How to check if all elements of a list matches a condition

Question

I have a list consisting of like 20000 lists  I use each list s 3rd element as a flag  I want to do some operations on this list as long as at least one element s flag is 0  it s like   my list      a    b   0     c    d   0     e    f   0            In the beginning  all flags are 0  I use a while loop to check if at least one element s flag is 0   def check list        for item in list           if item 2     0              return True     return False   If check my list  returns True  then I continue working on my list   while check my list       for item in my list          if condition              item 2    1         else              do sth     Actually  I wanted to remove an element in my list as I iterated over it  but I m not allowed to remove items as I iterate over it   Original my list didn t have flags   my list      a    b      c    d      e    f             Since I couldn t remove elements as I iterated over it  I invented these flags  But the my list contains many items  and while loop reads all of them at each for loop  and it consumes lots of time  Do you have any suggestions

User · Answer

If you want to check if any item in the list violates a condition use all:

if all([x[2] == 0 for x in lista]):
    # Will run if all elements in the list has x[2] = 0 (use not to invert if necessary)

To remove all elements not matching, use filter

# Will remove all elements where x[2] is 0
listb = filter(lambda x: x[2] != 0, listb)

User · Answer

The best answer here is to use all    which is the builtin for this situation  We combine this with a generator expression to produce the result you want cleanly and efficiently  For example    gt  gt  gt  items     1  2  0    1  2  0    1  2  0    gt  gt  gt  all flag    0 for        flag  in items  True  gt  gt  gt  items     1  2  0    1  2  1    1  2  0    gt  gt  gt  all flag    0 for        flag  in items  False   Note that all flag    0 for        flag  in items  is directly equivalent to all item 2     0 for item in items   it s just a little nicer to read in this case   And  for the filter example  a list comprehension  of course  you could use a generator expression where appropriate     gt  gt  gt   x for x in items if x 2     0    1  2  0    1  2  0     If you want to check at least one element is 0  the better option is to use any   which is more readable    gt  gt  gt  any flag    0 for        flag  in items  True

User · Answer

You could use itertools s takewhile like this  it will stop once a condition is met that fails your statement  The opposite method would be dropwhile  for x in itertools takewhile lambda x  x 2     0  list      print x

User · Answer

Another way to use itertools ifilter  This checks truthiness  and process  using lambda   Sample-  for x in itertools ifilter lambda x  x 2     0  my list       print x

User · Answer

this way is a bit more flexible than using all     my list     1  2  0    1  2  0    1  2  0   all zeros   False if False in  x 2     0 for x in my list  else True any zeros   True if True in  x 2     0 for x in my list  else False   or more succinctly   all zeros   not False in  x 2     0 for x in my list  any zeros   0 in  x 2  for x in my list

[python] How to check if all elements of a list matches a condition?

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