Question

I have written a pair of methods in python:

1. number_combinations() which obtains all possible combinations of 3 numbers
2. checkAllValuesAreDifferent() which checks that all the values in the list are different

–

temp = []

def number_combinations(listOfElements,index):

    if index==3:
        #if checkAllValuesAreDifferent(listOfElements):
        #print "Going to append the following list to the sat tuple list "      
        print listOfElements

        #print "Temp is ",temp

        temp.append(listOfElements)
        print "Temp is ",temp
        print "exit from function " 
        return listOfElements

    else:
        for value in range(3):
            listOfElements[index]=value
            #print "LIST OF ELEMENTS IS ",listOfElements
            if checkAllValuesAreDifferent(listOfElements,index,value)==True:
                #print "RECURSIVE CALL "                
                (number_combinations(listOfElements,index+1))
                #print "THE LIST IS ",temp
                #print "APPENDING TO SAT TUPLES "               
                #print "BACKTRACK HAPPEND ",listOfElements
                #print "AND INDEX IS ",index        

                 #recursive call to next level              

        return #backtrack since no number found

def checkAllValuesAreDifferent(list_of,index,value):

    if index==0:
        return True
    else:
        for i in range(index):
            #print "Entered loop with index ",index
            if(list_of[i]==value):
                return False
        return True

When I just try printing the listOfElements, I get the right answer i.e all sets of 3 digits with each distinct. However, when I try appending to the list temp, I end up getting duplicate entries. So temp should be : [0,1,2],[0,2,1][1,0,2][1,2,0][2,0,1][2,1,0]
(which is the result of printing alone).However,temp turns out to be [[2, 2, 2], [2, 2, 2], [2, 2, 2], [2, 2, 2], [2, 2, 2], [2, 2, 2]]

I cannot figure out the problem with this. Could anyone give me some idea as to how to successfully append the right lists being generated to the temp list.

Answer 1

Because you are passing the same listOfElements back to the function, and it will be modified in place.

You can use the [:] idiom to shallow-copy a list:

temp.append(listOfElements[:])