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ord() and chr() in Python

ord("A") returns the number code of a character (65). chr(65) turns a number code back into its character ("A"). Ciphers use them to change letters to numbers and back.

Part of: ACARA Digital Technologies Years 9–10 — Cyber Security Threats and Vulnerabilities — Module 121, QCE Digital Solutions Unit 4 — Prototype Digital Data Exchanges — Module 106

Understanding ord() and chr() in Python in ACARA Digital Technologies Years 9–10

What it does: ord("D") is 68 and ord("A") is 65, so the result is 3, the position of D in the alphabet starting from 0. 2 + 65 is 67, and chr(67) is "C", so the position 2 becomes the letter C. ord() turns a character into its number code. The code for capital A is 65. Think of ord as 'order number' of a character. chr goes the other way, from number to character. chr() takes a number and gives a character. ord() needs a character, so ord(72) causes an error. A is 65, so subtracting 65 makes A = 0, B = 1 and D = 3.

Common mistakes: chr() turns a number into a character, so it cannot take the letter "A". To get the number code of a letter, use ord(letter). ord() only accepts a single character. Passing the whole string "HI" causes a TypeError. It should be ord(first), which is the single letter "H".

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This module's key terms are also in a printable crossword (Modules 860–875) and word search (Modules 860–944).

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Learning

Duo

What does ord("A") return in Python?

65"A"

Matches

ord(65)

↓

ord() needs a character such as "A", not a number. To turn 65 into a letter, use chr(65).

Applying

Meanings

Term

ord() and chr()

Meaning

ord("A") returns the number code of a character (65). chr(65) turns a number code back into its character ("A"). Ciphers use them to change letters to numbers and back.

Practice

letter = "D"

Testing

Gaps

print(("A"))

Check Answer

Spots

1letter = "A"

2code = chr(letter)

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Quick Questions

1. What does ord("A") return in Python?

A. 65 ✓

B. "A"

ord() turns a character into its number code. The code for capital A is 65.

2. What does chr(66) return?

A. "B" ✓

B. 66

chr() turns a number code back into a character. Code 66 is capital B.

3. Which function changes a letter into a number?

A. ord() ✓

B. chr()

Think of ord as 'order number' of a character. chr goes the other way, from number to character.

4. Which function changes the number 72 into the letter "H"?

A. ord(72)

B. chr(72) ✓

chr() takes a number and gives a character. ord() needs a character, so ord(72) causes an error.

5. Which line gives the position 0-25 of the letter "D" in the alphabet?

A. ord("D") + 65

B. ord("D") - 65 ✓

A is 65, so subtracting 65 makes A = 0, B = 1 and D = 3.

Match the Pairs

ord("A") → Returns the number code of the character A, which is 65

chr(66) → Returns the character for the number code 66, which is "B"

Convert a letter into a position from 0 to 25 by taking away the code of "A" → ord("D") - ord("A")

Turn a position from 0 to 25 back into an uppercase letter by adding the code of "A" and using chr() → chr(2 + ord("A"))

ord(65) → ord() needs a character such as "A", not a number. To turn 65 into a letter, use chr(65).

chr("A") → chr() needs a number code such as 65, not a letter. To get the code of "A", use ord("A").

Key Term

ord() and chr()

ord("A") returns the number code of a character (65). chr(65) turns a number code back into its character ("A"). Ciphers use them to change letters to numbers and back.

Fill in the Blanks

1. print(ord("A"))

Hint: Type the function that turns a character into its number code.

2. print(chr(65))

Hint: Type the function that turns a number code back into a character.

3. pos = ord("D") - 65 print(pos)

Hint: Type the number code of the first letter of the alphabet, so the position starts at 0.

4. letter = "C" num = ord(letter) - 65 print(num)

Hint: Type the variable name that holds the 0-25 position of the letter.

5. print(chr(ord("A") + 1))

Hint: Type the operator that moves the code up to the next letter.

Spot the Error

Question 1

A letter = "A"

B code = chr(letter)

C print(code)

Line B — chr() turns a number into a character, so it cannot take the letter "A". To get the number code of a letter, use ord(letter).

Question 2

A word = "HI"

B first = word[0]

C code = ord(word)

D print(code)

Line C — ord() only accepts a single character. Passing the whole string "HI" causes a TypeError. It should be ord(first), which is the single letter "H".

Question 3

A pos = ord("C") + 65

B print(pos)

C print(chr(pos + 65))

Line A — To turn a letter into a position from 0 to 25, subtract 65 (the code for "A"). Adding 65 gives 132 instead of 2. It should be ord("C") - 65.

Question 4

A pos = 5

B code = pos + 65

C letter = ord(code)

D print(letter)

Line C — ord() changes a character into a number, but code is already a number. To change a number code into a letter you need chr(code).

Question 5

A letter = "Z"

B pos = ord(letter) - 64

C print(pos)

Line B — "A" has code 65, so subtract 65 to start counting at 0. Subtracting 64 makes Z equal 26 instead of 25. It should be - 65.

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