This code finds the longest alphabetical substring in a string (s).
letter = s[0]
best = ''
for n in range(1, len(s)):
if len(letter) > len(best):
best = letter
if s[n] >= s[n-1]:
letter += s[n]
else:
letter = s[n]
It works most of the time, but occasionally it gives wrong answers and I am confused why it only works sometimes. for example when:
s='maezsibmhzxhpprvx'
It said the answer was "hpprv" while it should have been "hpprvx".
In another case, when
s='ysdxvkctcpxidnvaepz'
It gave the answer "cpx", when it should have been "aepz".
Can anyone make sense of why it does this?
Your routine was almost ok, here's a little comparison between yours, the fixed one and another possible solution to your problem:
def buggy(s):
letter = s[0]
best = ''
for n in range(1, len(s)):
if len(letter) > len(best):
best = letter
if s[n] >= s[n - 1]:
letter += s[n]
else:
letter = s[n]
return best
def fixed(s):
letter = s[0]
best = ''
for n in range(1, len(s)):
if s[n] >= s[n - 1]:
letter += s[n]
else:
letter = s[n]
if len(letter) > len(best):
best = letter
return best
def alternative(s):
result = ['' for i in range(len(s))]
index = 0
for i in range(len(s)):
if (i == len(s) - 1 and s[i] >= s[i - 1]) or s[i] <= s[i + 1]:
result[index] += s[i]
else:
result[index] += s[i]
index += 1
return max(result, key=len)
for sample in ['maezsibmhzxhpprvx', 'ysdxvkctcpxidnvaepz']:
o1, o2, o3 = buggy(sample), fixed(sample), alternative(sample)
print "buggy={0:10} fixed={1:10} alternative={2:10}".format(o1, o2, o3)
As you can see in your version the order of the inner loop conditional is not good, you should move the first conditional to the end of loop.