I saw this question on Stack Overflow: What’s an efficient way to fill missing
values with previous nonmissing value?
.
I’ll answer in the following.
The following answer is entirely based on the discussions in this thread: Julia DataFrame Fill NA with LOCF . More specifically, it is based on the answers by Danish Shrestha, Dan Getz , and btsays .
As laborg
implies, the accumulate
function in Base Julia will do the job.
Suppose we have an array: a = [1, missing, 2, missing, 9]
. We want to replace the 1st missing
with 1
and the second with 2
: a = [1, 1, 2, 2, 9], which is a = a[[1, 1, 3, 3, 5]]
([1, 1, 3, 3, 5] here are indexes).
This function will do the job:
ffill(v) = v[accumulate(max, [i*!ismissing(v[i]) for i in 1:length(v)], init=1)]
BTW, “ffill” means “forward filling”, a name I adopted from Pandas.
I’ll explain in the following.
What the accumulate
function does is that it returns a new array based on the array we input.
For those of you who are new to Julia like me: in Julia’s mathematical operations, i*true = i
, and i*false=0
. Therefore, when an element in the array is NOT missing, then i*!ismissing() = i
; otherwise, i*!ismissing() = 0
.
In the case of a = [1, missing, 2, missing, 9]
, [i*!ismissing(a[i]) for i in 1:length(a)]
will return [1, 0, 3, 0, 5]
. Since this array is in the accumulate
function where the operation is max
, we’ll get [1, 1, 3, 3, 5]
.
Then a[[1, 1, 3, 3, 5]]
will return [1, 1, 2, 2, 9]
.
That’s why
a = ffill(a)
will get [1, 1, 2, 2, 9]
.
Now, you may wonder why we have init = 1
in ffill(v)
. Say, b = [missing, 1, missing, 3]
. Then, [i*!ismissing(b[i]) for i in 1:length(b)]
will return [0, 2, 0, 4]
. Then the accumulate
function will return [0, 2, 2, 4]. The next step, b[[0, 2, 2, 4]] will throw an error because in Julia, index starts from 1
not 0
. Therefore, b[0]
doesn’t mean anything.
With init = 1
in the accumulate
function, we’ll get [1, 2, 2, 4] rather than [0, 2, 2, 4] since 1 (the init
we set) is larger than 0 (the first number).
We can go further from here. The ffill()
function above only works for a single array. But what if we have a large dataframe?
Say, we have:
using DataFrames
a = ["Tom", "Mike", "John", "Jason", "Bob"]
b = [missing, 2, 3, missing, 8]
c = [1, 3, missing, 99, missing]
df = DataFrame(:Name => a, :Var1 => b, :Var2 => c)
julia> df
5×3 DataFrame
Row │ Name Var1 Var2
│ String Int64? Int64?
─────┼──────────────────────────
1 │ Tom missing 1
2 │ Mike 2 3
3 │ John 3 missing
4 │ Jason missing 99
5 │ Bob 8 missing
Here, Dan Getz’s answer comes in handy:
nona_df = DataFrame([ffill(df[!, c]) for c in names(df)], names(df))
julia> nona_df
5×3 DataFrame
Row │ Name Var1 Var2
│ String Int64? Int64?
─────┼─────────────────────────
1 │ Tom missing 1
2 │ Mike 2 3
3 │ John 3 3
4 │ Jason 3 99
5 │ Bob 8 99
Reflections #

Two questions to think about:

In
nona_df = ...
, is there any difference between usingffill(df[!, c])
and usingffill(df[:, c])
? 
When we use
ffill(df[!, c])
, will values in the originaldf
be changed as well?


Answers to the above two questions:

!
and:
are different when accessing a column.!
references directly todf
whereas:
makes a copy of that column. In the case offfill
, the function basically creates a new array based on the array we input. Therefore, no matter how we modify the result offfill(df[!, c])
orffill(df[:, c])
,df
remains unchanged. So practically speaking, there is no difference between usingffill(df[!, c])
and usingffill(df[:, c])
. 
No.
df
will remain the same.

Last modified on 20211005