I am trying to find a character in an alphabetized string... Here is the code
def isIn(char, aStr):
middleChar = len(aStr)//2
if char == aStr[middleChar]:
return True
elif char < aStr[middleChar]:
LowerHalf = aStr[:middleChar]
return isIn(char, LowerHalf)
elif char > aStr[middleChar]:
UpperHalf = aStr[middleChar:]
return isIn(char, UpperHalf)
else:
return False
print(isIn('a', 'abc'))
It returns True. But When I put
print(isIn('d', 'abc'))
it returns this error: maximum recursion depth exceeded in comparison; instead of False.
I don't understand whats wrong. Please tell me where is the logical mistake I am doing.
With d, The program splits the string from abc and picks out UpperHalf bc. Then it searches the new string bc. It then returns 'c' from 'bc' as expected. Since d > c, the program goes chooses that condition and once again returns the upper half of string 'c', which is c. Hence the recursion. To fix this, you need a separate way of handling length 1 strings.
The last else is useless - it will never be executed.
The end of binary search is when the array becomes of one item - if this item isn't the searched one, the searched item isn't in the array.
Related
In this problem, I take two strings from the user, the first string being s and the second string being t. If t is the reverse of s, I print "YES" else I print "NO".
Here is my code which gives me expected outputs:
s = input()
t = input()
if t == s[::-1]:
print("YES")
else:
print("NO")
But I found another approach that I am curious to understand, but the slicing part is making me confused. Here the code goes:
print("YNEOS"[input()!=input()[::-1]::2])
Trying to find a good explanation, so StackOverflow is what came to my mind before anything else.
Let's first extract the parts of that expression that concern the input/output and the string reversal. We then get this solution:
s = input()
t = input()
trev = t[::-1]
result = "YNEOS"[s != trev::2]
print(result)
The focus of the question is on the expression "YNEOS"[s != trev::2]
Now we get to the "trick" that is performed here. The expression s != trev can be either False or True. This boolean value becomes the first part in the slicing. You'd expect to have the start index of the slice at this position. But the boolean value will also work, as booleans are a subclass of integers, and False is 0 and True is 1. So the above expression evaluates to either:
"YNEOS"[0::2]
or
"YNEOS"[1::2]
The 2 serves as the step, and so "YNEOS"[0::2] will take the characters at indices 0, 2 and 4 ("YES"), while "YNEOS"[1::2] takes the characters at indices 1 and 3 ("NO").
I hope this clarifies it.
I'm writing a program to check if a given user input is a palindrome or not. if it is the program should print "Yes", if not "no". I realize that this program is entirely too complex since I actually only needed to check the whole word using the reversed() function, but I ended up making it quite complex by splitting the word into two lists and then checking the lists against each other.
Despite that, I'm not clear why the last conditional isn't returning the expected "Yes" when I pass it "racecar" as an input. When I print the lists in line 23 and 24, I get two lists that are identical, but then when I compare them in the conditional, I always get "No" meaning they are not equal to each other. can anyone explain why this is? I've tried to convert the lists to strings but no luck.
def odd_or_even(a): # function for determining if odd or even
if len(a) % 2 == 0:
return True
else:
return False
the_string = input("How about a word?\n")
x = int(len(the_string))
odd_or_even(the_string) # find out if the word has an odd or an even number of characters
if odd_or_even(the_string) == True: # if even
for i in range(x):
first_half = the_string[0:int((x/2))] #create a list with part 1
second_half = the_string[(x-(int((x/2)))):x] #create a list with part 2
else: #if odd
for i in range(x):
first_half = the_string[:(int((x-1)/2))] #create a list with part 1 without the middle index
second_half = the_string[int(int(x-1)/2)+1:] #create a list with part 2 without the middle index
print(list(reversed(second_half)))
print(list(first_half))
if first_half == reversed(second_half): ##### NOT WORKING BUT DONT KNOW WHY #####
print("Yes")
else:
print("No")
Despite your comments first_half and second_half are substrings of your input, not lists. When you print them out, you're converting them to lists, but in the comparison, you do not convert first_half or reversed(second_half). Thus you are comparing a string to an iterator (returned by reversed), which will always be false.
So a basic fix is to do the conversion for the if, just like you did when printing the lists out:
if list(first_half) == list(reversed(second_half)):
A better fix might be to compare as strings, by making one of the slices use a step of -1, so you don't need to use reversed. Try second_half = the_string[-1:x//2:-1] (or similar, you probably need to tweak either the even or odd case by one). Or you could use the "alien smiley" slice to reverse the string after you slice it out of the input: second_half = second_half[::-1].
There are a few other oddities in your code, like your for i in range(x) loop that overwrites all of its results except the last one. Just use x - 1 in the slicing code and you don't need that loop at all. You're also calling int a lot more often than you need to (if you used // instead of /, you could get rid of literally all of the int calls).
I want to reverse the string using the Loop & Function. But when I use the following code, it is output the exact same string again. But it suppose to reverse the string. I can't figure out why.
def reversed_word(word):
x=''
for i in range(len(word)):
x+=word[i-len(word)]
print(i-len(word))
return x
a=reversed_word('APPLE')
print(a)
If you look at the output of your debug statement (the print in the function), you'll see you're using the indexes -5 through -1.
