Kermit Rose wrote: > # def insertw(j1,k,w1,w2,jar,limit): > # trace = 1 > # if k == 0: > # jar[k][0] = k > # jar[k][1] = w1 > # jar[k][2] = w2 > # return > # if j1 > k: > # jar[j1][0] = k > # jar[j1][1] = w1 > # jar[j1][2] = w2 > # return > # > # for j2 in range(j1+1,k+1): > # j3 = k + j1 - j2 > # if trace > 0: > # print " insertw: move jar[",j3,"] up one"," j1 = ",j1," k = " > k," w1 = ",w1," w2 = ",w2 > # f = jar[j3] > # jar[j3+1] = f
I think you want to copy jar[j3] here. In your code, jar[j3] and jar[j3+1] both refer to the same list! If you change the list, you will see it in both places. Assignment in Python is not a copy, it is a name binding. Assignment creates a name for an object. If you assign the same object to two names, they both are bound to the same thing. If the object is mutable, like a list, changes to the object will be seen regardless of which name you use to refer to it. For example: In [14]: d = [1, 2, 3] In [15]: e=d d and e are now references to the same list In [18]: d[0]=55 The changes the list, the change can be seen regardless of which reference to the list is used to access it. In [19]: d Out[19]: [55, 2, 3] In [20]: e Out[20]: [55, 2, 3] Here is one way to make a copy; d and f refer to different lists: In [21]: f=d[:] In [22]: f Out[22]: [55, 2, 3] Changing f doesn't affect d or e: In [23]: f[1]=23 In [24]: f Out[24]: [55, 23, 3] In [25]: d Out[25]: [55, 2, 3] In [26]: e Out[26]: [55, 2, 3] This may help: http://www.effbot.org/zone/python-objects.htm Kent > # if trace > 0: > # print " insertw: jar[",j3+1," is now ",jar[j3+1] > # > # > # jar[j1][0] = k > # jar[j1][1] = w1 > # jar[j1][2] = w2 > # > # if trace > 0: > # for j in range(k+1): > # print " insertw: jar[",j,"] = ",jar[j] > # return > # > > > debug trace shows the following puzzling behavior. > > > fermat2: before insertw: jar[ 0 ] = [0, 2, 4] > fermat2: before insertw: jar[ 1 ] = [1, 4, 16] > fermat2: before insertw: jar[ 2 ] = [-1, -1, -1] > > > I show the array jar before going into insert. > > remember the heading of insertw is > > # def insertw(j1,k,w1,w2,jar,limit): > > > > insertw: move jar[ 1 ] up one j1 = 1 k = 2 w1 = 16 w2 = 13 > > This shows that insert made one shift, and sifted jar[1] to jar[2]. > j1 = 1 means that insertw was supposed to insert new value into jar[1] > > insertw: jar[ 2] is now [1, 4, 16] > > I print out jar[2] to show that insertw really did shift jar[1] to jar[2]. > > > > > insertw: jar[ 0 ] = [0, 2, 4] > insertw: jar[ 1 ] = [2, 16, 13] > insertw: jar[ 2 ] = [2, 16, 13] > > Now, outside the loop, > I set jar[j1] to the new values. > > And I print the resulting array, still within the routine insertw. > > jar[1] has been set to the new values. > > BUT, and this is the problem, > > jar[2] has been also set to the new values. > > WHY??????????? > > > > > > > > > _______________________________________________ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > > _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
