library(tidyr)Missing Values - NA
packages
NA/NULL
One common scenario when working with real-world data is that we want to extract all elements of a vector that are not NA (i.e. missing data).
Recall that is.na(x) yields a vector of logical values the same length as x
- with TRUEs corresponding to NA values in x
- FALSEs corresponding to non-NA values in x
Data
Create vector
If you remember how to create a vector
x <- c(44, NA, 5, NA)
x * 3[1] 132 NA 15 NArnorm
The random distribution funtion used to create a sample. rnorm and sample are used here to generate data, will explain them in another post.
#lets create a vector containing 1000 draws from a standard normal distribution with
y <- rnorm(1000)
# let's create a vector containing 1000 NAs with
z <- rep(NA, 1000)
#let's select 100 elements at random from these 2000 values
my_data <- sample(c(y, z), 100)
my_data [1] NA -0.149080130 0.611913417 0.792317020 NA
[6] NA -0.443881467 NA NA NA
[11] NA 0.131915146 -0.655590374 1.625254541 1.476649483
[16] 0.077669540 0.683838937 -0.341545294 NA NA
[21] NA 1.458865592 -1.716902683 0.311653852 NA
[26] 0.438143543 2.632704989 -1.885218450 NA NA
[31] -0.182403988 NA -0.366120784 -0.637607563 NA
[36] NA NA NA NA 0.887597603
[41] NA NA NA NA 0.399891715
[46] 1.215637115 -0.002129227 NA NA NA
[51] 0.853127710 NA -0.937588465 -0.146712302 -1.945789440
[56] NA -0.182059167 -0.152109019 NA NA
[61] 1.246470992 NA NA 2.273235432 NA
[66] NA NA 0.114473671 NA NA
[71] NA NA -0.599098347 0.889928281 -0.039066527
[76] NA 0.606582498 NA -1.014450742 NA
[81] 0.442974358 NA NA 0.261724214 0.514009056
[86] 0.617626441 NA 0.292849743 NA NA
[91] 2.689840150 NA NA -0.170027290 NA
[96] NA NA 1.027216782 NA 0.351250291Look for NA
is.na
Where are our NAs located in our data? The is.na() function tells us whether each element of a vector is NA
my_na <- is.na(my_data)
my_na [1] TRUE FALSE FALSE FALSE TRUE TRUE FALSE TRUE TRUE TRUE TRUE FALSE
[13] FALSE FALSE FALSE FALSE FALSE FALSE TRUE TRUE TRUE FALSE FALSE FALSE
[25] TRUE FALSE FALSE FALSE TRUE TRUE FALSE TRUE FALSE FALSE TRUE TRUE
[37] TRUE TRUE TRUE FALSE TRUE TRUE TRUE TRUE FALSE FALSE FALSE TRUE
[49] TRUE TRUE FALSE TRUE FALSE FALSE FALSE TRUE FALSE FALSE TRUE TRUE
[61] FALSE TRUE TRUE FALSE TRUE TRUE TRUE FALSE TRUE TRUE TRUE TRUE
[73] FALSE FALSE FALSE TRUE FALSE TRUE FALSE TRUE FALSE TRUE TRUE FALSE
[85] FALSE FALSE TRUE FALSE TRUE TRUE FALSE TRUE TRUE FALSE TRUE TRUE
[97] TRUE FALSE TRUE FALSEcount
- Another way of counting how many rows have NA values is by using the sum() function. Since TRUE and FALSE are represented by 1 and 0 using the sum(of true values) would give us the count.
- Underneath the surface, R represents TRUE as the number 1 and FALSE as the number 0.
- Therefore, if we take the sum of a bunch of TRUEs and FALSEs, we get the total number of TRUEs or the COUNT of NA or non-NA values/rows based on how it’s used
sum(my_na)[1] 53Extract
!is.na
- Just as we learned to use is.na we can use !is.na which is the negativity of it to extract what IS NOT NA from the sample and store it in non_na
non_na <- my_data[!is.na(my_data)]
non_na [1] -0.149080130 0.611913417 0.792317020 -0.443881467 0.131915146
[6] -0.655590374 1.625254541 1.476649483 0.077669540 0.683838937
[11] -0.341545294 1.458865592 -1.716902683 0.311653852 0.438143543
[16] 2.632704989 -1.885218450 -0.182403988 -0.366120784 -0.637607563
[21] 0.887597603 0.399891715 1.215637115 -0.002129227 0.853127710
[26] -0.937588465 -0.146712302 -1.945789440 -0.182059167 -0.152109019
[31] 1.246470992 2.273235432 0.114473671 -0.599098347 0.889928281
[36] -0.039066527 0.606582498 -1.014450742 0.442974358 0.261724214
[41] 0.514009056 0.617626441 0.292849743 2.689840150 -0.170027290
[46] 1.027216782 0.351250291Remove
As part of tidyr: “drop_na() drops rows where any column specified by … contains a missing value” ### drop_na
drop_na(data, ...)- data is data frame
- … Columns to inspect for missing values. If empty, all columns are used.
