Develop Reference

How to count words that end with a letter? (python)

I am a beginner and this is what I came up with so far. However, it does not output the correct number of words that end with "a" or "b." Any tips on how to correct this code?
names = input("Enter list of names: ")
name = names.split(" ")
num = len(name)
ab = ""
print("Number of words:", num)
for i in range(num):
if name[i] == ' ':
if name[i-1] == a:
ab.append() + " "
elif name[i-1] == b:
ab.append() + " "
a_b = ab.split(' ')
print("Number of words that end with a or b: ",len(a_b))

In Python boolean (True and False) are the same as the integers 1 and 0. This means you can use sum() to count True booleans. str.endswith() returns a boolean. This means you can just do this:
words = ["stab", "drama", "hello", "magma", "test"]
a_b = sum(word.endswith(('a', 'b')) for word in words)
# 3
z_c = sum(word.endswith(('z', 'c')) for word in words)
# 0

Any tips on how to correct this code?
Others have answered alternative, working solutions, but I want to try to point out the specific things wrong in your code snippet and tips to correct.
First here's a copy of your snippet with some simple debugging prints along with their output (running in python 3.10).
names = "Barry Lima Bab"
name = names.split(" ")
print(f"{name=}") # name=['Barry', 'Lima', 'Bab']
num = len(name)
print(f"{num=}") # num=3
ab = ""
print("Number of words:", num)
for i in range(num):
print(name[i]) # Barry -> Lima -> Bab
if name[i] == ' ':
if name[i-1] == a:
ab.append() + " "
elif name[i-1] == b:
ab.append() + " "
print(f"{ab=}") # ab=''
a_b = ab.split(' ')
print(f"{a_b=}") # a_b=['']
Breaking things down step by step like this is a great starting point for better understanding what's going on.
if name[i] == ' ': Notice the value of name[i] so this check never resolves to True meaning the code inside never runs
if the inner code did run, you'd hit a NameError: name 'a' is not defined. Did you mean: 'ab'? because a is undefined. Probably meant 'a' here. same for b
In my example, name[i-1] would be 'Bab' -> 'Barry' -> 'Lima' which is probably not what you're expecting. This is because you're getting the -1th, 0th, 1st items in name. to get the last letter from the name, you want name[i][-1] here
if you then get into either of the furthest inside conditions, you'd encounter AttributeError: 'str' object has no attribute 'append' which happens because append is for list not str. You couldchange ab to be a list to use this in which case you'd want ab.append(name[i]) or you can use string concatenation like ab += " " + name[i] or using str.concat
1 last note of interest, you may have noticed your code as-is really likes to return and say that there's 1 item. This is because the above issues always (if the program doesn't break) leaves ab == '' and ''.split(' ') => [''] and thus len(ab.split(" ")) => 1
1 tip that I think would help in code comprehension is that the name variable here is not a single name string like it implies. It's actually a list[str]. I'd probably denote the variables something more like names_str: str vs names: list[str] or raw_names vs names. Then just use something like for name in names: and not worry about indexes. You can also use name.endswith('a') instead of name[-1] == 'a' for better readability.
Eventually you can combine these into a list comprehension for maximal succinctness -> len([name for name in names if name.endswith('a') or name.endswith('b')]).

words = ["ab", "bab", "pa", "pap"]
result = 0
for word in words:
if word[-1] in "ab":
result += 1
print(result)

As a list comprehension:
words = ["word1a", "word2b", "word3", "word4", "word5a"] # ['word1a', 'word2b', 'word5a']
filtered_words = [word for word in words if word[-1] in "ab"]
filtered_words = [word for word in words if word.endswith(("a", "b"))] # better, if you have multiple endings you want to search for with different lengths
len(filtered_words) # 3

name[i-1] is the previous name in the list, not the last character of the current name in the loop.
There's no need to append the matching names to a string. If you just need the count of matches, increment a counter variable.
You need to put a and b in quotes to make them strings, not variables.
names = input("Enter list of names: ")
name = names.split(" ")
matches = 0
print("Number of words:", len(name))
for cur_name in name:
if cur_name.endswith('a') or cur_name.endswith('b'):
matches += 1
print("Number of words that end with a or b: ", matches)

