Using the .find() function in a loop [closed]

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I was trying to get the last word of the sentence using the .find() method in a loop. My code:

sentence = "please write a program which keeps asking the user for words"
index = 0
substring = sentence.find(" ")
while index < len(sentence):
    substring = sentence.find(" ")
    sentence = sentence[substring+1:]
    index += 1
print(sentence)

This code works when the sentence is:

sentence = "it was a dark and stormy python"

but it doesn't work when the sentence is:

sentence = "please write a program which keeps asking the user for words"

I want to know why that is and what will be the correct way to do this without using .split().

I expect the code to output the last word in the sentence.

You are comparing the number of words (index) to the number of characters in the string (sentence). — Guy
– Guy, Commented Jul 14 at 10:20
What is a debugger and how can it help me diagnose problems? How to debug small programs: StackOverflow is a question-and-answer site for specific questions about actual code; “I wrote some buggy code that I can’t fix” is not a question, it’s a story, and not even an interesting story. — user16540390
– user16540390, Commented Jul 14 at 11:04
if you want to find last word then you could use one .rfind(" ") to find last " " — furas
– furas, Commented Jul 14 at 11:49
Maybe first use print() (and print(type(...)), print(len(...)), etc.) to see which part of code is executed and what you really have in variables. It is called "print debugging" and it helps to see what code is really doing. — furas
– furas, Commented Jul 14 at 11:58

furas · Accepted Answer · 2025-07-14 14:49:09Z

If you want to find last word then you could use .rfind() which searchs from right to left.

It doesn't need to use loop for this. And it keeps original sentence.

sentence = "please write a program which keeps asking the user for words"
pos = sentence.rfind(" ")     # 54
last_word = sentence[pos+1:]  # [55:]

In original code problem is that you change sentence (it is shorter and shorter) but in every loop it calculates again len(sentence) and it has different value but index is bigger and bigger.

If you would add print() then you would see it.

print(f"{index} | {len(sentence)} | {sentence:60} | {substring} | {sentence[substring+1:]}")

# index | len | sentence | substring | new sentence

0 | 60 | please write a program which keeps asking the user for words | 6 | write a program which keeps asking the user for words
1 | 53 | write a program which keeps asking the user for words        | 5 | a program which keeps asking the user for words
2 | 47 | a program which keeps asking the user for words              | 1 | program which keeps asking the user for words
3 | 45 | program which keeps asking the user for words                | 7 | which keeps asking the user for words
4 | 37 | which keeps asking the user for words                        | 5 | keeps asking the user for words
5 | 31 | keeps asking the user for words                              | 5 | asking the user for words
6 | 25 | asking the user for words                                    | 6 | the user for words
7 | 18 | the user for words                                           | 3 | user for words
8 | 14 | user for words                                               | 4 | for words
sentence = 'for words'

Full code for previous result:

sentence = "please write a program which keeps asking the user for words"
index = 0

substring = sentence.find(" ")

#length = len(sentence)
#while index < length:
while index < len(sentence):
    substring = sentence.find(" ")
    print(f"{index} | {len(sentence)} | {sentence:60} | {substring} | {sentence[substring+1:]}")
    sentence = sentence[substring+1:]
    index += 1

print(f"{sentence = }")

You should get len(sentence) before loop and assign to variable.

But it may have other problem with value.
It repeates the same code after it already found last word.

# index | len | sentence | substring | new sentence

0 | 60 | please write a program which keeps asking the user for words | 6 | write a program which keeps asking the user for words
1 | 53 | write a program which keeps asking the user for words        | 5 | a program which keeps asking the user for words
2 | 47 | a program which keeps asking the user for words              | 1 | program which keeps asking the user for words
3 | 45 | program which keeps asking the user for words                | 7 | which keeps asking the user for words
4 | 37 | which keeps asking the user for words                        | 5 | keeps asking the user for words
5 | 31 | keeps asking the user for words                              | 5 | asking the user for words
6 | 25 | asking the user for words                                    | 6 | the user for words
7 | 18 | the user for words                                           | 3 | user for words
8 | 14 | user for words                                               | 4 | for words
9 | 9 | for words                                                    | 3 | words
10 | 5 | words                                                        | -1 | words
11 | 5 | words                                                        | -1 | words
12 | 5 | words                                                        | -1 | words
13 | 5 | words                                                        | -1 | words
14 | 5 | words                                                        | -1 | words
15 | 5 | words                                                        | -1 | words
16 | 5 | words                                                        | -1 | words
17 | 5 | words                                                        | -1 | words
18 | 5 | words                                                        | -1 | words
.
.
.
57 | 5 | words                                                        | -1 | words
58 | 5 | words                                                        | -1 | words
59 | 5 | words                                                        | -1 | words
sentence = 'words'