- In other words it only keeps the COMPLETE ROWS (where no rows contain missing values)
- Similar in a sense to complete.cases()
library(tidyr)
df <- data.frame(x = c(1,2, NA), y = c("a", NA, "b"))
#__________This will drop NA from all columns
df1 <- df |> drop_na()
df1 x y
1 1 a#__________To remove NA from column x
df2 <- df |> drop_na(x)
df2 x y
1 1 a
2 2 <NA>You have to be cautious as to when to use an operation on df if you are using any of the functions that remove NA. For example we might want to calculate the mean of column (x). In that case we don’t use drop_na() because it will remove rows from x that have values. See example below in na.omit()
na.omit()
If df is oz, self explanatory really, this function just omits all rows that have NA as value. So similar to drop_na it omits the entire rows that have at least one NA
oz <- na.omit(oz)Here is another way to use na.omit(), inside another function.
- Let’s say we want to bind two dfs together and then
- we want to calculate the mean of column (x)
- If my dataset has millions of rows, I prefer to remove the NA from the column I will use to calculate the mean on before I bind them together.
- If you have a few thousands rows and a couple columns you might want to consider the time element of running the operation is all I’m saying.
# Let's say we have 2 dfs and want to calculate the sum() of column x, we also need to remove NAs from it prior
df1 <- data.frame(x = c(1,2, NA), y = c("a", NA, "b"))
df2 <- data.frame(x = c(24,NA, -12), y = c(NA, 4, "z"))
df1 x y
1 1 a
2 2 <NA>
3 NA bdf2 x y
1 24 <NA>
2 NA 4
3 -12 z# Let's rbind the two while we remove NA values and take the sum of all what's left in x
bound <- na.omit(rbind(df1,df2))
bound x y
1 1 a
6 -12 zsum(bound$x) [1] -11Look in theHow To page for a real project using both na.omit() [mean function example] and complete.cases() in the [complete case example] ### na.rm = TRUE
This once again is another way to remove NA from your data. This could be used inside another function, just like you can use na.omit() inside another function.. Let’s say we want to get the sum() or mean() or whatever of column (x), omitting NA values in that specific column ONLY, column (x). Here is a short way to do it. As you see the value is not -11.
#___________Same dfs as before, we'll rbind them first, then we'll sum column(x) after na.rm=TRUE is executed
df1 <- data.frame(x = c(1,2, NA), y = c("a", NA, "b"))
df2 <- data.frame(x = c(24,NA, -12), y = c(NA, 4, "z"))
boundall <- rbind(df1,df2)
boundall x y
1 1 a
2 2 <NA>
3 NA b
4 24 <NA>
5 NA 4
6 -12 z# see the value of boundall compared to bound above
sum(boundall$x, na.rm = TRUE)[1] 15complete.cases
complete.cases(…) returns a logical vector indicating which cases are complete, i.e., have no missing values. Args: a sequence of vectors, matrices and data frames
#_________Use same data as above
df1 <- data.frame(x = c(1,2, NA), y = c("a", NA, "b"))
df2 <- data.frame(x = c(24,NA, -12), y = c(NA, 4, "z"))
df1 x y
1 1 a
2 2 <NA>
3 NA bdf2 x y
1 24 <NA>
2 NA 4
3 -12 z#__Let's look at what we get for each if we run complete.cases()
ok1 <- complete.cases(df1)
ok2 <- complete.cases(df2)
#__________So in df1 only 1 complete case, and same in df2
ok1 + ok2[1] 1 0 1#____________if we bind them we get
bind_them <- rbind(ok1,ok2)
bind_them [,1] [,2] [,3]
ok1 TRUE FALSE FALSE
ok2 FALSE FALSE TRUE- We only want rows that are not missing values?
- We did it above with na.omit can we do it with complete cases?
- So let’s go back to the beginning and bind df1 & df2 into boundall, then
- We use df[complete.case(df), ] that tells R we want to scan through the complete.cases rows and all columns
boundall <- rbind(df1,df2)
boundall x y
1 1 a
2 2 <NA>
3 NA b
4 24 <NA>
5 NA 4
6 -12 zclean_bind <- boundall[complete.cases(boundall), ]
clean_bind x y
1 1 a
6 -12 zna.fail
Returns the object if it does not contain any missing values, and signals an error otherwise. Just as na.omit returns the object with incomplete cases removed. na.pass returns the object unchanged.