Counting the number of True Booleans in a Python List

Question

I have a list of Booleans    True  True  False  False  False  True    and I am looking for a way to count the number of True in the list  so in the example above  I want the return to be 3    I have found examples of looking for the number of occurrences of specific elements  but is there a more efficient way to do it since I m working with Booleans  I m thinking of something analogous to all or any

User · Answer

I prefer len  b for b in boollist if b is True    or the generator-expression equivalent   as it s quite self-explanatory  Less  magical  than the answer proposed by Ignacio Vazquez-Abrams   Alternatively  you can do this  which still assumes that bool is convertable to int  but makes no assumptions about the value of True  ntrue   sum boollist    int True

User · Answer

Just for completeness  sake  sum is usually preferable   I wanted to mention that we can also use filter to get the truthy values  In the usual case  filter accepts a function as the first argument  but if you pass it None  it will filter for all  truthy  values  This feature is somewhat surprising  but is well documented and works in both Python 2 and 3   The difference between the versions  is that in Python 2 filter returns a list  so we can use len    gt  gt  gt  bool list    True  True  False  False  False  True   gt  gt  gt  filter None  bool list   True  True  True   gt  gt  gt  len filter None  bool list   3   But in Python 3  filter returns an iterator  so we can t use len  and if we want to avoid using sum  for any reason  we need to resort to converting the iterator to a list  which makes this much less pretty     gt  gt  gt  bool list    True  True  False  False  False  True   gt  gt  gt  filter None  bool list   lt builtins filter at 0x7f64feba5710 gt   gt  gt  gt  list filter None  bool list    True  True  True   gt  gt  gt  len list filter None  bool list    3

User · Answer

True is equal to 1    gt  gt  gt  sum  True  True  False  False  False  True   3

User · Answer

After reading all the answers and comments on this question  I thought to do a small experiment   I generated 50 000 random booleans and called sum and count on them   Here are my results    gt  gt  gt  a    bool random getrandbits 1   for x in range 50000    gt  gt  gt  len a  50000  gt  gt  gt  a count False  24884  gt  gt  gt  a count True  25116  gt  gt  gt  def count it a         curr   time time         counting   a count True        print  Count it       str time time   - curr         return counting       gt  gt  gt  def sum it a         curr   time time         counting   sum a        print  Sum it       str time time   - curr         return counting       gt  gt  gt  count it a  Count it   0 00121307373046875 25015  gt  gt  gt  sum it a  Sum it   0 004102230072021484 25015   Just to be sure  I repeated it several more times    gt  gt  gt  count it a  Count it   0 0013530254364013672 25015  gt  gt  gt  count it a  Count it   0 0014507770538330078 25015  gt  gt  gt  count it a  Count it   0 0013344287872314453 25015  gt  gt  gt  sum it a  Sum it   0 003480195999145508 25015  gt  gt  gt  sum it a  Sum it   0 0035257339477539062 25015  gt  gt  gt  sum it a  Sum it   0 003350496292114258 25015  gt  gt  gt  sum it a  Sum it   0 003744363784790039 25015   And as you can see  count is 3 times faster than sum  So I would suggest to use count as I did in count it   Python version  3 6 7 CPU cores  4 RAM size  16 GB OS  Ubuntu 18 04 1 LTS

User · Answer

list has a count method    gt  gt  gt   True True False  count True  2   This is actually more efficient than sum  as well as being more explicit about the intent  so there s no reason to use sum   In  1   import random  In  2   x    random choice  True  False   for i in range 100    In  3    timeit x count True  970 ns    41 1 ns per loop  mean    std  dev  of 7 runs  1000000 loops each   In  4    timeit sum x  1 72   s    161 ns per loop  mean    std  dev  of 7 runs  100000 loops each

User · Answer

It is safer to run through bool first  This is easily done    gt  gt  gt  sum map bool  True  True  False  False  False  True    3   Then you will catch everything that Python considers True or False into the appropriate bucket    gt  gt  gt  allTrue  True  not False  True 1  0        1   0    0 0   set  0     gt  gt  gt  list map bool allTrue    True  True  True  True  True  True  True  True  True    If you prefer  you can use a comprehension    gt  gt  gt  allFalse           False 0 set       not True  True-1   gt  gt  gt   bool i  for i in allFalse   False  False  False  False  False  False  False  False  False

User · Answer

You can use sum      gt  gt  gt  sum  True  True  False  False  False  True   3

User · Answer

If you are only concerned with the constant True  a simple sum is fine   However  keep in mind that in Python other values evaluate as True as well   A more robust solution would be to use the bool builtin    gt  gt  gt  l    1  2  True  False   gt  gt  gt  sum bool x  for x in l  3   UPDATE  Here s another similarly robust solution that has the advantage of being more transparent    gt  gt  gt  sum 1 for x in l if x  3   P S  Python trivia  True could be true without being 1   Warning  do not try this at work    gt  gt  gt  True   2  gt  gt  gt  if True  print  true        true  gt  gt  gt  l    True  True  False  True   gt  gt  gt  sum l  6  gt  gt  gt  sum bool x  for x in l  3  gt  gt  gt  sum 1 for x in l if x  3   Much more evil   True   False

[python] Counting the number of True Booleans in a Python List

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