← QCE Digital Solutions Unit 2

Aggregating a stream of records

To aggregate is to combine many records into one summary value, such as a running total, a count of matching records, or an average worked out after the loop.

Part of: QCE Digital Solutions Unit 2 — Data and Programming Techniques — Module 58

Understanding Aggregating a stream of records in QCE Digital Solutions Unit 2

What it does: The if limits the counting to matching records, and adding 1 each time combines them into a single count. An aggregate needs a starting value. A total starts at 0 so the first record is added to nothing. With no if, every record adds 1 to the count. The second option only counts records that match the condition. The total and count are only complete once the loop ends. Calculate the average after the loop for the final value. An empty stream leaves count at 0. Check that count > 0 before dividing.

Common mistakes: A running total must start at 0. Starting at 1 makes every final total one too high. It should be total ← 0. The file name is text, so it must be in quotes. It should be marks ← readFile("marks.csv").

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Learning

Duo

A loop adds each record's price to a variable called total. What does this build?

A running total that combines many records into one valueA copy of every record in the stream

Matches

for each record in records total <- 0 total <- total + record.mark

↓

The total is reset to 0 inside the loop, so every pass wipes out the earlier marks and only the last record is kept.

Applying

Meanings

Term

aggregate

Meaning

To aggregate is to combine many records into one summary value, such as a running total, a count of matching records, or an average worked out after the loop.

Practice

records = [("Ava", 88), ("Liam", 65), ("Mia", 95)]

Testing

Gaps

total ←

Check Answer

Spots

1sales ← readFile("sales.csv")

2total ← 1

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Quick Questions

1. A loop adds each record's price to a variable called total. What does this build?

A. A copy of every record in the stream

B. A running total that combines many records into one value ✓

Aggregating turns many records into one summary value. A running total starts at 0 and adds each record's value as the loop goes.

2. Which line sets up a running total correctly BEFORE the loop starts?

A. total <- 0 ✓

B. total <- price

The total must start at 0 before the loop. If it starts at a record's price, the first value is counted twice or the total is wrong.

3. You want to count how many records have mark >= 50. Where does count <- count + 1 go?

A. Outside the loop after it finishes

B. Inside an if mark >= 50 inside the loop ✓

Counting matching records means adding 1 only when the condition is true. That needs an if inside the loop.

4. Which code counts ALL the records in the stream, not just the matching ones?

A. for each record: count <- count + 1 ✓

B. for each record: if mark > 50 then count <- count + 1

With no if, every record adds 1 to the count. The second option only counts records that match the condition.

5. When should the average be worked out for a stream of records?

A. After the loop: average <- total / count ✓

B. Inside the loop on every record: average <- total / count

The total and count are only complete once the loop ends. Calculate the average after the loop for the final value.

Match the Pairs

total <- total + record.mark → Adds each record's mark to a running total as the loop visits every record

average <- total / count → Works out the average after the loop has finished, using the total and the number of records

Count only the records that match a condition by adding 1 to a counter inside an if statement → if record.mark >= 50 then count <- count + 1

Set the running total to 0 before the loop so each record can be added to it → total <- 0

for each record in records total <- 0 total <- total + record.mark → The total is reset to 0 inside the loop, so every pass wipes out the earlier marks and only the last record is kept.

for each record in records total <- total + record.mark average <- total / count → The average is calculated inside the loop, before all records have been added and counted, so it is wrong until the last pass.

Key Term

aggregate

To aggregate is to combine many records into one summary value, such as a running total, a count of matching records, or an average worked out after the loop.

Fill in the Blanks

1. total ← 0 for each sale in sales total ← total + sale.amount

Hint: Type the starting value a running total must have before any records are added.

2. total ← 0 for each sale in sales total ← total + sale.amount

Hint: Type the arithmetic operator that adds each record's amount onto the running total.

3. count ← 0 for each s in students if s.mark >= 50 then count ← count + 1

Hint: Type the number added to the counter each time a record matches the condition.

4. end for average ← total / count print average

Hint: Type the variable holding how many records were seen, which the total is divided by after the loop.

5. total ← 0 for each sale in sales total ← total + sale.amount

Hint: Type the loop keywords that visit every record in the stream one at a time.

Spot the Error

Question 1

A sales ← readFile("sales.csv")

B total ← 1

C for each s in sales

D total ← total + s.amount

E print total

Line B — A running total must start at 0. Starting at 1 makes every final total one too high. It should be total ← 0.

Question 2

A marks ← readFile(marks.csv)

B total ← 0

C for each m in marks

D total ← total + m

E print total

Line A — The file name is text, so it must be in quotes. It should be marks ← readFile("marks.csv").

Question 3

A count ← 0

B for each s in students

C if s.mark >= 50 then

D count ← 1

E print count

Line D — This line sets count to 1 each time instead of adding to it, so the count never goes above 1. It should be count ← count + 1.

Question 4

A total ← 0

B for each m in avg

C total ← total + m

D avg ← total / length(marks)

E print avg

Line B — The loop must go through the list of records, marks. The variable avg has not been created yet. It should be for each m in marks.

Question 5

A total ← ""

B for each o in orders

C total ← total + o.price

D avg ← total / length(orders)

E print avg

Line A — The accumulator must start as the number 0, not text. Adding a number to text will not work. It should be total ← 0.

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