Search & Count: in, index, count, len

By the end of this lesson you can:

• Test membership with in instead of writing a search loop.
• Find a position with index(), guarding against the ValueError.
• Summarise a list of numbers with len, sum, max and min.

Six lines to answer one yes-or-no question. Read it, then predict the output.

club = ["Mia", "Leo", "Ana"]

found = False
for name in club:
    if name == "Ana":
        found = True

print(found)

True

Like asking the class monitor rather than counting heads yourself. Is Ana here? Where is she sitting? How many people brought a calculator? One question, one answer.

in — is it there?

club = ["Mia", "Leo", "Ana", "Hugo"]

print("Ana" in club)
print("Zara" in club)
print("Zara" not in club)
print("ana" in club)

Output

True
False
True
False

The last line matters: matching is exact, so case counts. not in reads better than not (x in y).

index() — where is it?

club = ["Mia", "Leo", "Ana", "Hugo", "Ana"]

print(club.index("Ana"))
print(club.index("Ana", 3))

Output

2
4

club = ["Mia", "Leo", "Ana"]
wanted = "Zara"

if wanted in club:
    print(wanted, "is at seat", club.index(wanted))
else:
    print(wanted, "is not in the club")

Output

Zara is not in the club

count() — how many?

votes = ["pizza", "pancake", "pizza", "noodles", "pizza"]

print(votes.count("pizza"))
print(votes.count("ice cream"))
print(len(votes))

Output

3
0
5

count() returns 0 for something absent rather than raising — so unlike index(), it needs no guard.

Summarising numbers

marks = [78, 91, 65, 88, 72]

print("How many:", len(marks))
print("Total:   ", sum(marks))
print("Highest: ", max(marks))
print("Lowest:  ", min(marks))
print("Average: ", sum(marks) / len(marks))
print(f"Average: {sum(marks) / len(marks):.2f}")

Output

How many: 5
Total:    394
Highest:  91
Lowest:   65
Average:  78.8
Average: 78.80

These are built-in functions, so they wrap the list: sum(marks), not marks.sum(). max() and min() work on text too, comparing alphabetically.

Why it matters. Vote tallies, high scores, class averages, checking a name is registered before letting them in — every one is a built-in rather than a loop.

Thirteen votes, counted and reported without a single counting loop.

# canteen_vote.py - tally a vote using built-ins only

votes = ["pizza", "pancake", "pizza", "noodles",
         "pizza", "ice cream", "pancake", "pizza",
         "noodles", "pancake", "pizza", "ice cream", "noodles"]

options = ["pizza", "pancake", "noodles", "ice cream", "hot dog"]

print(f"{len(votes)} votes cast")
print()
print(f"{'Dish':<14}{'Votes':>6}{'Share':>9}")
print("-" * 29)

counts = []
for dish in options:
    n = votes.count(dish)
    counts.append(n)
    share = n / len(votes) * 100
    print(f"{dish:<14}{n:>6}{share:>8.1f}%")

print("-" * 29)

winner_votes = max(counts)
winner = options[counts.index(winner_votes)]

print(f"Winner: {winner} with {winner_votes} votes")
print(f"Dishes nobody chose: {counts.count(0)}")

if "hot dog" in votes:
    print("Hot dog got at least one vote")
else:
    print("Hot dog got no votes at all")

Output

13 votes cast

Dish           Votes    Share
-----------------------------
pizza         5    38.5%
pancake         3    23.1%
noodles         3    23.1%
ice cream             2    15.4%
hot dog              0     0.0%
-----------------------------
Winner: pizza with 5 votes
Dishes nobody chose: 1
Hot dog got no votes at all

The winner line is the pattern worth keeping:

• max(counts) finds the biggest number.
• counts.index(...) finds where it sits.
• That position reads the matching name out of options, because the two lists were built together.

in answers whether an item is present, index() says where — and raises ValueError if it is absent, so check with in first. count() is safe and returns 0. len, sum, max and min are built-in functions that wrap the list, and none of them survive an empty one except len and sum.