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I'm trying to code in python to filter a list of the first n integers that when divided have a remainder of 1. However, it looks like it's not checking every integer in my list according to the print statements I have. I have no idea why. I'm just starting to learn code, so please be patient with me. I suspect its something obvious I'm not seeing.

Below is my code I have so far:

def main():
    checkUpTo = 20
    listOfInts = list(range(checkUpTo))
    filterList = [3]
    boolList = []
    
    for d in filterList:
        print("checking list: " + str(listOfInts) + " for rem=1 when divided by " + str(d))
        for x in listOfInts:
            print("checking: " + str(x))
            isDivisible = checkIfDivisible(x,d)
            if isDivisible==False:
                listOfInts.remove(x)
        print("got list: " + str(listOfInts))

    print(listOfInts)

def checkIfDivisible(number,divisor):
    remainder = number % divisor
    print(str(number) + "%" + str(divisor) + "=" + str(remainder))
    if number % divisor == 1:
        return True
    if number % divisor != 1:
        return False

if __name__ == "__main__":
    main()

Thanks for the help

edit: my output:

checking list: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] for rem=1 when divided by 3
checking: 0
0%3=0
checking: 2
2%3=2
checking: 4
4%3=1
checking: 5
5%3=2
checking: 7
7%3=1
checking: 8
8%3=2
checking: 10
10%3=1
checking: 11
11%3=2
checking: 13
13%3=1
checking: 14
14%3=2
checking: 16
16%3=1
checking: 17
17%3=2
checking: 19
19%3=1
got list: [1, 3, 4, 6, 7, 9, 10, 12, 13, 15, 16, 18, 19]
[1, 3, 4, 6, 7, 9, 10, 12, 13, 15, 16, 18, 19]

not sure why it's not checking 1,3, etc.

nessieTuxedo
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    Can you provide an example input and the expected output? – Justin Mar 10 '21 at 17:19
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    a list of ints from 0 to n and a filter list. for example, if the filter list is [2,3] and i'm checking the integers 1 to 10. first i get [3,5,7,9] because each one is one more than a divisor by 2. then, checking 3, it would be [7] because it is one more than a divisor of 7. I'll add the output i get into the post. – nessieTuxedo Mar 10 '21 at 17:29
  • You should not modify the list while you are iterating over it. Have a look at this question and ist answers: https://stackoverflow.com/questions/1207406/how-to-remove-items-from-a-list-while-iterating – Donat Mar 10 '21 at 20:43

2 Answers2

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listOfInts.remove(x) changes the object (listOfInts) you're iterating over in for, which will change the number of iterations in the loop.

Consider:

l = [1, 2, 3, 4, 5]

for index, value in enumerate(l):
    print(l)
    l.remove(value)

print(index + 1) # 3, not 5.

Rather than removing elements from listOfInts, iterate over x in range(checkUpTo), and if you find a value that is divisible by d, add it to a set.

crcvd
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  • I see that listOfInts.remove(x) changes the list, but I'm not worried about the number of iterations changing. but from output it looks like it's not checking 3%3=0 when it does the inner for loop over listOfInts. – nessieTuxedo Mar 10 '21 at 17:44
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Look at For loop, listOfInts.remove(x) removes element from list. Therefore, your list dose not have those element whose remainder is one.

 for x in listOfInts:
        print("checking: " + str(x))
        isDivisible = checkIfDivisible(x, d)
        if isDivisible == False:
             listOfInts.remove(x)

Lets understand what is hapening in for loop,

  • List contain element from 0 to 19.

  • for loop takes 0 and pass it to checkIfDivisible function.

  • 0%3=0 returns False.

  • isDivisible == False, so listOfInts.remove(x) removes element from position 0.

  • for loop now fetches next item, since we have removed item from 0 position now at 0 position we have element 1 at 1 position we have elemt 2 and so on.

  • therefore print statement doesnot display element 1.

     for x in listOfInts:
     print("checking: " + str(x))
     isDivisible = checkIfDivisible(x, d)
     print(isDivisible)
     if isDivisible == False:
         print(x)
         print(listOfInts)
         listOfInts.remove(x)

    checking: 0 0%3=0 False 0 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] checking: 2 2%3=2 False 2 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

Rima
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