Removing words in list - loop malfunction

Question

I have written a simple for loop meant to check if all of the words in one list are found in a larger list. It is a practice question found here.

I do not understand why my function does not execute properly - After running it, what is left in the sample list "note2" is ['one','today'], so it seems like the loop is somehow skipping over those words. Why is that?! I do not understand it conceptually.

Thank you for your help on this.

Example lists (two pairs of examples):

mag1=['two', 'times', 'three', 'is', 'not', 'four']
note1=['two', 'times', 'two', 'is', 'four']

mag2=['give', 'me', 'one', 'grand', 'today', 'night']
note2=['give','one','grand','today']

Function:

def checkMagazine(magazine, note):
    #Counter(note1).max
    for word in note:
        if word in magazine:
            magazine.remove(word)
            note.remove(word)
            #print(magazine)
            #print(note)
        #print(note)
    if len(note)>0:
        #ans="No"
        print("No")
    else:
        #ans="Yes"
        print("Yes")

I don't know for *absolute sure* that that's your problem, but it definitely sounds like it is. You can't reliably change the size of a collection (like removing from it) while iterating it. That can cause weird behavior (like elements getting skipped). Change to a list comprehension, or, have a second list that's a copy of the original. — Carcigenicate, Nov 16 '19 at 17:17
Actually, arguably [this](https://stackoverflow.com/questions/742371/why-does-python-skip-elements-when-i-modify-a-list-while-iterating-over-it) is a better dupe. — Carcigenicate, Nov 16 '19 at 17:18
If you do not want to iterate you can use something like this:`final = [word for word in mag1 if word not in note1]` — LocoGris, Nov 16 '19 at 17:30

seralouk · Accepted Answer · 2019-11-16T18:34:44.013

2

In the loop you remove elements from the list and then word is looking at the elements of the reduced list.

Copy the lists in the for loop line using note[:]:

def checkMagazine(magazine, note):
    for word in note[:]:
        if word in magazine:
            magazine.remove(word)
            note.remove(word)

mag2=['give', 'me', 'one', 'grand', 'today', 'night']
note2=['give','one','grand','today']

checkMagazine(mag2, note2)

edited Nov 16 '19 at 18:34

answered Nov 16 '19 at 17:22

seralouk

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Yes this is what I said. It does not need to be copied. I have edited it. – seralouk Nov 16 '19 at 17:24
Just saw the duplicate indication. Feel free to close and delete it – seralouk Nov 16 '19 at 17:25
Thank you, makis. A question: what does the addition of the [:] actually do? Why does this make it work? – Z_D Nov 16 '19 at 18:01
read the first sentence of my answer – seralouk Nov 16 '19 at 18:21
to answer also here, `[:]` copies the list. – seralouk Nov 16 '19 at 18:35

dpant · Answer 2 · 2019-11-16T17:34:26.023

As explained in this answer https://stackoverflow.com/a/58718886/313935 removing items from a list inside a loop is always a no-go. You can find out yourself by inspecting the result of the print statements you've commented out in your code (i.e. the position of items in the list changes every time an item is removed).

Here's another approach:

def checkMagazine(magazine, note):    
    matches=0
    for word in note:
        if word in magazine:
            matches+=1

    if matches==len(note):
        print("Yes")
    else:
        print("No")

Removing words in list - loop malfunction

2 Answers2