa
4          4.6         3.1         11.5        10.2  setosa
5          5.0         3.6         11.4        10.2  setosa
6          5.4         3.9         11.7        10.4  setosa


Cheers
Petr


> -----Original Message-----
> From: R-help <r-help-bounces using r-project.org> On Behalf Of Luigi Marongiu
> Sent: Thursday, September 2, 2021 3:35 PM
> To: r-help <r-help using r-project.org>
> Subject: [R] Loop over columns of dataframe and change values condtionally
> 
> Hello,
> it is possible to select the columns of a dataframe in sequence with:
> ```
> for(i in 1:ncol(df)) {
>   df[ , i]
> }
> # or
> for(i in 1:ncol(df)) {
>   df[ i]
> }
> ```
> And change all values with, for instance:
> ```
> for(i in 1:ncol(df)) {
>   df[ , i] <- df[ , i] + 10
> }
> ```
> Is it possible to apply a condition? What would be the syntax?
> For instance, to change all 0s in a column to NA would `df[i][df[i == 0] =
NA`
> be right?
> Thank you
> 
> 
> --
> Best regards,
> Luigi
> 
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