Remove Null Elements from List of Lists

Python remove empty elements from list of lists

Your attempt will remove empty lists from the list of lists, not empty elements from the sublists. Instead, apply the filter to the sublists:

str_list = [list(filter(None, lst)) for lst in list_of_lists]

The call to filter() is wrapped with list() in case you happen to try this in python3 later, as filter() returns an iterator in py3.

Note that since you tagged this as csv you might have to be careful as filtering might produce rows with differing lengths and items in wrong columns. If you know that for each row the 2 last items will always be empty, you could slice them out:

str_list = [row[:-2] for row in list_of_lists]

R: removing NULL elements from a list

The closest you'll be able to get is to first name the list elements and then remove the NULLs.

names(x) <- seq_along(x)

## Using some higher-order convenience functions
Filter(Negate(is.null), x)
# $`11`
# [1] 123
# 
# $`13`
# [1] 456

# Or, using a slightly more standard R idiom
x[sapply(x, is.null)] <- NULL
x
# $`11`
# [1] 123
# 
# $`13`
# [1] 456

Recursive remove NULL elements of list of lists

One approach is to use rrapply in the rrapply-package (extension of base rapply):

library(rrapply)

x <- list(a=1, b=2, c=list(ca=1, cb=2, cc=NULL), d=NULL)

rrapply(x, condition = Negate(is.null), how = "prune")
#> $a
#> [1] 1
#> 
#> $b
#> [1] 2
#> 
#> $c
#> $c$ca
#> [1] 1
#> 
#> $c$cb
#> [1] 2

---

Benchmark timings

Benchmarking the computation time of rrapply against rlist's list.clean function for some large nested lists, I get the following results:

## recursively create nested list with dmax layers and 50% NULL elements
f <- function(len, d, dmax) {
  x <- vector(mode = "list", length = len)
  for(i in seq_along(x)) {
    if(d + 1 < dmax) {
      x[[i]] <- Recall(len, d + 1, dmax)
    } else {
      x[[i]] <- list(1, NULL)
    }
  }
  return(x)
}

## long shallow list (3 layers, total 5e5 nodes)
x_long <- f(len = 500, d = 1, dmax = 3)

microbenchmark::microbenchmark(
  rlist = rlist::list.clean(x_long, recursive = TRUE),
  rrapply = rrapply::rrapply(x_long, condition = Negate(is.null), how = "prune"),
  check = "equal",
  times = 5L
)
#> Unit: milliseconds
#>     expr       min        lq      mean    median        uq       max neval
#>    rlist 2331.4914 2343.3001 2438.9674 2441.3850 2512.3484 2566.3121     5
#>  rrapply  353.7169  393.0646  400.8198  399.7971  417.7235  439.7972     5

## deeply nested list (18 layers, total 2^18 nodes)
x_deep <- f(len = 2, d = 1, dmax = 18)

microbenchmark::microbenchmark(
  rlist = rlist::list.clean(x_deep, recursive = TRUE),
  rrapply = rrapply::rrapply(x_deep, condition = Negate(is.null), how = "prune"),
  check = "equal",
  times = 5L
)
#> Unit: milliseconds
#>     expr       min        lq      mean    median        uq       max neval
#>    rlist 2167.2946 2251.5203 2279.9963 2292.5045 2332.4432 2356.2188     5
#>  rrapply  268.9463  274.7437  325.9585  292.4559  354.1607  439.4857     5

Remove empty strings from a list of strings

I would use filter:

str_list = filter(None, str_list)
str_list = filter(bool, str_list)
str_list = filter(len, str_list)
str_list = filter(lambda item: item, str_list)

Python 3 returns an iterator from filter, so should be wrapped in a call to list()

str_list = list(filter(None, str_list))

Java - Remove null from list of Objects

Mutable list

You can use List::removeIf with a predicate detecting null items.

List<Object[]> mutableList = new ArrayList<>(Arrays.asList(
    new Object[] {}, 
    null, 
    new Object[] {}));

mutableList.removeIf(Objects::isNull);

Immutable list

In this case you have to use Stream API or a for-loop to find non-null elements and add them to a new list.

List<Object[]> immutableList = Arrays.asList(
    new Object[] {}, 
    null, 
    new Object[] {});

List<Object[]> newList = immutableList.stream()
                                      .filter(Objects::nonNull)
                                      .collect(Collectors.toList());

Remove values / nan / empty string( ) from list of lists in python

Here is the corrected version.

from math import isnan  # numpy.isnan can also be used instead

# If the list contains only strings, `if e != ""` can be simplied to `if e`
a = [[e for e in l if e != ""] for l in a]

# Use isnan function to check for nan values
b = [[e for e in l if not isnan(e)] for l in b]

How to delete nan/null values in lists in a list in Python?

IF you want to keep rows with nans you can do it like this:

In [5457]: df.T.dropna(how='all').T                                                                                                                                                            
Out[5457]: 
         Index           1           2           3           4
0 20000765.000  624380.000         nan         nan         nan
1 20000766.000  624380.000         nan         nan         nan
2 20000768.000 1305984.000 1305985.000 1305983.000 1306021.000

if you don't want any columns with nans you can drop them like this:

In [5458]: df.T.dropna().T                                                                                                                                                                     
Out[5458]: 
         Index           1
0 20000765.000  624380.000
1 20000766.000  624380.000
2 20000768.000 1305984.000

To create the array:

In [5464]: df.T.apply(lambda x: x.dropna().tolist()).tolist()                                                                                                                                  
Out[5464]: 
[[20000765.0, 624380.0],
 [20000766.0, 624380.0],
 [20000768.0, 1305984.0, 1305985.0, 1305983.0, 1306021.0]]

or

df.T[1:].apply(lambda x: x.dropna().tolist()).tolist()                                                                                                                              

Out[5471]: [[624380.0], [624380.0], [1305984.0, 1305985.0, 1305983.0, 1306021.0]]

depending on how you want the array

Python: How to remove empty lists from a list?

Try

list2 = [x for x in list1 if x != []]

If you want to get rid of everything that is "falsy", e.g. empty strings, empty tuples, zeros, you could also use

list2 = [x for x in list1 if x]

---

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