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Python for kids · Module 2

Lesson 30

Contest tasks with conditions

After this lesson: You'll solve real contest tasks: list all the cases and test every road of the program.

Video

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The video for this lesson is coming soon.

Explanation

You already know everything you need for real contest tasks with conditions - like the ones at OIJ or Logia! 🏆 Today you'll learn the method: how to read a task, list all the cases and miss nothing.

Bajtek's plan for a task 🎬

Click Next - four steps that work for every task with conditions.

?Bajtek
🪨📄✂️
1

Step 1: read carefully

The task: Two players play rock, paper, scissors. Print 1 if the first one wins, 2 if the second one does, and REMIS for a draw. Bajtek underlines: what is read, what is printed and what the rules are.

Step 1 / 4

A table of cases

For tasks with many possibilities draw a little table - then no case gets away:

first \ secondrock 🪨paper 📄scissors ✂️
rock 🪨REMIS21
paper 📄1REMIS2
scissors ✂️21REMIS

Rock, paper, scissors 🪨📄✂️

Type the first player's move 🪨 and the second player's move ✂️ (kamien, papier or nozyce - rock, paper, scissors in Polish), Enter ⏎ after each.

✊1first = input()
2second = input()
if first == second
4print("A draw 🤝")
🤝
elif first == "kamien" and second == "nozyce" or first == "nozyce" and second == "papier" or first == "papier" and second == "kamien"
6print("Player 1 wins")
1️⃣
elsein every other case
8print("Player 2 wins")
2️⃣

Click the arrow - I'll walk the program's path, line by line.

Code
1first = input()
2second = input()
3if first == second:
4    print("A draw 🤝")
5elif first == "kamien" and second == "nozyce" or first == "nozyce" and second == "papier" or first == "papier" and second == "kamien":
6    print("Player 1 wins")
7else:
8    print("Player 2 wins")
Screen
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Tickets for a trip 🎬

Click Next - a task where you have to pick the cheapest option.

?Bajtek
🎟️10 zloty📦10 pcs = 80 zloty
1

Two kinds of tickets

13 people are going to the museum. A single ticket costs 10 zloty, and a pack of 10 tickets - 80 zloty. What's the cheapest way to buy them?

Step 1 / 3

The cheapest tickets on the podium

The three ideas are three bars - the crown goes on the lowest.

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🎟️
?
📦🎟️
?
📦📦
📦 singles📦 mixed📦 packs
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Click the arrow - the numbers will stand on the podium as bars.

Code
1people = 13
2singles = people * 10
3mixed = people // 10 * 80 + people % 10 * 10
4packs = (people // 10 + 1) * 80
5cheapest = singles
6if mixed < cheapest:
7    cheapest = mixed
8if packs < cheapest:
9    cheapest = packs
10print(cheapest)
Screen
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Work out the tickets for your class 🎟️

Type a number of people 🧒 (e.g. 13, 19 or 27) and press Enter ⏎.

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🎟️
?
📦🎟️
?
📦📦
📦 singles📦 mixed📦 packs
?

Click the arrow - the numbers will stand on the podium as bars.

Code
1people_str = input()
2people = int(people_str)
3singles = people * 10
4mixed = people // 10 * 80 + people % 10 * 10
5packs = (people // 10 + 1) * 80
6cheapest = singles
7if mixed < cheapest:
8    cheapest = mixed
9if packs < cheapest:
10    cheapest = packs
11print(cheapest)
Screen
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The kind of triangle

A contest classic: three sides a, b, c can make an equilateral triangle (all sides equal), an isosceles one (two equal) or a scalene one (none equal). But first a guard: is it a triangle at all? (from lesson 25: any two sides together are longer than the third).

The order matters: an equilateral triangle also has two equal sides - so we check it before isosceles.

What kind of triangle is it?

Sides 5, 5 and 8: the guard lets them through, equilateral - no, two equal - yes!

📐1a = 5
2b = 5
3c = 8
if a + b <= c or a + c <= b or b + c <= a
5print("Not a triangle")
🚫
elif a == b and b == c
7print("Equilateral")
🔺
elif a == b or b == c or a == c
9print("Isosceles")
🔻
elsein every other case
11print("Scalene")
📐

Click the arrow - I'll walk the program's path, line by line.

Code
1a = 5
2b = 5
3c = 8
4if a + b <= c or a + c <= b or b + c <= a:
5    print("Not a triangle")
6elif a == b and b == c:
7    print("Equilateral")
8elif a == b or b == c or a == c:
9    print("Isosceles")
10else:
11    print("Scalene")
Screen
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How many situations are there in rock-paper-scissors for two players?

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Which triangle should be checked first?

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Which tests do you pick for a task with three roads (if, elif, else)?

Remember

  • The plan: read (what's read, what's printed) ➡️ list the cases ➡️ write the ifs ➡️ test every road.
  • With many possibilities draw a table of cases.
  • First the guard (does the data make sense), then the most demanding cases, the else at the end.
  • When looking for the cheapest / best option - work out all of them and hold a tournament of ifs.

Time for real contest tasks! Remember Bajtek's plan: read, list the cases, write, test.

Tasks from the video

We solve them together in the video - pause it and try first on your own.

1Rock, paper, scissors2The kind of triangle3Three numbers in order4How many days has the month?5Tickets for a trip6A meeting on the number line7Parking8The tournament winner

Practice tasks

Try to solve them without the video. Stuck? Every task has hints and an editorial.

9A square in a box10Rounding to tens11Tomorrow12A dice duel

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