How can I find the last time a character was used in a txt file

Question

I apologize if I didn't ask this question properly. It was kind of confusing. But for a visual example if I have this txt file:

| $100 on the first line
| $654 on the second line
| $123 on the third line
| $111 on the fourth line

How could I find the last time $ was used to print out 111?

How large does the text file get? Would it fit in memory? What have you tried yourself? Please share your code and the problems you had with it. — Grismar, Jan 15 '21 at 04:55

Akshay Sehgal · Accepted Answer · 2021-01-15T05:46:18.517

0

You can use str.rfind() to get the last instance of a string (reversed first find) and then use that to index the string.

import io

file = "| $100 on the first line\n| $654 on the second line\n| $123 on the third line\n| $111 on the fourth line"
txt = io.StringIO(file).getvalue()

idx = txt.rfind('$')

#txt[idx+1:idx+4]
txt[idx+1:].split()[0]  #takes the complete first token instead of 3 characters

'111'

edited Jan 15 '21 at 05:46

answered Jan 15 '21 at 05:10

Akshay Sehgal

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I have a question. What is the `idx+1` and `idx+4` part? – WaddyBox Jan 15 '21 at 05:14
index of the last `$` character is `78`. You want the part of the string that exists between characters `79 to 79+4` which is '111'. You index a string as `string[start:stop]`. (in python start is inclusive and stop is exclusive) – Akshay Sehgal Jan 15 '21 at 05:16
Do you want to print `111` or you just want the position of the last `$`? – Akshay Sehgal Jan 15 '21 at 05:19
I just want to print out whatever is after the last `$` whether it be `111` or `111.46` – WaddyBox Jan 15 '21 at 05:30
does that mean the complete sentence? `$111 on the fourth line` or just the token attached to `$`? – Akshay Sehgal Jan 15 '21 at 05:36
Updated my answer, try now. Now it simply takes the first attached word/number to `$` and returns it. – Akshay Sehgal Jan 15 '21 at 05:39
do let me know if that solved your question. thanks! – Akshay Sehgal Jan 15 '21 at 05:46

score 0 · Answer 2 · answered Jan 15 '21 at 07:35

0

Well, depending of your needs bash may be more efficient for this. from your shell command line:

$ grep "$" file.txt | cut -d "$" -f2 | cut -d " " -f1 | tail -1

Output:

answered Jan 15 '21 at 07:35

Synthase

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score -1 · Answer 3 · answered Jan 15 '21 at 04:59

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for small files see this: How to read a file in reverse order?

for large files such that you need to consider diskIO, maybe try to seek() to end of file and read back (in chunks)

answered Jan 15 '21 at 04:59

Bing Wang

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How can I find the last time a character was used in a txt file

3 Answers3