Question

If I have a dictionary as follows (with some lists):

units = ['a','b']
nums = ['1','2']
ratios = ['alpha', 'beta']

d = {'a_1_alpha':4, 'a_1_beta' :1, 'a_2_alpha' :2, 'a_2_beta': 3, 'b_1_alpha':2}

How do i from a new dictionary which:

• forms a key comprising of tuple (num,ratio) #items from list nums & ratios
• the value would be the sum of the earlier dictionary (d) value.

i.e.

new_d = { ('1','alpha'): 6, ('1','beta'): 1, ('2','alpha'): 2, ('2','beta'): 3}

I have the following code, but doesn’t seem right.

new_d = {}
for num in nums:
    for ratio in ratios:
        for k,v in d.items():
            if ratio in k:
                try:
                    oldval = dict[num,ratio]
                except:
                    oldval = 0
                new_d[(num,ratio)] = oldval + v

for p,q in new_d.items():
    print p,q

Please help to comment/advice.
Thanks :).

Answer 1

The two outer loops are redundant, simply iterate over the key-value pairs of d. You can easily extract the three components of a key using split(). Here’s the code:

new_d = {}
for k, v in d.items():
   u, n, r = k.split('_')
   new_d[(n, r)] = v + new_d.get((n, r), 0)