Question

This code count the letters but i don’t want it to rewrite the same letter twice. What can i do?

d=list('banttnay')
for letter in d:
    print letter, d.count(letter)

Output:

b 1
a 2
n 2
t 2
t 2
n 2
a 2
y 1

I don’t want it to rewrite ‘a 2’ or ‘t 2’ twice

Answer 1

use set():

In [40]: d='banttnay'

In [41]: for x in set(d):
   ....:     print x,d.count(x)
   ....:     
a 2
y 1
b 1
t 2
n 2

or to maintain the order use OrderedDict from collections module:

In [67]: from collections import *

In [68]: c=OrderedDict()

In [69]: for x in d:
    c[x]=c.get(x,0)+1
In [71]: for x in c:
   ....:     print x,c[x]
   ....:     
b 1
a 2
n 2
t 2
y 1

or you can also use the unique_everseen recipe from the itertools Recipes:

>>> d='banttnay'
>>> from itertools import *
>>> def unique_everseen(iterable, key=None):
...     seen = set()
...     seen_add = seen.add
...     if key is None:
...         for element in ifilterfalse(seen.__contains__, iterable):
...             seen_add(element)
...             yield element
...     else:
...         for element in iterable:
...             k = key(element)
...             if k not in seen:
...                 seen_add(k)
...                 yield element
... 
>>> for x in unique_everseen(d):
...     print x,d.count(x)
... 
b 1
a 2
n 2
t 2
y 1