I am not getting the desired output with my python code

Question

<code>def CheckNumber(MyList, number):
    counter=0
    while number!=0:
        for i,element in enumerate(MyList):
            if number%10==element:
                del MyList[i]
            else:
                continue
        number = number/10
    if len(MyList)==0:
        return 1
    else:
        return 2

print("Program to print all the possible combinations of a number")
MyNumber = int(input("Enter the number: "))
MyList = []
while MyNumber!=0:
    MyList.append(MyNumber%10)
    MyNumber=int(MyNumber/10)
MyLimit = 10**(len(MyList)-1)
for i in range(MyLimit, MyLimit*10):
    answer = CheckNumber(MyList, i)
    if answer == 1:
        print(i)
    else:
        continue`</code>

I am a beginner at programming and I was trying to write a code to print all the possible combinations of a number. If user enters a 3 digit number the program will check all the three digit numbers to find possible combinations but instead it gives all the numbers as output. For example if user enters 12 then the output should be 12 21 but instead it shows every number from 10 to 99.

As far as I know everything is working fine but the results are not as I expect.

score 1 · Answer 1 · answered Jan 24 '23 at 23:40

This is a pass-by-reference vs pass-by-value problem. What that means is when you pass a list to a function in python you are not passing the values in that list, you are passing the list itself, or rather its location in memory. So when you are modifying MyList in your CheckNumber function you are actually modifying the MyList variable globally. This is not true for primitive types which is why modifying number does not change i in the for loop. Quick example:

def foo(my_list):
    my_list.append('world')
    print(my_list)

a = []
foo(a) # this will print out 'world'
print(a) # this will print out 'world'

b = 'hello'
foo(b.copy()) # This will print out 'hello world'
print(b) # Here we have not passed b directly into foo, 
# but instead passed a copy, so this will just print out 'hello' as b
# has not been modified

To summarize variable are stored in a specific location in memory. When you pass-by-reference you are passing a long that location in memory so you variable will be mutated. If you pass-by-value, you function will create a new variable and store a copy of the data so you will not mutate your outer variable. In other languages you can specify which way to pass in a variable but afaik you cannot in python.

With that out of the way this is a very easy fix. You don't want to modify your original MyList so just make a copy of it and pass that into the function. You also forgot to cast number/10 to int in the CheckNumber function. The working code should look like this:

def CheckNumber(MyList, number):
counter=0
while number!=0:
    for i,element in enumerate(MyList):
        if number%10==element:
            del MyList[i]
        else:
            continue
    number = int(number/10)
if len(MyList)==0:
    return 1
else:
    return 2

print("Program to print all the possible combinations of a number")
MyNumber = int(input("Enter the number: "))
MyList = []
while MyNumber!=0:
    MyList.append(MyNumber%10)
    MyNumber=int(MyNumber/10)
MyLimit = 10**(len(MyList)-1)
for i in range(MyLimit, MyLimit*10):
    answer = CheckNumber(MyList.copy(), i)
    if answer == 1:
        print(i)
    else:
        continue

More info on pass-by-reference:

What's the difference between passing by reference vs. passing by value?

https://blog.penjee.com/passing-by-value-vs-by-reference-java-graphical/

https://courses.washington.edu/css342/zander/css332/passby.html

Thank you so much. Actually I knew about the pass by reference and to avoid that I made a CopiedList in the CheckNumber function and did all the operations on that list but it still didn't work since I didn't cast number/10 to int. Now the code is working completely fine. Thank you very much.\ — Dilraj Singh, Jan 25 '23 at 04:29

I am not getting the desired output with my python code

1 Answers1