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F3.6 Records

Strings, lists and records · GCSE · OCR J277 2.2.3, AQA 8525 3.2.6, Edexcel 1CP2 6.3.1 · about 15 min

BugBotLab

What this lesson is about

Grouping related fields: a sensor reading as a record, and a list of records as a table.

Questions 5 marks in all

  1. [1 mark]What makes a record different from an array?

    1. AA record's fields can hold different data types, reached by name
    2. BA record can only hold numbers
    3. CA record is always sorted
    4. DAn array can hold different types, a record cannot
    Answer: A. An array holds items of one type by index; a record groups related fields of any types by name.

  2. [1 mark]What does this program print?

    reading = {"heading": 90, "distance": 51.0}
reading["heading"] = 180
print(reading["heading"], reading["distance"])
    Answer:
    180 51.0

    A field is reached by its name, and can be changed like a variable.

  3. [1 mark]What does this program print?

    log = [{"h": 0, "d": 31}, {"h": 90, "d": 51}, {"h": 180, "d": 15}]
best = log[0]
for r in log:
    if r["d"] > best["d"]:
        best = r
print(best["h"])
    Answer:
    90

    Keeping the whole record means the heading of the largest distance comes with it.

  4. [1 mark]A robot log stores a time, a heading, a distance and a note for each reading. Which data types suit the fields?

    Tick every answer that is true.

    1. Atime: real
    2. Bheading: integer
    3. Cdistance: real
    4. Dnote: string
    Answer: A, B, C, D. All four are sensible: that mix of types in one record is exactly what records are for.

  5. [1 mark]In a table of records, what is each row?

    1. AOne record
    2. BOne field
    3. COne data type
    4. DOne array index
    Answer: A. Each record is a row; each field is a column.

The task: reading records

The robot is boxed in on three sides. Look in four directions, 90 degrees apart, storing each look as a record with the fields heading and distance in a list. Then print one line per record as heading 0: 31.0 cm, and finally print closest heading: <heading> for the record with the smallest distance.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

log = []
for i in range(4):
    turn_right(30, angle=90)

The hint students can ask for: Store each reading as a record with the direction it was taken in, and add it to a list before turning. Afterwards, loop over the list to print, and keep the record with the smallest distance.

A solution

from bugbot import *
connect()
log = []
for i in range(4):
    log.append({"heading": i * 90, "distance": distance()})
    turn_right(30, angle=90)
for r in log:
    print(f"heading {r['heading']}: {r['distance']} cm")
closest = log[0]
for r in log:
    if r["distance"] < closest["distance"]:
        closest = r
print("closest heading:", closest["heading"])

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.