Project 3: a marks report

What you will be able to do

After this lesson you can

• combine a dictionary, the statistics module, sorting and searching in one tool,
• handle an empty class without a crash,
• rank with a two-part key, and
• test the edges of a report.

The plan

A class report has to do four jobs: summarise the marks, rank the students, answer a question such as "who is above the mean?", and search for a mark. The marks live in a dictionary: the key is the name, and the value is the mark.

Step 1: the expected numbers

Five students have the marks Asha 92, Ravi 78, Meera 85, Kabir 64 and Zoya 78. Work out by hand.

• The sum is 92 + 78 + 85 + 64 + 78 = 397, so the mean is 397 divided by 5, which is 79.4.
• In order the marks are 64, 78, 78, 85, 92, so the median is the middle one, 78.
• The mean is a little higher than the median, because the 92 is far above the others.
• The ranking: Asha 92, Meera 85, then Ravi and Zoya with 78 (Ravi first, because R comes before Z), then Kabir 64.
• Above the mean (79.4): Asha and Meera.

Step 2: the report

Predict first. Will the mean be higher or lower than the median? What will the program say about an empty class? Then run it.

Step 3: search for a mark

A sorted list can be searched fast. Binary search looks at the middle and throws away half of the list in each step.

sorted() makes a new list, so the marks as typed stay in their order for the rest of the report.

Step 4: test the edges

The last-but-one row is a surprise: if all the marks are equal, then nobody is more than the mean, so the list of those above is empty. That is the correct answer, but a user may not expect it.

Extend it

Pick one or two, and do them without looking at a solution. Write the expected answers first.

1. A letter grade. Give each student a letter from the mark.
2. The top three. Show the three best marks with the names.
3. A new student. Add Dev with 70, and say who is above the mean now (hand first!).
4. Ranks with ties. Equal marks share a rank, as in the search and sort course.
5. Pass rate. The percentage of students with 40 or more.

Common mistakes

• Dividing by zero (or calling mean) for an empty class. Catch the StatisticsError.
• Forgetting the minus sign and ranking from the lowest.
• Binary search on a list that is not in order.
• Mixing up the mean and the median when someone asks "the average".
• Using the marks and forgetting the names, after sorting only the marks.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Marks report tools. You write the summary and the ranking, and the checker tries them, including an empty class.
2. Build. Open the problem The class report and the problem An empty class.
3. Stretch. Open the problem The ranking with a tie and the problem Binary search in the marks.

Remember these

• A dictionary of marks: the name is the key.
• statistics gives the mean, the median and the standard deviation. An empty list raises StatisticsError.
• sorted(..., key=...) ranks, and a minus sign reverses a number.
• Binary search needs a sorted list.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is -pair[1] used in the key of the ranking?

2. How many marks must a sorted list have, at most, for binary search to need 3 steps?

3. What does the summary give for an empty class?