Python: delete item in a list using a for-loop

Question

Hello. I have an array with subjects and every subject has connected time. I want to compare every subjects in the list. If there are two of the same subjects, I want to add the times of both subjects, and also want to delete the second subject information (subject-name and time). But If I delete the item, the list become shorter, and I get an out-of-range-error. I tried to make the list shorter with using subjectlegth-1, but this also don't work.

   ...
   subjectlegth = 8
   for x in range(subjectlength):
        for y in range(subjectlength):
            if subject[x] == subject[y]:
                if x != y:
                    #add
                    time[x] = time[x] + time[y]
                    #delete
                    del time[y]
                    del subject[y]
                    subjectlength = subjectlength - 1

Answer 1

Iterate backwards, if you can:

for x in range(subjectlength - 1, -1, -1):

and similarly for y.

Answer 2

The best practice is to make a new list of the entries to delete, and to delete them after walking the list:

to_del = []
subjectlength = 8
for x in range(subjectlength):
    for y in range(x):
        if subject[x] == subject[y]:
            #add
            time[x] = time[x] + time[y]
            to_del.append(y)

to_del.reverse()
for d in to_del:
    del subject[d]
    del time[d]

Answer 3

If the elements of subject are hashable:

finalinfo = {}

for s, t in zip(subject, time):
  finalinfo[s] = finalinfo.get(s, 0) + t

This will result in a dict with subject: time key-value pairs.

Answer 4

An alternate way would be to create the subject and time lists anew, using a dict to sum up the times of recurring subjects (I am assuming subjects are strings i.e. hashable).

subjects=['math','english','necromancy','philosophy','english','latin','physics','latin']
time=[1,2,3,4,5,6,7,8]
tuples=zip(subjects,time)
my_dict={}
for subject,t in tuples:
    try:
        my_dict[subject]+=t
    except KeyError:
        my_dict[subject]=t
subjects,time=my_dict.keys(), my_dict.values()
print subjects,time