问题描述:

how can I calculate the number of gaps in sequences:

for example:

`s1='G _ A A T T C A G T T A'`

s2='G G _ A _ T C _ G _ _ A'

s3='G A A T T C A G T _ T _'

her the number of `'_'`

is **8**

I try the following:

`def count():`

gap=0

for i in range(0, len(s1), 3):

for x,y,z in zip(s1,s2,s3):

if (x=='_') or (y=='_')or (z=='_') :

gap=gap+1

return gap

it gives 6 not 8

Your code returns **7** which is the total count of all the underscores minus the extra underscore in the third to last position. You can fix that by removing the or-test (which short-circuits the tests when a match is found).

Also note there is no need to triple-zip the code or to loop with a stride-of-three.

Here is a cleaned-up version of your original code:

```
def count():
gap=0
for x,y,z in zip(s1,s2,s3):
if (x == '_'): # these if-stmts don't short-circuit
gap += 1
if (y == '_'):
gap += 1
if (z == '_'):
gap += 1
return gap
```

There are other ways to do this faster (i.e. the **str.count** method) but I wanted to show you how to repair and clean-up your original logic. That ought to put you on the right track when you do other analytics.

Strings have a count() method:

```
s1.count('_') + s2.count('_') + s3.count('_')
```

The two `_`

's in the 10th position only get counted twice. You should get 7, rather than 6.

The simple solution is `sum([item.count('_') for item in [s1,s2,s3]])`