R help - Sep 2021 - how to combine logic test on vectors in R?

Hello,
I have two data frames, each with three rows:
```
df_a <- data.frame(a = letters[1:3], b = LETTERS[1:3], q = c("",
"", ""),
stringsAsFactors = FALSE)
df_b <- data.frame(a = letters[4:6], b = LETTERS[4:6], q = c("",
"", "1.5"),
stringsAsFactors = FALSE)
```
I need to test whether the dataframe has been selected and if there is
a value in the q column. I combined in the following test:
```
if (nrow(df_a) == 0 || unique(df_a$q) == "") {
print("empty")
}
if (nrow(df_b) == 0 || unique(df_b$q) == "") {
print("empty")
}
```
The test for df_a worked as expected:
```
> nrow(df_a) == 0
[1] FALSE
> unique(df_a$q) == ""
[1] TRUE
> (nrow(df_a) == 0 || unique(df_a$q) == "")
[1] TRUE
> if (nrow(df_a) == 0 || unique(df_a$q) == "") {
+ print("empty")
+ }
[1] "empty"
```
but the one for df_b did not:
```
> nrow(df_b) == 0
[1] FALSE
> unique(df_b$q) == ""
[1]  TRUE FALSE
> (nrow(df_b) == 0 || unique(df_b$q) == "")
[1] TRUE
> unique(df_b$q)
[1] ""    "1.5"
```
I say that it did not work because unique(df_b$q) IS NOT "", hence
`(nrow(df_b) == 0 || unique(df_b$q) == "")` should be FALSE, instead R
evaluated the first element of unique(df_b$q) == "", which is TRUE.
How can I properly implement a logic test on vectors?
 Thank you

Hi,

The OR operator you used is working as expected: || starts from the
left and evaluates only enough of the options to determine the
results. The first test is TRUE,  so the result is TRUE. It sounds
like you might actually want an AND operator, & or &&, which will
only
return TRUE if all elements are TRUE,

More on logical operators:
https://stat.ethz.ch/R-manual/R-devel/library/base/html/Logic.html

Sarah

On Thu, Sep 30, 2021 at 9:07 AM Luigi Marongiu <marongiu.luigi at
gmail.com> wrote:
>
> Hello,
> I have two data frames, each with three rows:
> ```
> df_a <- data.frame(a = letters[1:3], b = LETTERS[1:3], q =
c("", "", ""),
> stringsAsFactors = FALSE)
> df_b <- data.frame(a = letters[4:6], b = LETTERS[4:6], q =
c("", "", "1.5"),
> stringsAsFactors = FALSE)
> ```
> I need to test whether the dataframe has been selected and if there is
> a value in the q column. I combined in the following test:
> ```
> if (nrow(df_a) == 0 || unique(df_a$q) == "") {
> print("empty")
> }
> if (nrow(df_b) == 0 || unique(df_b$q) == "") {
> print("empty")
> }
> ```
> The test for df_a worked as expected:
> ```
> > nrow(df_a) == 0
> [1] FALSE
> > unique(df_a$q) == ""
> [1] TRUE
> > (nrow(df_a) == 0 || unique(df_a$q) == "")
> [1] TRUE
> > if (nrow(df_a) == 0 || unique(df_a$q) == "") {
> + print("empty")
> + }
> [1] "empty"
> ```
> but the one for df_b did not:
> ```
> > nrow(df_b) == 0
> [1] FALSE
> > unique(df_b$q) == ""
> [1]  TRUE FALSE
> > (nrow(df_b) == 0 || unique(df_b$q) == "")
> [1] TRUE
> > unique(df_b$q)
> [1] ""    "1.5"
> ```
> I say that it did not work because unique(df_b$q) IS NOT "",
hence
> `(nrow(df_b) == 0 || unique(df_b$q) == "")` should be FALSE,
instead R
> evaluated the first element of unique(df_b$q) == "", which is
TRUE.
> How can I properly implement a logic test on vectors?
>  Thank you
>
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-- 
Sarah Goslee (she/her)
http://www.sarahgoslee.com