Python For Loop, How To Find Next Value(object)?

HI, I'm trying to use for loop to find the difference between every two object by minus each other. So, how can I find the next value in a for loop? for entry in entries: first

Solution 1:

It should be noted that none of these solutions work for generators. For that see Glenn Maynards superior solution.

use zip for small lists:

 for current, last in zip(entries[1:], entries):
     diff = current - last

This makes a copy of the list (and a list of tuples from both copies of the list) so it's good to use itertools for handling larger lists

import itertools as it

items = it.izip(it.islice(entries, 1, None), entries)
for current, last in items:
    diff = current - last

This will avoid both making a copy of the list and making a list of tuples.

Another way to do it without making a copy is

entry_iter = iter(entries)
entry_iter.next() # Throw away the first version
for i, entry in enumerate(entry_iter):
    diff = entry - entries[i]

And yet another way is:

for i in xrange(len(entries) - 1):
    diff = entries[i+1] - entries[i]

This creates an iterator that indexes entries and advances it by one. It then uses enumerate to get an indice with the item. The indice starts at 0 and so points to the previous element because we the loop one item in.

Also, as Tyler pointed out in the comment, a loop might be overkill for such a simple problem if you just want to iterate over the differences.

diffs = (current - last for current, last in 
         it.izip(it.islice(entries, 1, None), entries))

Solution 2:

zip works for lists, but for the general case:

def pairs(it):
    it = iter(it)
    prev = next(it)
    for v in it:
        yield prev, v
        prev = v

a = [1,2,3,4,5]
for prev, cur in pairs(a):
    print cur - prev

import itertools as it
for prev, cur in pairs(it.cycle([1,2,3,4])):
    print cur - prev

This works efficiently for large containers, and more importantly, it works for iterators:

for prev, cur in pairs(open("/usr/share/dict/words").xreadlines()):
    print cur, prev,

Edit: I changed the generator to omit the first value with no previous value, since that fits the original question better ("finding differences between adjacent pairs"), and I added an example case showing that it works for an infinitely-repeating iterator.

Solution 3:

i don't know exactly what are you looking but maybe this can help :

first = entries[0]
for entry in entries[1:]:
    last = entry       
    diff = last - first 
    first = entry

Solution 4:

very simply, using enumerate, no fancy stuff

>>> entries=[10,20,30,40]
>>> for n,i in enumerate(entries):
...     try:
...        print entries[n+1] - i
...     except IndexError:
...        print entries[-1]
...
10
10
10
40