Code:

sentence = "please write a program which keeps asking the user for words"
index = 0

substring = sentence.find(" ")

length = len(sentence)
while index < length:
#while index < len(sentence):
    substring = sentence.find(" ")
    print(f"{index} | {len(sentence)} | {sentence:60} | {substring} | {sentence[substring+1:]}")
    sentence = sentence[substring+1:]
    index += 1

print(f"{sentence = }")

I would check if find() returns -1 which means it didn't find space in sentence

sentence = "please write a program which keeps asking the user for words"

while True:
    substring = sentence.find(" ")
    if substring < 0:
        break
    print(f" - | {len(sentence)} | {sentence:60} | {substring} | {sentence[substring+1:]}")
    sentence = sentence[substring+1:]

print(f"{sentence = }")

Result:

# no index | len | sentence | substring | new sentence

 - | 60 | please write a program which keeps asking the user for words | 6 | write a program which keeps asking the user for words
 - | 53 | write a program which keeps asking the user for words        | 5 | a program which keeps asking the user for words
 - | 47 | a program which keeps asking the user for words              | 1 | program which keeps asking the user for words
 - | 45 | program which keeps asking the user for words                | 7 | which keeps asking the user for words
 - | 37 | which keeps asking the user for words                        | 5 | keeps asking the user for words
 - | 31 | keeps asking the user for words                              | 5 | asking the user for words
 - | 25 | asking the user for words                                    | 6 | the user for words
 - | 18 | the user for words                                           | 3 | user for words
 - | 14 | user for words                                               | 4 | for words
 - |  9 | for words                                                    | 3 | words
sentence = 'words'

The same without changing sentence but using find(..., start_position)


sentence = "please write a program which keeps asking the user for words"

start = 0
while True:
    pos = sentence.find(" ", start)
    if pos < 0:
        last_word = sentence[start:]
        break
    start = pos+1
    print(f" - | {len(sentence)} | {sentence:60} | {start} | {sentence[start:]}")

print(f"{last_word = }")

Result:

# no index | len | sentence | start | sentence[start:]

 - | 60 | please write a program which keeps asking the user for words | 7 | write a program which keeps asking the user for words
 - | 60 | please write a program which keeps asking the user for words | 13 | a program which keeps asking the user for words
 - | 60 | please write a program which keeps asking the user for words | 15 | program which keeps asking the user for words
 - | 60 | please write a program which keeps asking the user for words | 23 | which keeps asking the user for words
 - | 60 | please write a program which keeps asking the user for words | 29 | keeps asking the user for words
 - | 60 | please write a program which keeps asking the user for words | 35 | asking the user for words
 - | 60 | please write a program which keeps asking the user for words | 42 | the user for words
 - | 60 | please write a program which keeps asking the user for words | 46 | user for words
 - | 60 | please write a program which keeps asking the user for words | 51 | for words
 - | 60 | please write a program which keeps asking the user for words | 55 | words
last_word = 'words'

All versions in one file:

(so everyone can simply copy and test it)

print('\n--- variable `length` ---\n')

sentence = "please write a program which keeps asking the user for words"
index = 0

substring = sentence.find(" ")

length = len(sentence)
while index < length:
#while index < len(sentence):
    substring = sentence.find(" ")
    print(f"{index:2} | {len(sentence):2} | {sentence:60} | {substring:2} | {sentence[substring+1:]}")
    sentence = sentence[substring+1:]
    index += 1

print()
print(f"{sentence = }")

print('\n--- rfind() ---\n')

sentence = "please write a program which keeps asking the user for words"
pos = sentence.rfind(" ")
last_word = sentence[pos+1:]

print(f"{last_word = }")
print(f"{sentence = }")

print('\n--- find() < 0 ---\n')

sentence = "please write a program which keeps asking the user for words"

while True:
    substring = sentence.find(" ")
    if substring < 0:
        break
    print(f" - | {len(sentence):2} | {sentence:60} | {substring:2} | {sentence[substring+1:]}")
    sentence = sentence[substring+1:]

print()
print(f"{sentence = }")

print('\n--- find(..., start) < 0 ---\n')

sentence = "please write a program which keeps asking the user for words"

start = 0
while True:
    pos = sentence.find(" ", start)
    if pos < 0:
        last_word = sentence[start:]
        break
    start = pos+1
    print(f" - | {len(sentence):2} | {sentence:60} | {start:2} | {sentence[start:]}")

print()
print(f"{last_word = }")
print(f"{sentence = }")

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