CODEWITHSUNDEEP
pythonlist
forest
forest

Reputation: 1464

Unexpected output: code returning double row output whereas only one row output is expected

This code counts the number of pairs in a list and returns the total number of pairs:

ar = [10, 20, 20, 10, 10, 30, 50, 10, 20]
parsed = []
total_pairs = 0

for i in ar:

# if a list item has been parsed, put it in this list
# so that it does not reoccur
    if i in parsed:
        continue
    else:

        # if it's a new item, append it to parsed list
        parsed.append(i)

        # if there is only one item, skip it:
        if ar.count(i) == 1:
            continue
        else:
            # count the number of pairs
            pairs = ar.count(i) // 2

            # add this to the total number of pairs
            total_pairs = total_pairs + pairs
            print(total_pairs)

It is expected to return 3, but the output is:

2
3

I can't understand where the 2 is coming from.

Upvotes: 1

Views: 45

Answers (3)

RoadRunner
RoadRunner

Reputation: 26335

Adding to the other answers, you could also use collections.Counter to count the total number of pairs:

>>> from collections import Counter
>>> ar = [10, 20, 20, 10, 10, 30, 50, 10, 20]
>>> sum(v // 2 for v in Counter(ar).values())
3

Or using the fast collections.defaultdict:

from collections import defaultdict

ar = [10, 20, 20, 10, 10, 30, 50, 10, 20]

counts = defaultdict(int)
for item in ar:
    counts[item] += 1

print(sum(v // 2 for v in counts.values()))
# 3

Upvotes: 2

wjandrea
wjandrea

Reputation: 33159

print(total_pairs) is inside the loop. Simply move it after the loop:

            total_pairs = total_pairs + pairs

print(total_pairs)

BTW you can simplify the loop:

for i in ar:
    if i in parsed:
        continue

    parsed.append(i)

    if ar.count(i) == 1:
        continue

    total_pairs += ar.count(i) // 2

And FWIW you can do the whole thing more easily with a Counter:

from collections import Counter

counts = Counter(ar)
total_pairs = sum(i//2 for i in counts.values())
print(total_pairs)  # -> 3

Upvotes: 2

ngShravil.py
ngShravil.py

Reputation: 5058

It's an indentation issue.

ar = [10, 20, 20, 10, 10, 30, 50, 10, 20]
parsed = []
total_pairs = 0

for i in ar:

# if a list item has been parsed, put it in this list
# so that it does not reoccur
    if i in parsed:
        continue
    else:

        # if it's a new item, append it to parsed list
        parsed.append(i)

        # if there is only one item, skip it:
        if ar.count(i) == 1:
            continue
        else:
            # count the number of pairs
            pairs = ar.count(i) // 2

            # add this to the total number of pairs
            total_pairs = total_pairs + pairs
print(total_pairs) # This should be outside the loop.

Upvotes: 2

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