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Multi Way Selection • • You can choose statement(s) to run from many sets of choices. There are two cases for this: (a) Multi way selection by nested IF structure (b) Multi way selection by SWITCH structure Multi Way Selection by Nested IF Structure • The structure that contains another structure of the same type is called a nested structure. • In the Nested IF structure, the statements that exists between IF and ELSE or between IF and END IF can contain IF statement. Multi Way Selection by Nested If Structure .. Cont. • Syntax of one possible structure: IF (condition1) THEN Statements1 ELSE IF (condition2) THEN Statements2 ELSE IF (Condition3) THEN Statements3 ELSE IF (Condition4) THEN Statements4 END IF END IF END IF Note: The nest can be to many levels. The following figure shows the execution of this structure. Multi Way Selection by Nested If Structure .. Cont. True Statements1 Condition1 False True Statements2 Condition2 Rest of algorithm False True Condition3 Statements3 False Statements4 Multi Way Selection by Nested If Structure .. Examples • Example 4: Write an algorithm that inputs a student mark and outputs the corresponding grade, where grades are as follows: mark grade 90-100 A 80-89 B 70-79 C 60-69 D < 60 E Example 4 .. Cont. 1- Analysis stage: • Problem Input: - student’s mark, mark • Problem Output: - grade • Criteria - according to the previous grade table Example 4 .. Cont. 2- Algorithm Design ALGORITHM Grades INPUT mark IF ( mark < 0 OR mark > 100 ) THEN OUTPUT “ Mark out of range” ELSE IF ( mark 90 AND mark 100 ) THEN OUTPUT “A” ELSE IF ( mark 80 ) THEN OUTPUT “B” ELSE IF ( mark 70 ) THEN OUTPUT “C” ELSE IF ( mark 60 ) THEN OUTPUT “D” ELSE OUTPUT “E” END IF END IF END IF END IF END IF END Grades Comparison of Nested IF Structure and a Sequence of IF Structures • • Some times the nested IF structure is a good solution for a given problem than a sequence of IF structures. Consider the following example: A) Sequence case: IF ( x > 0 ) THEN pos_count pos_count + 1 END IF IF ( x < 0 ) THEN neg_count neg_count + 1 END IF IF ( x = 0 ) THEN zero_count zero_count + 1 END IF (B) Nested case: IF ( x > 0 ) THEN pos_count pos_count + 1 ELSE IF ( x < 0 ) THEN neg_count neg_count + 1 ELSE zero_count zero_count + 1 END IF END IF • In the previous example, only one statement should be executed for any value of x. • In the sequence case, the sequence does not show clearly that exactly one of the three statements is executed for any value of x. • In the nested case, only one statement is executed exactly. Therefore, it is better than the sequence case. Multi Way Selection by SWITCH Structure • Note that the nest can be done to many levels. • Practically, it is not preferable to have more than 3 nested levels. • The SWITCH control structure is the substitute for the nested IF structure of many levels of nesting. • The SWITCH control structure is used to select one of several paths. It is especially useful when the selection is based on the value of a single variable or a simple expression (called the case selector). • The case selector may be an integer, character, or Boolean variable or expression. Multi Way Selection by SWITCH Structure • Syntax SWITCH (selector) CASE label1: Statements1 BREAK CASE label2: Statements2 BREAK . . . DEFAULT: Statements_n END SWITCH Multi Way Selection by SWITCH Structure .. Cont. • The semantics (execution) of this statement: - If the value of “selector” equals to one of the labels values, the statements of that case are executed and the execution continues to the statement after END SWITCH. - If the value of the “selector” does not match with any of the labels values, the statements in the DEFAULT case are executed and the execution continues to the statement that follows END SWITCH. - Note that if you remove BREAK from the cases, the statements of the matched case are executed and the execution proceed to the cases that follow. - The following figure shows this execution: Multi Way Selection by SWITCH Structure .. Cont. selector label1 Statements1 BREAK DEFUALT label2 Statements2 BREAK Rest of algorithm … Statements_n BREAK Multi Way Selection by SWITCH Structure Examples • Example 5: Write an algorithm that inputs a character and displays a suitable musical note (i.e. do, re, mi, etc.) 1- Analysis stage: • Problem Input: - a character, musical_note Problem Output: - a message showing the corresponding musical note Multi Way Selection by SWITCH Structure Examples.. Cont. 2- Algorithm Design ALGORITHM Music INPUT musical_note SWITCH (musical_note) CASE ‘c’ : OUTPUT “ do “ BREAK CASE ‘d’ : OUTPUT “ re “ BREAK CASE ‘e’ : OUTPUT “ mi “ BREAK CASE ‘f’ : OUTPUT “ fa “ BREAK CASE ‘g’ : OUTPUT “ sol “ BREAK CASE ‘a’ : OUTPUT “ la “ BREAK CASE ‘b’ : OUTPUT “ ti “ BREAK DEFAULT: OUTPUT “ Invalid note was read “ END SWITCH END Music Multi Way Selection by SWITCH Structure Examples.. Cont. • Example 6 Write an algorithm to use SWITCH statement to present menu asking the user to enter a specified letter to perform the corresponding task. The algorithm is to calculate the area of the circle if the letter a is read, and to calculate the circumference if letter c is read. 1- Analysis stage: • Problem Input: - radius of the circle - a character to perform a specified task • Problem Output: depending on the character read, the output is: - area of the circle or - circumference of the circle Multi Way Selection by SWITCH Structure Examples.. Cont. 2- Algorithm Design ALGORITHM Circle OUTPUT “ Enter the radius of a circle: “ INPUT radius OUTPUT “ Enter a to calculate the area of a circle or c to calculate its circumference:” INPUT ch SWITCH (ch) CASE ‘a’ : area 3.14 * radius * radius OUTPUT “ Area = “ , area BREAK CASE ‘c’ : circum 2 * radius * 3.14 OUTPUT “ Circumference = “ , circum BREAK DEFAULT: OUTPUT “ Invalid letter was read “ END SWITCH END Circle