tame dagger
I have the following recursion ... but i need help to return " * " if the first char of both lists are the same, and ' . ' if they are not
def compare_str(str1,str2):
if str1 == "":
return 0
print(str1,str2)
return compare_str(str1[1:],str2[1:])
print(compare_str("AABBBBAAB","AABBBBAAB"))
" ********* "
daring grailBOT
I have the following recursion ... but i need help to return " * " if the first...
@tame dagger
Python help channel opened
cobalt steppe
iterate over the strings and do a match for match for each?
tame dagger
this time round i am doing it recursively
both strings get smaller by [-1] each call
cobalt steppe
so you only want the very 1st char to match?
tame dagger
and i want to compare the first char of both strings, if they match or don't return a string value
so comparing 1st char each time and shortening the string each time by [1:]
i have trouble creating the leaf case in this instnace
cobalt steppe
your basically saying what you want.. just need to construct the if statements to say it
autumn breach
You know how to check the first index of a string or no
tame dagger
this is how i did it
if str1[0] == str2[0]:
return '*'
if str1 == "":
return 0
if str1[0] == str2[0]:
return '*'
if str1[0] != str2[0]:
print(str1[0])
return '.'
return compare_str(str1[1:],str2[1:])```Well you only want to return * or . When you’ve checked the entire string right?
tame dagger
i want a ******** or as many * or . for every match or no-match
autumn breach
Oh I see
cobalt steppe
1st if str == ' ' .. acutally means.. if str == None .. so easier to put as, if not str1
and your 2nd ifs.. should be forloops done as if and elif
tame dagger
i think i need to store the result somewhere and return it all in the end
autumn breach
An empty string is False not None
cobalt steppe
meh false, none.. none is false.. but for clarity yes
tame dagger
let say im given
str2: 'ACTCATGTAA'
output: '*.****.**.'
So you want to start with an empty string
And add either * or . To it every time you call the function
tame dagger
yes sir
autumn breach
What’s your current code?
tame dagger
the question is where do i do a res = "", and how do it return res + " * "
if not str1:
return 0
elif str1[0] == str2[0]:
return '*'
elif str1[0] != str2[0]:
print(str1[0])
return '.'
return compare_str(str1[1:],str2[1:])
print(compare_str("AABBBBAAB","AABBBBAAB"))```yes - my output was only 1 *
tough pike
the question is where do i do a res = "", and how do it return res + " * "
dont introduce an additional variable, just define it as an expression
precondition: length of strings matches
compare("", _) = ""
# (x:xs) as in, first character is x, rest are xs
compare((x:xs), (y:ys)) = ("*" if x==y else ".") + compare(xs, ys)
I think you want those to be return “*” + compare_str(str1[1:0], str2[1:0]
cobalt steppe
dont introduce an additional variable, just define it as an expression
```py
pre...
as your name suggests, i bet you frieghtend many new comers :/
tame dagger
dont introduce an additional variable, just define it as an expression
```py
pre...
too advanced for me... how do it in recursion 😦
tough pike
this is recursion, in the case where strings are not empty, compare compares the first characters of them, and adds that with the result of comparing the rest of the string
i just wrote it in uhh, something inbetween python and haskell
tame dagger
if not str1:
return 0
return ("*" if str1[0]==str2[0] else ".") + compare_str(str1[1:],str2[1:])```i think it doesnt work because i concat a str with function..
int to str because it returns 0
tough pike
no, it is because in the end, you add a string with 0, like, "." + 0
return an empty string in the base case
autumn breach
I think you want those to be return “*” + compare_str(str1[1:0], str2[1:0]
Did you see this
Oh you did that nevermind
Instead of zero you just want an empty string
tame dagger
return ''
ahhhhhhhhh
if not str1:
return ''
return ("*" if str1[0]==str2[0] else ".") + compare_str(str1[1:],str2[1:])
print(compare_str("AABBBBAAB","AABBBBAAB"))```("*" if str1[0]==str2[0] else ".") this part is really helpful guys
over here i am assuming both strings are equal length, hence if not str1 or 2 is fine
tough pike
!e
def f(xs: str, ys: str) -> str:
n = len(xs)
m = len(ys)
if n != m:
raise ValueError
return g(n, bytearray(b'.'*n), xs, ys, 0).decode()
def g(n: int, b: bytearray, xs: str, ys: str, i: int) -> bytearray:
if i >= n:
return b
if xs[i] == ys[i]:
b[i] = 42
return g(n, b, xs, ys, i+1)
xs = ("A"*500)+("B"*500)
ys = ("A"*250)+("B"*250)+("A"*250)+("B"*250)
print(f(xs, ys))
:x: Your 3.12 eval job has completed with return code 1.
001 | Traceback (most recent call last):
002 | File "/home/main.py", line 17, in <module>
003 | print(f(xs, ys))
004 | ^^^^^^^^^
005 | File "/home/main.py", line 6, in f
006 | return g(n, bytearray(b'.'*n), xs, ys, 0).decode()
007 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
008 | File "/home/main.py", line 13, in g
009 | return g(n, b, xs, ys, i+1)
010 | ^^^^^^^^^^^^^^^^^^^^
... (truncated - too many lines)
Full output: https://paste.pythondiscord.com/XYL63YDVNZ4I7Q76XMYASNDCIQ
tough pike
oops it has too small of a stack size
!e
import sys
sys.setrecursionlimit(1100)
def f(xs: str, ys: str) -> str:
n = len(xs)
m = len(ys)
if n != m:
raise ValueError
return g(n, bytearray(b'.'*n), xs, ys, 0).decode()
def g(n: int, b: bytearray, xs: str, ys: str, i: int) -> bytearray:
if i >= n:
return b
if xs[i] == ys[i]:
b[i] = 42
return g(n, b, xs, ys, i+1)
xs = ("A"*500)+("B"*500)
ys = ("A"*250)+("B"*250)+("A"*250)+("B"*250)
print(f(xs, ys))
:white_check_mark: Your 3.12 eval job has completed with return code 0.
**********************************************************************************************************************************************************************************************************************************************************....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................**********************************************************************************************************************************************************************************************************************************************************
... (truncated - too long)
Full output: https://paste.pythondiscord.com/PCLOBWMQVJ4CTD4YWBXVTNM654
tough pike
if only python had tail call optimization
tame dagger
!e ```def compare_str(str1,str2):
if not str1:
return ''
return ("*" if str1[0]==str2[0] else ".") + compare_str(str1[1:],str2[1:])
print(compare_str("AABBBBAAB","AABBBBAAB"))```
daring grailBOT
!e ```def compare_str(str1,str2):
if not str1:
return ''
return ...
:white_check_mark: Your 3.12 eval job has completed with return code 0.
*********
i dont know what that means but let me google it
tough pike
basically, it is an optimization, that lets avoid allocating a new stack frame when you have a recursive function with a return that is like return function(but, with, new, arguments) - you can kinda just replace the arguments in the current frame, and start from the body again
python doesnt have it, and as you can see, trying to use our recursive function on a string the length of which was greater than the stack size resulted in an error
it is a pretty essential thing for tail recursive functions that end up reaching a lot of depth
tame dagger
thank you !
daring grailBOT
Python help channel closed