Why is this happening to my nested list? (Python)-CodePudding

if __name__ == '__main__':
    records = []
    smallest = 0
    fat = int(input())
    for x in range(fat):
        name = input()
        score = float(input())
        records.append([name, score])
    records.sort(key=lambda x:x[1])
    print(records)
    for i in range(len(records)-3):
        if records[i][1] <= records[i 1][1]:
            del records[i]
        else:
            ''
print(records)

When I input this:

6
Jorge
3
Mia
3
Gus
5
Myles
4
Gerald
9
Fred
8

The output is [['Mia', 3.0], ['Gus', 5.0], ['Gerald', 9.0]]. I want to remove all the parts of the list with the lowest value. Why isn't this code working?

CodePudding user response：

You have a sorted list of half a dozen records, with some scores being tied. Essentially you're doing this:

for i in range(3):
    if {i-th score smaller than next score}:
        del records[i]

That is very weird. Why? First, you are iterating over just half of the records. But worse, you are mutating the list you're iterating over, so it's unclear whether a given score will be compared with next score. Some scores will be compared, and some won't due to deletion advancing to next index. That effectively skips an entry, skips the comparison of a pair of records.

Rather than deleting 2-element records in that way, consider defining 3-tuples with 3rd element initialized to False. Then set it True in the case where you want to delete, based on a comparison. Finally, return just those records that still have a False tombstone value.

Your <= comparsion will always be true, given that it is examining records that have already been placed into sorted order.

Perhaps you wanted < strictly less than, instead?

CodePudding user response：

Since you sort the records, you can just take a slice of the resulting list that is from the 4th entry on, assuming that what you want is to remove the 3 entries with the smallest scores:

if __name__ == '__main__':

    records = [['Jorge', 3], ['Mia', 3], ['Gus', 5], ['Myles', 4], ['Gerald', 9], ['Fred', 8]]

    # records = []
    # fat = int(input())
    # for x in range(fat):
    #     name = input()
    #     score = float(input())
    #     records.append([name, score])

    records.sort(key=lambda x:x[1])

    print(records)

    records = records[3:]

    print(records)

Result:

[['Jorge', 3], ['Mia', 3], ['Myles', 4], ['Gus', 5], ['Fred', 8], ['Gerald', 9]]
[['Gus', 5], ['Fred', 8], ['Gerald', 9]]

I took out your input code in favor of a hard coded value to demonstrate the specific case you give in a reproducible way. I hate having to enter values to play around with code like this. You can easily put your input code back in by uncommenting it.

CodePudding user response：

this is because u cant edit a list in side a loop, such as del.. so which operations that are all leading to the change in length wont work.. if the loop is working for a length of list

for this you can create a temp list and store the required values there.