7. Case
Match-case statements are in python from 3.10.
Python refers to this as structural pattern matching.
Match-case statements can be used for what is known as switch-case statements in general use in other languages.
7.1. Match-case
Multiple cases can be used.
Each case is checked in order until a matching case is found.
The simple pattern in python is:
match subject:
case <pattern_1>:
<action_1>
case <pattern_2>:
<action_2>
An example in python is:
age_flag = False
match age_flag:
case True:
print("Entry permitted")
case False:
print("No entry until you reach 13 years of age.")
Pseudocode. The equivalent pseudocode is:
BEGIN
age_flag ← FALSE
CASE age_flag:
TRUE: OUTPUT "Entry permitted"
FALSE: OUTPUT "No entry until you reach 13 years of age."
ENDCASE
END
7.2. Alternatives
Several alternatives can be used in a single pattern using the pipe symbol for
or: |.grade = "A"
match grade:
case "A+" | "A" | "B+" | "B" | "C+" | "C":
print("Acceptable standard.")
case "D+" | "D" | "NP" | "UG":
print("Retest required.")
Pseudocode. The equivalent pseudocode is:
BEGIN
grade ← "A"
CASE grade:
"A+" | "A" | "B+" | "B" | "C+" | "C": OUTPUT "Acceptable standard."
"D+" | "D" | "NP" | "UG": OUTPUT "Retest required."
ENDCASE
END
7.3. Wildcard
If an exact match is not confirmed, the last case, if provided, will be used as the matching case.
The wildcard
_ is usually used when it is not referred to again in the code block.Another variable name, such as
other, is used if it is referred to in the following case block code.grade = "A-"
match grade:
case "A+" | "A" | "B+" | "B" | "C+" | "C":
print("Acceptable standard.")
case "D+" | "D" | "NP" | "UG":
print("Retest required.")
case _:
print("Not a valid grade")
Pseudocode. The equivalent pseudocode is:
BEGIN
grade ← "A-"
CASE grade:
"A+" OR "A" OR "B+" OR "B" OR "C+" OR "C": OUTPUT "Acceptable standard."
"D+" OR "D" OR "NP" OR "UG": OUTPUT "Retest required."
OTHERWISE: OUTPUT "Not a valid grade"
ENDCASE
END
7.4. Matching tuples for coordinates
Other objects, apart from strings can be matched.
An example with a tuple is below.
point = (2, 3)
match point:
case (0, 0):
print("Origin")
case (0, y):
print(f"{y} on Y axis")
case (x, 0):
print(f"{x} on X axis")
case (x, y):
print(f"{x} on X axis, {y} on Y axis")
case _:
print("Invalid point")
Pseudocode. The equivalent pseudocode is:
BEGIN
point ← (2, 3)
CASE point:
(0, 0): OUTPUT "Origin"
(0, y): OUTPUT f"{y} on Y axis"
(x, 0): OUTPUT f"{x} on X axis"
(x, y): OUTPUT f"{x} on X axis, {y} on Y axis"
OTHERWISE: OUTPUT "Invalid point"
ENDCASE
END
7.5. Pseudocode alternatives
In pseudocde, the code for each case can be on the same line as the case value.
The keywords: OF, OTHERS, can be used.
The OTHERS clause with its default sequence is optional.
The general pattern may be:
CASE expression OF
condition 1 : sequence 1
condition 2 : sequence 2
...
condition n : sequence n
OTHERS : default sequence
ENDCASE
Example:
BEGIN
CASE Title OF
Mr : OUTPUT "Mister"
Mrs : OUTPUT "Missus"
Ms : OUTPUT "Miss"
OTHERS : OUTPUT "Form of address not recognised"
ENDCASE
END
Python. The equivalent python is:
title = "Ms"
match title:
case "Mr":
print("Mister")
case "Mrs":
print("Missus")
case "Ms":
print("Miss")
case other:
print("Form of address not recognised")