This exercise involves a codeacademy problem to write a function which takes a string as an input and outputs the string in reverse, I've found solutions as to how to do it online, I'm just confused as to why mine doesnt work:
def reverse(c):
empty=[]
stringsize= len(c)
for x in range(stringsize):
empty[x]=c[stringsize-x]
return empty
You need to start at indexing your string from -1 through to -stringsize, and use empty.append() to add values:
for x in range(stringsize):
empty.append(c[stringsize - x - 1])
Python indexing starts at 0, making stringsize - 1 the last index. Because empty is an empty list, you cannot index into it. Using the list.append() method adds new values at the end instead.
You don't really need the stringsize reference there, because negative indices automatically are subtracted from the length for you:
for x in range(len(c)):
empty.append(c[-x-1])
Since this is supposed to return a string, not a list, you need to join the characters again at the end:
return ''.join(empty)
The easiest way to reverse a list is to use a negative slice stride:
def reverse(c):
return c[::-1]
empty beforehand: empty = [None] * len(stringsize) ;-)
empty is empty. So there are two alternative solutions; prepopulate or list.append(). The latter is easier to explain.empty.append(…) seems to be a sin as well.One solution :
def reverse(c):
empty=[]
stringsize= len(c)
for x in range(stringsize):
empty.append(c[-(x+1)])
return ''.join(empty)
print reverse('string')
Another:
def reverse(c):
empty=[]
stringsize= len(c)
for x in range(stringsize):
empty.append(c[stringsize - (x+1)])
return ''.join(empty)
print reverse('string')
Using recursion:
def reverse(string,start,stop):
if start < stop - 1:
string[start],string[stop - 1] = string[stop - 1],string[start]
reverse(string,start+1, stop-1)
return ''.join(string)
print reverse(list('string'), 0, len('string'))
In Python a string is an iterable so iterable functions can be used with it. For example reversed function:
>>> "".join(reversed("123"))
'321'
The least changes necessary to make your code run seem to be:
def reverse(c):
stringsize= len(c)
empty=[None] * stringsize
for x in range(stringsize):
empty[x]=c[stringsize-x-1]
return empty
But you should reconsider your name empty (as Martijn pointed out) because that thing isn't empty (at least in the end), so the name is misleading.
The classic answer to this problem is to use a slice with a negative step to get what you want:
def reverse(c):
return c[::-1]
...but maybe that's not allowed for you?
IndexErrorbecauseemptylist you have does not have anything in it, it is basically empty. You shouldappendto it. Check python docs.reversedfunction to reverse your string. something like this:"".join(list(reversed("foo"))), though there may exist even simpler solutions. I'm callinglist()becausereversedreturns a "reversed object", which I then need to turn back into a string.IndexErrorbecausec[stringsize]is out of bounds; the last index isc[stringsize - 1].str.join()calls list on thereversed()iterator for you..