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F7.1 Reading and writing files

Files and databases · GCSE · OCR J277 2.2.3, Edexcel 1CP2 6.4.2 · about 15 min

BugBotLab

What this lesson is about

Open, read, write, append and close: a run log in a CSV file.

Questions 6 marks in all

  1. [1 mark]Put the steps for reading a file in order.

    Number the lines 1 to 3 to put them in the right order.

    1. Open the file
    2. Read from the file
    3. Close the file
    Answer:
    Open the file
Read from the file
Close the file

    Open, use, close: the same in every language.

  2. [1 mark]What does opening a file with mode "w" do to a file that already exists?

    1. AEmpties it before writing
    2. BAdds to the end of it
    3. CReads it
    4. DNothing until close is called
    Answer: A. "w" starts the file again; use "a" to add to the end.

  3. [1 mark]Which mode adds new lines to the end of a file and keeps what was there?

    1. A"a"
    2. B"w"
    3. C"r"
    4. D"x"
    Answer: A. a stands for append.

  4. [1 mark]What does this program print?

    line = "3,square,80,9.4\n"
fields = line.strip().split(",")
print(fields[1], float(fields[2]) + 1)
    Answer:
    square 81.0

    strip removes the newline, split makes a list of strings, and float converts the distance.

  5. [1 mark]Why use with open(...) as f:?

    1. AThe file is closed automatically at the end of the block
    2. BIt reads the file faster
    3. CIt makes the file bigger
    4. DIt is the only way to write
    Answer: A. Even if an error happens inside the block, the file is closed.

  6. [1 mark]Why store data in a file rather than only in variables?

    1. AVariables are lost when the program stops; a file keeps the data
    2. BFiles are faster than variables
    3. CVariables cannot hold numbers
    4. DFiles cannot be changed
    Answer: A. A file keeps data between runs of the program.

The task: log the run

Drive forward 25 cm, timing it with clock(). Then append a line to runs.csv for this run: run number 4, task wall, 25 cm, and the seconds rounded to one decimal place, in the same format as the other lines. Finally read the file and print 4 runs logged, counting the lines after the header rather than typing 4. The task starts from the original four-line runs.csv every time it runs, whatever earlier runs added to the copy above.

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

start = clock()
forward(50, distance=25)

The hint students can ask for: Time the run by taking a clock reading before and after. Open the file so that writing adds to the end rather than replacing it, and remember the header line when you count the rows afterwards.

A solution

from bugbot import *
connect()
start = clock()
forward(50, distance=25)
seconds = round(clock() - start, 1)
with open("runs.csv", "a") as f:
    f.write("4,wall,25," + str(seconds) + "\n")
with open("runs.csv") as f:
    lines = f.read().splitlines()
print(len(lines) - 1, "runs logged")

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