Answers

F3.2 Character codes and conversion

Strings, lists and records · GCSE · OCR J277 2.2.2, AQA 8525 3.2.8, Edexcel 1CP2 6.3.3 · about 15 min

What this lesson is about

ord and chr, ASCII, comparing strings, and a Caesar cipher.

• ord and chr
• Letters are in order
• Comparing strings
• Turning digits into numbers, and back
• A secret message

Questions 7 marks in all

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

   print(ord("A"), ord("a"))

   Answer:

   65 97

   In ASCII, A is 65 and lower case starts 32 later, at 97.

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

   print(chr(66) + chr(111) + chr(116))

   Answer:

   Bot

   chr turns each code back into its character: B, o, t.

3. [1 mark]ord("A") is 65. What is ord("E")?

   Answer: 69. The capital letters are in order, so E is 4 after A.

4. [1 mark]Why is "Zebra" < "apple" True?

   1. ACapital Z has a smaller character code than lower-case a
   2. BZebra is a shorter word
   3. CPython sorts words backwards
   4. DIt is False
   Answer: A. Strings compare by character code, and every capital letter comes before every lower-case letter.

5. [1 mark]What does this program print?

   letter = "Y"
   code = (ord(letter) - ord("A") + 3) % 26
   print(chr(code + ord("A")))

   Answer:

   B

   Y is letter 24; 24 + 3 is 27, and 27 % 26 is 1, which wraps round to B.

6. [1 mark]Which AQA pseudo-code function gives the code for a character?

   1. ACHAR_TO_CODE
   2. BASC
   3. Cord
   4. DSTRING_TO_INT
   Answer: A. AQA uses CHAR_TO_CODE and CODE_TO_CHAR. OCR's Reference Language uses ASC and CHR; Python uses ord and chr.

7. [1 mark]What does this program print?

   print("42".isdigit(), "4two".isdigit())

   Answer:

   True False

   isdigit is True only when every character is a digit.

The task: shift the letters

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

message = input("Message? ")
secret = ""

from bugbot import *
connect()
message = input("Message? ")
secret = ""
for letter in message:
    code = (ord(letter) - ord("A") + 1) % 26
    secret = secret + chr(code + ord("A"))
print(secret)