Use the endswith string method to detect if the last letter is "a" or "b"
names = ["bob","boba","larry","curly","moe"]
count = 0
for name in names:
if name.endswith("a") or name.endswith("b"):
count += 1
print(f"{count=}") # count=2

Str to int function

I am supposed to write a function that lets the user put in any string of numbers and then turns that input into an int list (e.g "12635 1657 132651627"). However, I don't know how to change this bit of code so that the user can actually put something into the console. When I try to introduce a new variable Python says that there is no value assigned to it, and I do not know how to work around that. I want everything to be within the function. I managed to introduce a new variable before the start of the function but that's not really my aim here.
def Int_Split(a):
List = []
last = 0
max = len(a)
for i in range (max):
if a[i] =="":
nmbr = a[last:i]
last = i+1
List.append(int(nbmr))
nmbr = a[last:]
List.append(int(nmbr))
return List
print(List)

It isn't clear what your issue is with adding a variable, or entirely what you're after. But if you are after converting "12635 1657 132651627" to [12635, 1657, 132651627], that can be done very simply with:
s = "12635 1657 132651627"
l = [int(x) for x in s.split()]
print(l)
Which yields:
[12635, 1657, 132651627]
Here is an example in a function:
def main():
print("input some numbers:")
s = input()
print([int(x) for x in s.split()])
if __name__ == "__main__":
main()
Here we print the request for input, set the value given to s, then use a list comprehension to say: split a string on any whitespace, then give the integer value for each.
Here is a method without using string.split():
def sep(s):
words = []
word = ""
for c in s:
if c != " ":
word += c
continue
words.append(word)
word = ""
else:
words.append(word)
return words
def main():
print("input some numbers:")
s = input()
print([int(x) for x in sep(s)])
if __name__ == "__main__":
main()
Here we have written a function called sep, which just iterates over characters in the given string until it finds a space or the end, each time adding that group of characters to a list of strings.

Receiving exit code (1) when trying to manipulate strings from codewars assignment? How can I do it differently and avoid Index Errors?

I have been working on this codewars assignment the entire day, but I keep failing the test on code wars. Running into index errors and exit code (1) even though I am passing every test and attempt, the exit code seems so illusive yet confusing. I understand if my program doesn't do what it is supposed to but it is passing the tests, and right now there are some errors I don't quite understand messing up my program in ways that I can't even comprehend. Because they are returning the strings exactly as requested by the assignment, so why... is it not working? What have I done wrong? Please help me almighty code wizards, I am a noob...
Working on Split Strings: Complete the solution so that it splits the string into pairs of two characters. If the string contains an odd number of characters then it should replace the missing second character of the final pair with an underscore ('_').
Here is my Code:
def solution(string):
join_string = "-".join(string[i:i+2] for i in range(0, len(string), 2))
x_string = join_string.split("-")
underscore = "_"
index = (len(string) - len(x_string))
if index % 2 == 0:
for i in range(0, index):
return x_string
break
else:
x_string[index] = x_string[index] + underscore
return x_string
elif index > 2:
x_string[index] = x_string[index] + underscore
return x_string
elif index < 1:
x_string = []
return x_string
The error comes up at
x_string[index] = x_string[index] + underscore
IndexError: list index out of range

This line
join_string = "-".join(string[i:i+2] for i in range(0, len(string), 2))
already does the entire job you want, except for the underscore at the end if there were an odd number of characters in the input. Your code is making very heavy weather of this special case. Check if the input string has an odd number of characters:
if len(string) % 2 == 1:
and if it has, append an underscore to the returned value:
join_string += "_"
Your question doesn't say what the expected output is, and it's hard to tell from the code, so, to keep to the essential point, the whole function becomes
def solution(string):
join_string = "-".join(string[i:i+2] for i in range(0, len(string), 2))
if len(string) % 2 == 1:
join_string += "_"
return join_string
I get the output
>>> solution("123465")
'12-34-65'
>>> solution("1234657")
'12-34-65-7_'
>>> solution("1")
'1_'
>>> solution("")
''
If instead you want, say, a list of 2-strings, then do
return join_string.split("-")

