How to count the average number of places between a letter in a string?

Question

I have strings like so:

F-F-F+F+F+F-F-F-F+F+F+F-F+F+F-F+F-F-F-F

And I would like to count the average number of places between instances of 'F', '+' and '-'.
So for this example it would be:

Average chars between Fs:   1
Average chars between +s:   2.25
Average chars between -s:   3

What would be the most efficient way of doing this?

Answer 1

this is a variant.

first i collect the index i of the occurrences of all char acters; then i calculate the mean of the differences:

from collections import defaultdict
from itertools import islice
from statistics import mean

strg = "F-F-F+F+F+F-F-F-F+F+F+F-F+F+F-F+F-F-F-F"

dct = defaultdict(list)

for i, char in enumerate(strg):
    dct[char].append(i)

for char, occurrences in dct.items():
    avg = mean(b - a for a, b in zip(occurrences, islice(occurrences, 1, None))) - 1
    print(f"Average chars between {char}s:  {avg}")

this prints:

Average chars between Fs:  1
Average chars between -s:  3
Average chars between +s:  2.25

after the first for loop there will be entries like this in dct :

'-': [1, 3, 11, 13, 15, 23, 29, 33, 35, 37]

and - as mentioned - the second for loop calculates the mean of the differences.

Answer 2

I would do that using regular expressions ( re module) following way:

import re
txt = "F-F-F+F+F+F-F-F-F+F+F+F-F+F+F-F+F-F-F-F"
chars = set(list(txt))
between = dict()
for i in chars:
    between[i] = re.findall('(?<='+re.escape(i)+').*?(?='+re.escape(i)+')',txt)
for i in chars:
    if len(between[i])==0:
        between[i] = 0.0
    else:
        between[i] = sum([len(i) for i in between[i]])/len(between[i])
print(between)

Output:

{'F': 1.0, '+': 2.25, '-': 3.0}

Explanation: I am looking for substrings between occurences given character (using zero length assertions) from left to right workng in non-greedy manner (hence "FFF" give ["-","-"] rather than ["-F-"] ) then simply calculate averages of their lengths. Note that I used re.escape to deal with characters of special meaning (like + ).

Answer 3

Another way without using any libraries:

string = 'F-F-F+F+F+F-F-F-F+F+F+F-F+F+F-F+F-F-F-F'
string = list(string)
chars = set(string)
for char in chars:
    ind = [i for i, x in enumerate(string) if x == char]
    diff = [ind[i+1]-ind[i] - 1 for i in range(len(ind)-1)]
    print(f'Average chars between {char}s:  {sum(diff) / len(diff)}')

Output:

Average chars between +s:  2.25
Average chars between Fs:  1.0
Average chars between -s:  3.0