Count vowels from raw input

Question

I have a homework question which asks to read a string through raw input and count how many vowels are in the string. This is what I have so far but I have encountered a pro

Answer 1

for i in range(0, len(string)):
    if string[i] == vowels[i]:

This actually has a subtler problem than only counting each vowel once - it actually only tests if the first letter of the string is exactly a, if the second is exactly e and so on.. until you get past the fifth. It will try to test string[5] == vowels[5] - which gives an error.

You don't want to use i to look into vowels, you want a nested loop with a second index that will make sense for vowels - eg,

for i in range(len(string)):
   for j in range(len(vowels)):
       if string[i] == vowels[j]:
          count += 1

This can be simplified further by realising that, in Python, you very rarely want to iterate over the indexes into a sequence - the for loop knows how to iterate over everything that you can do string[0], string[1] and so on, giving:

for s in string:
   for v in vowels:
      if s == v:
        count += 1

The inner loop can be simplified using the in operation on lists - it does exactly the same thing as this code, but it keeps your code's logic at a higher level (what you want to do vs. how to do it):

for s in string:
   if s in vowels:
       count += 1

Now, it turns out that Python lets do math with booleans (which is what s in vowels gives you) and ints - True behaves as 1, False as 0, so True + True + False is 2. This leads to a one liner using a generator expression and sum:

sum(s in vowels for s in string)

Which reads as 'for every character in string, count how many are in vowels'.