Lecture 5: Making Decisions

Transcription

1 Lecture 5: Making Decisions

2 Topics 5.1 Relational Operators 5.2 The if Statement 5.3 The if/else Statement 5.4 Logical Operators 5.5 Examples 4-2

3 3

4 5.1 Relational Operators Used to compare numbers to determine relative order Operators: > Greater than < Less than >= Greater than or equal to <= Less than or equal to == Equal to!= Not equal to 4-4

5 Relational Expressions Relational expressions are Boolean (i.e., evaluate to true or false) Examples: 12 > 5is true 7 <= 5is false if x is 10, then x == 10is true, x!= 8is true, and x == 8is false 4-5

6 Relational Expressions Can be assigned to a variable boolresult = (x <= y); Assigns 0for false, 1for true Do not confuse = (assignment) and == (equal to) 4-6

7 5.2 The ifstatement Allows statements to be conditionally executed or skipped over Models the way we mentally evaluate situations If it is cold outside, wear a coat and wear a hat. 4-7

8 Format of the ifstatement if (condition) { statement1; statement2; statementn; } The block inside the braces is called the body of the ifstatement. If there is only 1 statement in the body, the {}may be omitted. No ; goes here ; goes here 4-8

9 How the ifstatement Works If (condition)is true, then the statement(s) in the body are executed. If (condition)is false, then the statement(s) are skipped. 4-9

10 ifstatement Flow of Control condition true false 1 or more statements 4-10

11 Example ifstatements if (score >= 60) cout << "You passed.\n"; if (score >= 90) { grade = 'A'; cout << "Wonderful job!\n"; } 4-11

12 ifstatement Notes Do not place ;after (condition) Don't forget the { } around a multi-statement body Place each statement;on a separate line after (condition), indented 0 is false;any other value is true 4-12

13 What is trueand false? An expression whose value is 0 is considered false. An expression whose value is non-zero is considered true. An expression need not be a comparison it can be a single variable or a mathematical expression. 4-13

14 Comparisons with floating-point numbers It is difficult to test for equality when working with floating point numbers. It is better to use greater than,less than tests, or test to see if value is very close to a given value 4-14

15 5.3 The if/elsestatement Allows a choice between statements depending on whether (condition) is true or false Format: if (condition) { statement set 1; } else { statement set 2; } 4-15

16 How the if/elseworks If (condition) is true, statement set1is executed and statementset2is skipped. If (condition) is false, statement set1is skipped and statementset2is executed. 4-16

17 if/elseflow of Control true condition false statement set 1 statement set

18 Example if/elsestatements if (score >= 60) cout << "You passed.\n"; else cout << "You did not pass.\n"; 4-18

19 5.4 Logical Operators Used to create relational expressions from other relational expressions Operators, Meaning, and Explanation && AND New relational expression is true if both expressions are true OR New relational expression is true if! NOT either expression is true Reverses the value of an expression; true expression becomes false, false expression becomes true 4-19

20 Truth Tables

21 Logical Operator Examples int x = 12, y = 5, z = -4; (x > y) && (y > z) true (x > y) && (z > y) false (x <= z) (y == z) false (x <= z) (y!= z) true!(x >= z) false 4-21

22 Logical Precedence Highest! && Lowest Example: (2 < 3) (5 > 6) && (7 > 8) is true because AND is evaluated before OR 4-22

23 More on Precedence Highest Lowest arithmetic operators relational operators logical operators Example: 8 < == 6 is true 4-23

24 Checking Numeric Ranges with Logical Operators Used to test if a value is within a range if (grade >= 0 && grade <= 100) cout << "Valid grade"; Can also test if a value lies outside a range if (grade < 0 grade > 100) cout << "Invalid grade"; Cannot use mathematical notation if (0 <= grade <= 100) //Doesn t //work! 4-24

25 Examples Ex 5-1: Write a program that checks whether a given integer by user is divisible by both 5 and 7, without a remainder. Ex 5-2: Write a program that reads from the console three numbers of type intand prints their sum, average, largest, and smallest. 25

26 Examples Ex 5-3: Write a program that checks for a given point {x, y} if it is within the circle K({0, 0}, R=5). Explanation: the point {0, 0} is the center of the circle and 5 is the radius. Hint: Use Pythagorean theory. END OF LECTURE 26