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I had a text that I separated into individual sounds by means of graphemes. These graphemes are now part of a list, as can be seen below:

graphemes = ["th", "e", "g", "i", "r", "l", "th", "a", "t", "r", "ea", "d", "s", ...]

In addition to that, I also have a dictionary which links some of those graphemes to a number:

graph_nums = "th":1, "s":2, "t":3, ...

Finally, I have a set of conditions. For example, "if -s comes after a vowel" or "if -t comes before a consonant".

What I want to do is iterate over the list of graphemes and, if one of the conditions is met, then replace the grapheme with its corresponding number.

This is what I tried to do so far:

special_graphemes = ["s", "t"...] #a list with the characters that are mentioned in the conditions
vowels = ["a", "e", "i", "o", "u", ...] #a list with all the vowels and dipthongs 
consonants = ["b", "c", "d", ...] #a list of all consonants and groups of consonants
output = ""

for grapheme in graphemes: #iterate over each grapheme
 if grapheme in special_graphemes: #if the grapheme is one of the graphemes that needs to be replaced by a number
 if graphemes[grapheme-1] in vowels: #for a condition like "if -s comes after a vowel", it needs to be checked whether the previous grapheme is a vowel
 output += graph_nums.get(num) #if the previous condition applies, then replace the grapheme by its number, according to the dictionary
 elif XXXX #other conditions checked in a similar way
 else:
 output += grapheme #otherwise, just keep the grapheme as it is
print(output)

However, when I run this, I get an error concerning the indexes (i.e., this is wrong: graphemes[grapheme-1]). How could I then access the positions I'm interested in and replace them when necessary?

Also, I'm not sure either whether the way I'm accessing the dictionary and replacing the graphemes is correct either.

A loop like for e in a means that every e is an element of a, if you want the indexes you'll have to use the builtin enumerate. Your code will then become:

for i, grapheme in enumerate (graphemes): # Here i is the index and grapheme is the element at graphemes[i]
 if grapheme in special_graphemes:
 if i > 0 and graphemes[i-1] in vowels: # Now you can use i-1 to check the previous element, watch out for when i == 0
 output += graph_nums.get(num) # I assume this (and the rest) functions as requires
 elif XXXX
 else:
 output += grapheme

Using Python list comprehension

Try this :

graphemes = ["th", "e", "g", "i", "r", "l", "th", "a", "t", "r", "ea", "d", "s"]
graph_nums = "th":1, "s":2, "t":3
out_graphemes = [ x for x in (map(graph_nums.get, graphemes, graphemes)) ]
print (out_graphemes)

Output

[1, 'e', 'g', 'i', 'r', 'l', 1, 'a', 3, 'r', 'ea', 'd', 2]