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Lecture 4 Conditions and Logical Expressions Dr. Hebbat Allah A. Elwishy

Lecture 4 Conditions and Logical Expressions Dr. Hebbat Allah A. Elwishy Computer & IS Assistant Professor Email: hbbtllh_elwishy@yahoo.com. Lecture 4 Topics. Using Relational and Logical Operators to Construct and Evaluate Logical Expressions If-Else Statements. Flow of Control.

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Lecture 4 Conditions and Logical Expressions Dr. Hebbat Allah A. Elwishy

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  1. Lecture 4 Conditions and Logical Expressions Dr. Hebbat Allah A. Elwishy Computer & IS Assistant Professor Email: hbbtllh_elwishy@yahoo.com

  2. Lecture 4 Topics • Using Relational and Logical Operators to Construct and Evaluate Logical Expressions • If-Else Statements

  3. Flow of Control • the order in which program statements are executed WHAT ARE THE POSSIBILITIES. . .

  4. Flow of Control • is Sequential unless a “control structure” is used to change that • there are 2 general types of control structures: Selection (also called branching) Repetition(also called looping)

  5. C++ control structures • Selection if if . . . else switch • Repetition for loop while loop do . . . while loop

  6. Control Structures use logical expressions which may include: 6 Relational Operators < <= > >= == != 3 Logical Operators ! && ||

  7. 6 Relational Operators are used in expressions of form: ExpressionAOperatorExpressionB temperature > humidity B * B - 4.0 * A * C > 0.0 abs (number ) == 35 initial != ‘Q’

  8. int x, y ; x = 4; y = 6; EXPRESSIONVALUE x < y true x + 2 < y false x != y true x + 3 >= y true y == x false y == x+2 true

  9. Operator Meaning Associativity ! NOT Right *, / , % Multiplication, Division, Modulus Left + , - Addition, Subtraction Left < Less than Left <= Less than or equal to Left > Greater than Left >= Greater than or equal to Left == Is equal to Left != Is not equal to Left && AND Left || OR Left = Assignment

  10. LOGICAL EXPRESSION MEANING DESCRIPTION ! p NOT p ! p is false if p is true ! p is true if p is false p && q p AND q p && q is true if both p and q are true. It is false otherwise. p || q p OR q p || q is true if either p or q or both are true. It is false otherwise.

  11. Be Careful • What Numbers Satisfy: (x < 1) && (x > 10) (x < 1) || (x > 10) (x < 10) || (x > 1) (x < 10) && (x > 1)

  12. What is the value? int age, height; age = 25; height = 70; EXPRESSION VALUE !(age < 10) ? !(height > 60) ?

  13. Short-Circuit Example int age, height; age = 25; height = 70; EXPRESSION (age > 50) && (height > 60) false Evaluation can stop now because result of && is only true when both sides are true. It is already determined that the entire expression will be false.

  14. More Short-Circuiting int age, height; age = 25; height = 70; EXPRESSION (height > 60) || (age > 40) true Evaluation can stop now because result of || is true if one side is true. It is already determined that the entire expression will be true.

  15. What happens? int age, weight; age = 25; weight = 145; EXPRESSION (weight < 180) && (age >= 20) true Must still be evaluated because truth value of entire expression is not yet known. Why? Result of && is only true if both sides are true.

  16. What happens? int age, height; age = 25; height = 70; EXPRESSION ! (height > 60) || (age > 50) true false Does this part need to be evaluated?

  17. Write an expression for each taxRate is over 25% and income is less than 20000 temperature is less than or equal to 25 or humidity is less than 70% age is over 21 and age is less than 60 age is 21 or 22

  18. Some Answers (taxRate > .25) && (income < 20000) (temperature <= 25) || (humidity < .70) (age > 21) && (age < 60) (age == 21) || (age == 22)

  19. Operator precedence • ! • * / % • + - • < <= >= > • == != • && • ||

  20. If-Else Syntax if ( Expression ) StatementA else StatementB NOTE: StatementA and StatementB each can be a single statement, a null statement, or a block.

  21. if - else provides two-way selection between executing one of 2 clauses (the if clause or the else clause) TRUE FALSE expression if clause else clause

  22. Use of blocks recommended if ( Expression ) { } else { } “if clause” “else clause”

  23. Using the if Statement if ( boolean_expr ) statement1; [else statement2]; General : if (i % 2 == 0) cout << "Even"; else cout << “Odd"; … Examples: if (i % 2 == 0) { cout << i; cout << " is even"; }

  24. Example Write an interactive program that contains an if statements and may be used to compute the area of a square (area = side2 ) or triangle (area = 1/2 x base x height) after prompting the user to type the first character of the required figure name (t for triangle or s for square).

  25. Example #include <iostream> int main() { char c; float b, h, s, area;   cout << " Enter the figure name (t for triangle or s for square) : ";   cin >> c;   if ( c == 't' ) { cout << " Enter the base "; cin >> b; cout << "Enter the height"; cin >> h; area = 1/2 * h * b; cout << " area of triangle = " << area;   }

  26. Example else if ( c == 's') { cout << "Enter the side measure"; cin >> s; area = s * s; cout << " area of square = " << area; } else cout << "" error in figure name"; return 0; }

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