Since negative indexes specify the distance from the end of the string, -5 is the A, -4 is the first P, and so on. And, since you're appending these in turn to an originally empty string, you're just adding the letters in the same order they appear in the original.
To add them in the other order, you can simply use len(word) - i - 1 as the index, giving the sequence (len-1) .. 0 (rather than -len .. -1, which equates to 0 .. (len-1)):
def reversed_word(word):
result = ""
for i in range(len(word)):
result += word[len(word) - i - 1]
return result
Another alternative is to realise you don't need to use an index at all since iterating over a string gives it to you one character at a time. However, since it gives you those characters in order, you need to adjust how you build the reversed string, by prefixing each character rather than appending:
def reverse_string(word):
result = ""
for char in word:
result = char + result
return result
This builds up the reversed string (from APPLE) as A, PA, PPA, LPPA and ELPPA.
Of course, you could also go fully Pythonic:
def reverse_string(word):
return "".join([word[i] for i in range(len(word), -1, -1)])
This uses list comprehension to create a list of characters in the original string (in reverse order) then just joins that list into a single string (with an empty separator).
Probably not something I'd hand in for classwork (unless I wanted to annoy the marker) but you should be aware that that's how professional Pythonistas usually tackle the problem.
Let's say your word is python.
You loop will then iterate over the values 0 through 5, since len(word) == 6.
When i is 0, i-len(word) is -6 (note carefully that this value is negative). You'll note that word[-6] is the character six places to the left from the end of the string, which is p.
Similarly, when i is 1, i-len(word) is -5, and word[i-len(word)] is y.
This pattern continues for each iteration of your loop.
It looks like you intend to use positive indices to step backward through the string with each iteration. To obtain this behavior, try using the expression len(word)-i-1 to index your string.
def reversed_word(word):
reversed = ''
for i in range(len(word)-1, -1, -1):
reversed += word[i]
return reversed
print(reversed_word("apple"))
I just started to use python 3. I want to find specific characters inside a string that is part of a list. Here is my code:
num = ["one","two","threex"]
for item in num:
if item.find("x"):
print("found")
So, I want to print "found" if the character "x" is inside of one of the elements of the list. But when I run the code, it prints 3 times instead of one.
Why is printing 3 times? Can someone help me?
find() returns -1 if the character is not found in the string. Anything that is not zero is equal to True. try if item.find("x") > -1.
You can use in again for strings:
num = ["one","two","threex"]
for item in num:
if "x" in item:
print("found")
Think in Strings as a list of chars like "ext" -> ['e', 'x', 't']
so "x" in "extreme" is True
find returns Index if found and -1 otherwise.
num = ["one","two","threex"]
for item in num:
if item.find("x"):
print item.find("x")
i hope that you got the solution from above post ,here you know the reason why
You need to break out of looping through the strings if 'x' is found as otherwise, it may be found in other strings. Also, when checking if 'x' is in the string, use in instead.
num = ["one","two","threex"]
for item in num:
if "x" in item:
print("found")
break
which outputs:
found
And if I modify the num list so that it has no x in any of the elements:
num = ["one","two","three"]
then there is no output when running the code again.
But why was it printing 3 times before?
Well simply, using item.find("x") will return an integer of the index of 'x' in the string. And the problem with evaluating this with an if-statement is that an integer always evaluates to True unless it is 0. This means that every string in the num list passed the test: if item.find("x") and so for each of the 3 strings, found was printed. In fact, the only time that found wouldn't be printed would be if the string began with an 'x'. In which case, the index of 'x' would be 0 and the if would evaluate to False.
Hope this clears up why your code wasn't working.
Oh, and some examples of testing the if:
>>> if 0:
... print("yes")
...
>>> if 1:
... print("yes")
...
yes
>>> if -1:
... print("yes")
...
yes
I am trying to write a code which prints True if given string has at max 2 consecutive c, and at max 1 b. I am using recursion to reduce the string and check that at max 'c' is present in the same index twice.But my recursion is not stopping till it empties the whole list. Can you please suggest what's wrong with my code. Thanks!
def stringcond(N,count=0,k=0):
N=list(N)
if(N.count('b')>1):
return False
if len(N)<2:
return True
else:
for i,j in enumerate(N):
if(j=='c'):
del N[i]
count+=1
if(k==i and count>2):
return False
stringcond(N,count=count,k=i)
return True
You have several mistakes. First, why are you splitting the characters into a list? There is a perfectly good count method for strings.
Your recursion fails because you ignore the return value. You would want something like
if not stringcond(N,count=count,k=i):
return False
# I make no claim that this logic is correct.
In any case, there is no need to recur. Use count to check the quantity of "b" and many-'c' substrings:
def stringcond(s, c_max=0):
return s.count('b') <= 1 and \
s.count("c" * (c_max+1)) == 0
You have to use the result of the stringcond call. Now your function will only return whatever was determined on the top level call.