Replace NaNs with previous values

on 9 Oct 2012

Carlos Vladimir Rodriguez Caballero (view profile)

on 18 Jul 2017

Hello,

I have the following problem. I like to replace NaNs with the previous values.

A =

```     4     5     6     7     8
32   NaN   NaN    21   NaN
12   NaN    12   NaN   NaN
34   NaN   NaN   NaN   NaN```

B =

```     4     5     6     7     8
32     5     6    21     8
12     5    12    21     8
34     5    12    21     8```

I sloved it like this:

```for i = 2:5
```
```[r,c] = find(isnan(A(:,i)));
```
```while sum(isnan(A(:,i)))>0
```
```A(r,i) = A(r-1,i);
```
```end
```
```end
```

I'm sure there is a way avoiding the for and the while statement. I search for an "elegant" solution.

Someone's able to help me?

on 9 Oct 2012

What if a whole column is nan? Which value will fill it?

Johannes

Johannes (view profile)

on 9 Oct 2012

If the first value is NaN, everything should be NaN untill a different value appears in the column.

Thanks, Johannes

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on 9 Oct 2012

on 9 Oct 2012

Johannes, notice that your solution will fail if the first value in a column is nan. Rather than looking for a vectorized solution that may end up being rather convoluted (and being slower!), I would simply write a good FOR loop function that can handle all cases. For example, the following solution does not use the FIND function, and only uses simple loops and thus should be very fast:

```function A = fill_nans(A)
% Replaces the nans in each column with
% previous non-nan values.
```
```for ii = 1:size(A,2)
I = A(1,ii);
for jj = 2:size(A,1)
if isnan(A(jj,ii))
A(jj,ii) = I;
else
I  = A(jj,ii);
end
end
end
```

Jakob Hannibal

Jakob Hannibal (view profile)

on 16 Nov 2014

Yes, I thought about that. But I tried to use flipud before the loop and then reverse the flip after the operation. I think it works too! Thanks for feedback!!

Timothy Jackson

Timothy Jackson (view profile)

on 1 Apr 2016

Is there a way to do this both before and after values? For instance changing

A= NaN NaN 2 4 8 NaN NaN to A= 2 2 2 4 8 8 8 ?

on 1 May 2017

Good question Timothy Jackson. I hope someone can answer this

Wayne King (view profile)

on 9 Oct 2012
Edited by Wayne King

Wayne King (view profile)

on 9 Oct 2012

``` A = [ 4     5     6     7     8
32   NaN   NaN    21   NaN
12   NaN    12   NaN   NaN
34   NaN   NaN   NaN   NaN];
indices = isnan(A);
A(indices) = 0;
B = repmat([4 5 6 7 8],size(A,1),1);
A = A+B.*indices;```

1 Comment

on 9 Oct 2012

"Solution there:

A =

```     4     5     6     7     8
32     5     6    21     8
12     5    12     7     8
34     5     6     7     8```

Not good, would need the following: 4 5 6 7 8 32 5 6 21 8 12 5 12 21 8 34 5 12 21 8

Still thanks for you help!"

owr (view profile)

on 9 Oct 2012

I do this all the time, my code uses for loops, but I dont see anything wrong with for loops. Im sure there are more elegent solutions but this does the trick for me and is more than fast enough:

```function datai = backfillnans(data)
```
```% Dimensions
[numRow,numCol] = size(data);
```
```% First, datai is copy of data
datai = data;
```
```% For each column
for c = 1:numCol
% Find first non-NaN row
indxFirst = find(~isnan(data(:,c)),1,'first');
% Find all NaN rows
indxNaN = find(isnan(data(:,c)));
% Find NaN rows beyond first non-NaN
indx = indxNaN(indxNaN > indxFirst);
% For each of these, copy previous value
for r = (indx(:))'
datai(r,c) = datai(r-1,c);
end
end
```

on 9 Oct 2012

This seems to fail when a whole column of data is nan.

```A = [25   NaN    54    99    20
3   NaN    92    74    89
7   NaN   NaN   NaN    82
75   NaN    43    65    77
NaN   NaN    15   NaN    38]
```
owr

owr (view profile)

on 9 Oct 2012

Ah, good catch Matt, thanks for that. Ive been using this for almost 2 years multiple times a day and thats never come up - I guess I never have a full column of nans. It can be fixed I guess by putting an:

```if( ~isempty(indxFirst) )
```

after the line that calculates "indxFirst". Part of me would actually like the whole process to fail so I can figure out why I passed a full column of nans in the first place - that would be symptomatic of a much bigger issue...

Anyways, thanks for taking the time to run and test the code.

Carlos Vladimir Rodriguez Caballero (view profile)

on 18 Jul 2017

I found your procedure much more elegant and efficient. It was very helpful man.

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