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The CASE logic structure
This slide show describes the CASE logic
structure, its implemenation in C as the switch
statement, and how flowcharts are used to
document them.
Vocabulary:
case
default
switch
Christine S. Wolfe
Ohio University Lancaster
2008-Aug-01
In the IF miniLesson, we considered the problem below:
Calculate the interest charged on an invoice. The interest rate is
determined by the customer's credit status as determined by the vendor
as shown in the chart below:
Credit Status
Interest Rate
A
5%
B
10%
C
12%
Example 1: Customer Bob has a credit status of A and orders $1000.00 of
merchandise. The interest charged on his order is $50.00 (1000.00 * 0.05)
Example 2: Customer Alan has a credit status of C and orders $500.00 of
merchandise. The interest charged on his order is $60.00 (500.00 * 0.12)
©Christine S. Wolfe
In the earlier example, we developed an algorithm that included a series of IF
statements.
START
GET CreditStatus
GET TotalMerchandise
if CreditStatus == 'A'
CALCULATE Interest = TotalMerchandise * 0.05
end if
if CreditStatus == 'B'
CALCULATE Interest = TotalMerchandise * 0.10
end if
if CreditStatus == 'C'
CALCULATE Interest = TotalMerchandise * 0.12
end if
PRINT Interest
STOP
Notice that each if statement compares the variable CreditStatus to one of its
valid values.
main()
GET
CreditStatus
GET Total
Merchandise
IF
CreditStatus
== 'A'
N
A
B
IF
CreditStatus
== 'B'
IF
CreditStatus
== 'C'
N
Y
Y
Interest =
TotalMerchandise
* 0.10
Interest =
TotalMerchandise
* 0.12
B
PRINT
Interest
Y
Interest =
TotalMerchandise
* 0.05
A
STOP
N
Like the credit problem above, many solutions involve matching a
variable to one of a set of possible values.
These can always be solved by a series of IF statements. Each IF
statement compares the variable to one of the possible values.
A shortcut for this technique is the use of the CASE logic structure.
Many programming languages use the CASE logic structure. Your
textbook calls it a SWITCH because the word, switch, is the keyword
used in ANSI C. Most other languages will use the keyword, case. We
will use both terms to mean the exact same type of logic structure.
The CASE logic structure is used to execute one set of code out of
several options.
Let's look at how the customer interest problem can be solved with a CASE
structure. Here is the IF based solution. Click to see the CASE based
solution.
START
GET CreditStatus
GET TotalMerchandise
if
CreditStatus
== 'A'
CASE
CreditStatus
CALCULATE
case 'A': Interest = TotalMerchandise * 0.05
end if
CALCULATE Interest = TotalMerchandise * 0.05
if CreditStatus
== 'B'
case 'B':
CALCULATE Interest
= TotalMerchandise
* 0.10
CALCULATE
Interest = TotalMerchandise
* 0.10
end if
case 'C':
if CreditStatus == CALCULATE
'C'
Interest = TotalMerchandise * 0.12
CALCULATE Interest = TotalMerchandise * 0.12
end case
end if
PRINT Interest
STOP
When execution reaches the case statement, the key variable is compared to the first
case clause.
If the value of the key variable is equivalent to the value of the case clause, then the
instructions after the case are executed.
If the values of the key variable and the case clause do NOT match, then the key variable
is compared to the next case clause.
This process continues until either: 1) a matching case clause is found
or 2) no more case clauses exist.
If no more case clauses exist then execution looks for a default clause. If a default clause
is there, the instructions after the default are executed.
In any event, whether a clause is executed or not, execution continues with the first
statement after the end of the clause statement.
START
GET CreditStatus
GET TotalMerchandise
CASE CreditStatus
case 'A':
CALCULATE Interest = TotalMerchandise * 0.05
case 'B':
CALCULATE Interest = TotalMerchandise * 0.10
case 'C':
CALCULATE Interest = TotalMerchandise * 0.12
end case
PRINT Interest
STOP
Here are the 2 solutions so you can compare them and see that they give the same
result for any given Credit Status.
START
GET CreditStatus
GET TotalMerchandise
CASE CreditStatus
case 'A':
CALCULATE Interest = TotalMerchandise * 0.05
case 'B':
CALCULATE Interest = TotalMerchandise * 0.10
case 'C':
CALCULATE Interest = TotalMerchandise * 0.12
end case
PRINT Interest
START
STOP
GET CreditStatus
GET TotalMerchandise
if CreditStatus == 'A'
CALCULATE Interest = TotalMerchandise * 0.05
end if
if CreditStatus == 'B'
CALCULATE Interest = TotalMerchandise * 0.10
end if
if CreditStatus == 'C'
CALCULATE Interest = TotalMerchandise * 0.12
end if
PRINT Interest
STOP
Flowchart for Customer Interest Payment
C. S. Wolfe
01-Aug-2008
START
A
B
GET
CreditStatus
CASE OF
CreditStatus
PRINT
Interest
'A'
GET Total
Merchandice
'B'
A
'C'
B
Interest =
TotalMerchandice
* 0.05
Interest =
TotalMerchandice
* 0.10
Interest =
TotalMerchandice
* 0.12
STOP
Flowcharting a CASE (aka SWITCH) statement
Preferred method
Alternative method
CASE OF
variable
value1
value2
value3
default
statements
executed when
variable ==
value1
statements
executed when
variable ==
value2
statements
executed when
variable ==
value3
statements
executed when
variable != any
value listed
CASE OF
variable
value1
value2
value3
default
statements
executed when
variable ==
value1
statements
executed when
variable ==
value2
statements
executed when
variable ==
value3
statements
executed when
variable != any
value listed