Selection Structures II

Transcription

1 Lecture 7 Selection Structures II Nested if and switch statements CptS 121 Summer 2016 Armen Abnousi

2 If () if() Nested if statements An if- statement can be placed inside another if- statement. Every matches with the closest if statement above itself, unless the programmer uses curly braces to change this order. (another reason for indentation) If () if()

3 if () if() Nested if statements An if- statement can be placed inside another if- statement. Every matches with the closest unmatched if statement above itself, unless the programmer uses curly braces to change this order. (another reason for indentation) if () if() if () if() if() if()

4 Nested if if ( roadstatus == w ) /*road is wet*/ if ( temp > freezing ) printf( road is wait. Drive slowly ); printf( safe to drive );

5 Programming! Write a program that gets student s grade in scale and prints the grade point based on the chart below. Grade percentage Letter grade A B C D F

6 Compute the Letter Grade Program Nested if- statements: Sequence of if statements: if ( gradepoint >= 94 ) lettergrade = A ; if ( gradepoint >= 81 ) lettergrade = B ; if ( gradepoint >= 65 ) lettergrade = C ; if ( gradepoint >= 50 ) lettergrade = D ; lettergrade = F ; If ( 94 <= gradepoint ) lettergrade = A ; If ( 81 <= gradepoint && gradepoint < 94 ) lettergrade = B ; If ( 65 <= gradepoint && gradepoint < 81 ) lettergrade = C ; If ( 50 <= gradepoint && gradepoint < 65 ) lettergrade = D ; If ( gradepoint < 50 ) lettergrade = F ;

7 Nested If s, sequence of if s, and multiple alternative if s Nested if: We can use an if statement inside another if statement. A nested if can be written as multiple separate if statements. This is not efficient. Why? If each false task () is followed by an if-, use multiple-alternative if. This adds to readability and reduces confusion:

8 Nested If s, sequence of if s, and multiple alternative if s Nested if: We can use an if statement inside another if statement. A nested if can be written as multiple separate if statements. This is not efficient. Why? Sequence of if s is slower to execute. Computer will check all conditions, even if it already knows the letter grade. If each false task () is followed by an if-, use multiple-alternative if. This adds to readability and reduces confusion:

9 Multiple-alternative ifs If each false task () is followed by an if-, use multiple-alternative if. This adds to readability and reduces confusion: if( expression1 ) If exp1 evaluates to true this block executes. And then the program jumps to after the last block. if( expression2 ) If exp1 evaluates to false but exp2 evaluates to true, this block executes. And then the program jumps to after the last block. If none of the expressions for if s above evaluate to true, this block executes.

10 Mulitple-alternative if Grade letter computer: if ( 94 <= gradepoint ) lettergrade = A ; if ( 81 <= gradepoint ) lettergrade = B ; if ( 65 <= gradepoint ) lettergrade = C ; if ( 50 <= gradepoint ) lettergrade = D ; gradepoint = F ;

11 Switch Statment We saw that nested if s are not readable. The deeper the nest, the less readable the code. If the nested if depends only on one value (of type int or char), use switch statement. switch( expression ) First the expression is evaluated case label1 : statements to do break; If expression evaluates to label1, this block of codes is executed. Otherwise jumps to the next case! case label2 : statements to do default : break; If the program reaches a break, it exits the switch case statement. statements to do last case. If none of the cases evaluate to expression, the statements in default execute.

12 Switch Statements Break is a reserved word in C When reaching break, C exits the current switch statement. Break is optional but most of the times we need to use it. (what will happen if we don t use the break?) Default is the case that will evaluate to true if all other cases are false. Default is optional. Multiple cases can have one set of statements following them.

13 Programming A tennis player decides if he will go for a tennis session based on the following factors: Outlook, Wind and Humidity. The picture below shows his decision process. We are automating this process to make predictions about his schedule. Write a program to check the decision tree for the tennis player behavior prediction.

14 Switch ( outlook ) case o : case O : play = y ; break; case s : case S : if ( humidity > humiditythreshold ) play = n ; play = y ; break; case r : case R : if ( wind > windthreshold ) play = n ; play = y ; break; default : play = n ; Switch Case, example

15 Flowchart* Flowcharts are visual models used in software engineering to represent the flow of operation. Yes or True Display it s warm outside Temp > 32 No or False Display it s freezing * Derived from Andy O Fallon s lecture notes for CptS 121

16 Flowchart Use diamonds to show decision steps (ifs) Use oval to show start or end of the flow (algorithm). Use rectangle for executable statements (sometimes I/O is shown with a parallelogram)

17 References J.R. Hanly & E.B. Koffman, Problem Solving and Program Design in C (8thed.), Pearson, 2016 Andy O Fallon s lecture notes for CptS121 (