Question

I’ve got a database that I loop through, and based on certain conditions, I send the relevant database entries to a dictionary that I created. I then want to pick one randomly from this dictionary, store it in a list, clear the dictionary, and do the whole thing again with incremented variables. FYI, the [‘StartNoteNum’], etc. are just column names within the cost database.

What happens though, is that it works fine the first time thorough the loop, but if I try to clear the dictionary anywhere in the code (inside or outside the while loop), then the dictionary never re-fills based on the incremented values even though it should. To confirm that it should re-fill properly, I have set the initial values to be all possible values that it would encounter in the while loop, and each one works the first time through the loop, but fails once it tries to cycle. The error I get is that the random function can’t pull from an empty dictionary. Grr… here is the code.

def compute_policy(clean_midi, cost_database):
    note = 0             #Setting up starting variables.
    total_score = []
    current_finger = 1
    path = [1]
    next_move = {}
    while note <= 2:
        current_note = clean_midi[note]    #get note-pair for scoring
        dest_note = clean_midi[note+1]
        for each in cost_database:                            #find all relevant DB entries
            if (int(each['StartNoteNum']) == current_note
                and int(each['DestNoteNum']) == dest_note
                and int(each['StartFing']) == current_finger):
                next_move[int(each['DestFing'])] = int(each['Score'])  #move relevant entries to separate "bin"


        policy_choice = random.choice(next_move.keys())   #choose one at random
        total_score.append(next_move[policy_choice])       #track the scores for each choice in a list
        path.append(policy_choice)             #track the chosen finger
        current_finger = policy_choice        #update finger variable
        note += 1  
    path.append(current_finger)               #append last finger since the loop won't run again
    return total_score, path

any help here would be much appreciated. Thanks.

Answer 1

You are trying to use the cost_database iterator twice. After the first time you pass through it is exhausted and thus the second time you try to use it the whole for loop is skipped because it got an empty iterator.

>>> a = xrange(4)
>>> for i in a:
...    print(i)
0
1
2
3
4
>>> for i in a:
...    print(i)
>>> # Nothing since 'a' is already exhausted.