Think Python - Denison University

78 Chapter 8. StringsIf we test this function with the words “pots” and “stop”, we expect the return value True, but weget an IndexError:>>> is_reverse('pots', 'stop')...File "reverse.py", line 15, in is_reverseif word1[i] != word2[j]:IndexError: string index out of rangeFor debugging this kind of error, my first move is to print the values of the indices immediatelybefore thelinewhere the errorappears.while j > 0:print i, j# print hereif word1[i] != word2[j]:return Falsei = i+1j = j-1Now when I runtheprogram again, I get moreinformation:>>> is_reverse('pots', 'stop')0 4...IndexError: string index out of rangeThefirsttimethroughtheloop,thevalueofjis4,whichisoutofrangeforthestring'pots'. Theindex of thelastcharacter is3, sotheinitialvalue forjshould belen(word2)-1.IfIfix that error and runthe program again, I get:>>> is_reverse('pots', 'stop')0 31 22 1TrueThistimewegettherightanswer,butitlookslikethelooponlyranthreetimes,whichissuspicious.To get a better idea of what is happening, it is useful to draw a state diagram. During the firstiteration, theframeforis_reverselooks likethis:word1 ’pots’ word2 ’stop’i 0 j 3Itookalittlelicensebyarrangingthevariablesintheframeandaddingdottedlinestoshowthatthevalues ofiandjindicate characters inword1andword2.Exercise 8.8 Starting with this diagram, execute the program on paper, changing the values of iandjduring each iteration. Find and fixthe second error inthisfunction.