Week 4 Lec 1 PDF

Selection 1  Learning Outcomes At the end of this lecture, you should be able to: 1. Explain the concept of selection control structure 2. Use relational and logical operators 3. Use if, if..else, if..else if, switch..case, and nested if statements and condit...

Selection 1  Learning Outcomes At the end of this lecture, you should be able to: 1. Explain the concept of selection control structure 2. Use relational and logical operators 3. Use if, if..else, if..else if, switch..case, and nested if statements and conditional operators in program. 2  Control Structures Program is written using simple control structures to determine the flow of program execution. Three types of control structure: sequential, selection, repetition. SEQUENTIAL – is a series of statements that execute one after another. statement 1 statement 2 statement 3 …. 3  Control Structures SELECTION (branch) - is used to execute one set of statements if a given condition is TRUE, a different set of statements, or no statements at all, is executed if the condition is FALSE. statement set 1 …. condition …. statement set 2 4  3. SYSTEM DESIGN continued… Control Structures REPETITION (loop) - is used to execute a set of statements repeatedly as long as a given condition is TRUE. …. Condition …. statement set 5 The logic flow of Control Structures Sequential false ? true ? : boolean Selection expression false ? ? : boolean Repetition true expression 6  Why Selection? Selection structure allows the program to select an action (statements to execute), based upon a condition. For example, by using an if statement to check the password entered, your program can decide whether or not a user is allowed access to the program. The condition in this example is a valid password. Condition is evaluated (checked and determined) to be either true or false. 7  Understand TRUE and FALSE The operator will assign 1 if the evaluation is true, or 0 if the evaluation is false. true or false is known as boolean value (bool). An expression (program code that has a value) that has a boolean value is called a boolean expression. Thus, a condition is a boolean expression. 8  Boolean Expression There are a number of operators that allow checks on condition. There are two types of operators that could be used in boolean expression: relational operators and logical operators. Expression with a relational operator is called relational expression, and with logical operator is called logical expression. Relational and logical expressions are both boolean expressions, and can be used as condition. 9  Relational Operators Used to compare between two values. Operator Meaning Relational Result Expression < less than 1< 2 true 2 false >= greater than or equal to 1>=2 false == equal to 1==2 false != not equal to 1!=2 true 10  Relational Operators More examples: Assume int y=6, x=5; Relational Sample Relational Value Operators Expression > y>x T < y= x>=3 T = y ) && ( x ! = 3) 14  Logical Operator || (OR) Example : Assume int x = 4, y = 5, z = 8; expression1 expression2 expression1 | | Sample Expression expression2 F F F ( y > 10) | | ( z < = x ) F T T ( z < = y) | | ( x = = 4) T F T ( y ! = z) | | ( z < x ) T T T (z>=y)||(x!=3) 15 Logical Operator ! (NOT) Example : Assume int x = 50 expression ! expression Sample Expression F T ! (x = = 60) T F ! (x ! = 60) 16 Precedence of Logical Operator Highest ! && Lowest || Example: (2 < 3) || (5 > 6) && (7 > 8) 1 2 is true because AND is evaluated before OR 17  More on Operator Precedence Highest Arithmetic operators Relational operators Lowest Logical operators Example: 8 < 2 + 7 || 5 == 6 is true 1 2 3 4 16  Boolean Expressions Can be assigned to a variable Example : bool result = (x = 0 && grade