How To Fix String Index Out of Range in Python

I'm currently learning python. There is an exercise in the book which outlines code for decoding. I followed along with this code however I keep getting back an error in the middle of the program.
Here is the code that is causing the problem:
def decode(string_for_decoding):
result = ""
for i in range(0, len(string_for_decoding)):
if string_for_decoding[i+1].isdigit():
result += string_for_decoding[i] * int(string_for_decoding[i+1])
elif string_for_decoding[i].isalpha():
result += string_for_decoding[i]
return result
string_for_decoding = input("Enter a string to decode: ")
print(decode(string_for_decoding))

Check if the index from range is larger than the number of chars in the string. It might look like this:
def decode(string_for_decoding: str):
result = ""
for i in range(0, len(string_for_decoding)):
if len(string_for_decoding) > i + 1 and string_for_decoding[i + 1].isdigit():
result += string_for_decoding[i] * int(string_for_decoding[i + 1])
elif string_for_decoding.isalpha():
result += string_for_decoding[i]
return result
print(decode(input("Enter a string to decode: ")))

You are going from 0 to len(string) and inside for loop you are trying to access index: i+1
THis is the root cause! Either:
iterate till one length less e.g. till len(string) - 1
Or use indices
inside appropriately
Moreover it is highly discouraged to use range(len(x)). You can simply iterate over any collective datatype as for i in x: If you want indices too have a look at enumerate(). And you can forget about index errors all together.

You are trying to get index i+1 which cannot be equal or greater than len(string).

What did I do wrong with this function?

I don't know what I did - it's wrong .
Can someone help me?
def insert_sequence(dna1, dna2, number):
'''(str, str, int) -> str
Return the DNA sequence obtained by inserting the second DNA sequence
at the given index. (You can assume that the index is valid.)
>>> insert_sequence('CCGG', 'AT', 2)
'CCATGG'
>>> insert_sequence('TTGC', 'GG', 2)
'TTGGGC'
'''
index = 0
result = '';
for string in dna1:
if index == number:
result = result + dna2
result = result + string
index += 1
print(result)

Here's a solution:
def insert_sequence(dna1, dna2, number):
'''(str, str, int) -> str
Return the DNA sequence obtained by inserting the second DNA sequence
at the given index. (You can assume that the index is valid.)
>>> insert_sequence('CCGG', 'AT', 2)
'CCATGG'
>>> insert_sequence('TTGC', 'GG', 2)
'TTGGGC'
'''
return dna1[:number] + dna2 + dna1[number:]

you needed an if-else loop here :
def insert_sequence(dna1, dna2, number):
result = '';
#you can use enumerate() to keep track of index you're on
for ind,x in enumerate(dna1):
if ind == number: #if index is equal to number then do this
result = result + dna2 +x
else: #otherwise do this
result = result + x
print(result)
insert_sequence('CCGG', 'AT', 2)
insert_sequence('TTGC', 'GG', 2)
output:
CCATGG
TTGGGC

There are already right working functions in other answers (specially the comment from Rakesh Pandit and the answer from JeffS), but your actual question is "why my original function doesn't work".
I copied a working version of your function, comments below:
def insert_sequence(dna1, dna2, number):
index = 0
result = ''
for character in dna1:
if index == number:
result = result + dna2
result = result + character
index += 1
print(result)
Python considers indentation, so you should print only at the end of things, outside loops and ifs.
When you "increase" your result, you do this only inside the "if" on your function, when actually you should increase "for every character in dna1", and only when/"if index == number" you should put the middle string inside.
I believe you are very new to Python or to programming in general, being probably from a biological background, but you really shouldn't iterate to get this type of string operation done, as others have shown.
Hope this helps!

You're never splitting the string apart, so you'll always prepend dna2 to dna1.
You probably want to return dna1[:number] + dna2 + dna1[number:]

You do nothing if the index is not at the insertion point, including incrementing the index. Your code needs an else and you are also printing prematurely:
def insert_sequence(dna1, dna2, number):
index = 0
result = '';
for char in dna1:
if index == number:
result = result + dna2
result = result + char
index += len(dna2) + 1
else:
result = result + char
index += 1
print(result)

mistakes made: a) parameter index is initialised to 0. b) "for sting in dia1:" should have been "for dia1_position in range(len(dia1)):" c) print result indentation is wrong and function isn't just supposed to print. It should return result. d) index need not be incremented now.
Answers are already there. Above briefly lists the mistakes made. I guess you didn't see any error because you never called the function. First error should be "number" not defined (not any more as question has been updated and parameter has number defined).