get not working with dictionary comprehension

Question

Create a dictionary containing the frequency of each character in a given string


    str1 = "peter piper picked a peck of pickled peppers"
    freq = {}
    freq2 = {}
    for c in str1:
        freq[c] = freq.get(c, 0) + 1
    freq2 = {c: freq2.get(c, 0) + 1 for c in str1}
    print(freq)
    print(freq2)

Output

{'p': 9, 'e': 8, 't': 1, 'r': 3, ' ': 7, 'i': 3, 'c': 3, 'k': 3, 'd': 2, 'a': 1, 'o': 1, 'f': 1, >'l': 1, 's': 1}

{'p': 1, 'e': 1, 't': 1, 'r': 1, ' ': 1, 'i': 1, 'c': 1, 'k': 1, 'd': 1, 'a': 1, 'o': 1, 'f': 1, >'l': 1, 's': 1}

I just wanted to know why dictionary comprehension is not giving me the right answer?

`freq2.get(c, 0)` is always `0`, the new `freq2` dict is only bound *after* the dict comphrneions is completed — Chris_Rands, Sep 08 '20 at 13:37
Just a silly but perhaps interesting other way that does work: `freq2.update((c, freq2.get(c, 0) + 1) for c in str1)` — superb rain, Sep 08 '20 at 13:40
btw: if you have a line like `freq2 = { ...`, then you don't need the previous `freq2 = {}`. — quamrana, Sep 08 '20 at 14:17

score 1 · Accepted Answer · answered Sep 08 '20 at 13:37

When looping using the dictionary comprehension, freq2 is yet to be updated and all values are non-existent so get returns 0 (and you add 1 so assigned value to keys is 1 to a new dictionary).

Only after the dictionary comprehension, freq2 is updated (with the dictionary created by the dictionary comprehension).

The steps are:

At line 5; freq2 is empty
At line 6; first, a new dictionary created by looping on freq2 empty dictionary. Then, that new dictionary is assigned to freq2

get not working with dictionary comprehension

1 Answers1