def countchar() to find frequency of letters in one sentence [closed] - python-2.7

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def countchar(str):
list1 = [0]*26
for i in range(0,len(str)):
if (str[i] >= 'a' and str[i] <='z'):
***list1[ord(str[i])-0] += 1***
print list1
if __name__ == "__main__":
str = " GOOD morning and have a nice day"
str = str.lower()
print countchar(str)
there is an error in my code so I can achieve my goal.

Answer 1

Your main issue was that you needed to be subtracting ord("a") (which is 97) from each character to find its index in list1, not 0.
But I cleaned up the rest of your code too, since there were a lot of other inefficiencies and bad practices and stuff.
def countchar(sentence):
list1 = [0 for i in range(26)]
for c in sentence:
if 'a' <= c <= 'z':
list1[ord(c) - ord("a")] += 1
return list1
if __name__ == "__main__":
string = " GOOD morning and have a nice day"
string = string.lower()
print countchar(string)
In particular, it's bad practice to use keywords like str as variable names.
Also, depending on what you're planning to do with this, it's likely that a dictionary would work better for your purposes than a list.
Here's a quick rewrite (with the additional functionality that it'll count all characters, not just lowercase letters) using a dictionary:
def countchar(sentence):
char_dict = {}
for c in sentence:
if c in char_dict:
char_dict[c] += 1
else:
char_dict[c] = 1
return char_dict
if __name__ == "__main__":
string = " GOOD morning and have a nice day!!"
print "With uppercase (and punctuation, etc):"
print countchar(string)
print "All lowercase (and punctuation, etc):"
string = string.lower()
print countchar(string)
As requested in a comment, here is some clarification of the following line:
list1[ord(c) - ord("a")] += 1
First let's look at the inside, ord(c) - ord("a"). c is a string with only a single character in it, so ord(c) gives you the ASCII value for that string. Since you're mapping lowercase letters to the numbers 0, 1, ..., 25, we need to make sure that the letter "a" gets mapped to 0. Since, in ASCII, the letters are sequential (a=97, b=98, ..., z=122), then we can just subtract the smallest one from each one in order to map them:
a --> 97-97 = 0
b --> 98-97 = 1
c --> 99-97 = 2
...
z --> 122-97 = 25
So that's what ord(c) - ord("a") is doing. It's subtracting that 97 (which is the value of ord("a")) from each value, and giving a number between 0 and 25.
Then list1[ that number between 0 and 25 ] += 1 just increments the proper index in list1.