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Chapter 7 Conditional Statements

Chapter 7 Conditional Statements. 7.1 Conditional Expressions. Condition – any expression that evaluates to true/false value <expr1> <relational_operator> <expr2> Relational operators are BINARY X < 4 Y == a+b q+1 != s*2. 7.1.1 Relational Operators.

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Chapter 7 Conditional Statements

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  1. Chapter 7Conditional Statements

  2. 7.1 Conditional Expressions • Condition – any expression that evaluates to true/false value • <expr1> <relational_operator> <expr2> • Relational operators are BINARY X < 4 Y == a+b q+1 != s*2

  3. 7.1.1 Relational Operators * Single equal sign (=) is an assignment / does NOT compare values

  4. 7.1.1 Relational Operators constint CONST_iEXP = 9; int iexp1 = 0, iexp2 = 5; floatfexp = 9.0; char cexp = 'a'; bool result = true; result = iexp1 == 0;// true result = iexp2 >= iexp1;// true result = iexp1 > CONST_iEXP;// false result = fexp == CONST_iEXP;// true result = cexp <= iexp1;// false result = iexp1 != iexp2;// true result = cexp == 'a';// true

  5. 7.1.1 Relational Operators // ----- ILLEGAL OR MALFORMED CONDITIONS ---- result = 3 < X < Y; // Malformed condition. What does it mean? // 3 < X and X < Y ??? // 3 < X or X < Y ??? cexp == "a"; // Illegal. Attempting to compare a character// to a string literal.

  6. 7.1.2 Logical Operators • Logical operators - combine bool expressions • Operators: or: x || y (binary operator) • false only when both operands are false and: x && y (binary operator) • true only when both operands are true not: !x (unary operator) • false when operand is true; otherwise true.

  7. 7.1.2 Logical Operators • Truth table-displays Boolean results produced when the operator is applied to specified operands • Logical AND and OR truth table

  8. 7.1.2 Logical Operators • Logical NOT truth table • Order of precedence • ! (not) • && (and) • || (or)

  9. 7.1.2 Logical Operators • Misc Information: • Parentheses change the precedence • Parentheses can help clarify complicated conditions • Short-circuit evaluation - once the outcome of condition can be determined, evaluation ends

  10. 7.1.2 Logical Operators Various logical operators int iexp1 = 0, iexp2 = 5; float fexp = 9.0; charcexp = 'a'; constintCONST_iEXP = 9; bool result; result = iexp1 < iexp2 && fexp == 9.0; result = iexp1 > CONST_iEXP || fexp == 9.0;// true result = !(fexp == 9.0 || iexp1 > CONST_iEXP);// false // Short-Circuit Evaluation result = fexp == 9.0 || iexp1 > CONST_iEXP; // true

  11. 7.2 The if Statement • Selects actions to be taken only when a specific condition is satisfied • Syntax:if ( <condition> ) • <action> • Example:if ( N % 2 == 1) • cout << N << “ is ODD” << endl;

  12. 7.2 if (<condition>) <action> • <condition> -a valid expression that can be interpreted as a TRUE/FALSE value • Relational expression (e.g., x <= 2) • Logical expression ( x>5 && y != 2) • Integer-valued expression: 0=false; otherwise true. • <action> -a valid C++ statement or block • Single statement terminated with semicolon • Compound statement (block) enclosed in set braces { } • if (Age > 21) if (Income > 24000) • Sell_Beer = true; { • SSTax = 0.05 * Income; • FedTax = 0.075 * Income; • }

  13. 7.2 The if Statement • if (condition) statement; // Example 1 if ( test >= 80 && test < 90 ) cout << "You have earned a B" << endl; // Example 2 – relational expression/compound statement. if( test >= 90 ) { cout << "You have earned an A" << endl; cout << "Excellent work!" << endl; } // Example 3 – logical expression/compound statement. if( test >= 70 && test < 80 ) { cout << "You have earned a C" << endl; }

  14. 7.2.1 The else Statement • Optional part of an if statement • Can’t stand alone • Must be associated with an openif • if( <condition> ) • <action 1> // Consequence (true-part) • else • <action 2> // Alternative (false-part) • Means: “otherwise”, “catch-all”, “none of the above”

  15. 7.2.1 The else Statement • else <action> // consequence. • no condition or expression associated with it • relies on the value of the condition associated with the previousif(or chain of ifs) • executes action(s) only if the previous condition (or chain of ifs) is false • if more than one statement, the action must be enclosed in curly braces

  16. 7.2.1 The if-else Statement else Example if ( grade >= 60 ) pass = true; else { pass =false; cout << "Hope you do better next time" << endl; }

  17. 7.2.1 Multiple alternative if The alternative of an if is another decision: if( avg >= 90 ) cout << “Grade = A" << endl; else if ( avg >= 80 ) cout << "B" << endl; -------- NOTE indentation. if( avg >= 90 ) cout << “Grade = A" << endl; else if ( avg >= 80 ) cout << "B" << endl;

  18. 7.2.1 Selecting from multiple alternatives • Only ONE action is selected from a chain of decisions. • if ( Avg >= 90 ) • Grade = ‘A’; • else if ( Avg >= 80 ) • Grade = ‘B’; • else if ( Avg >= 70) • Grade = ‘C’; • else if ( Avg >= 60) • Grade = ‘D’; • else // Optional • Grade = ‘F’; • NOTE: The construct terminates when a choice is made. • QUESTIONS: # of decisions needed to assign A? C? F?

  19. 7.2.1 Sequence of if statements • Inefficient: how many decisions for A, C, F? if ( avg >= 90 ) cout << "A" << endl; if ( avg >= 80 && avg < 90 ) cout << "B" << endl; if ( avg >= 70 && avg < 80 ) cout << "C" << endl; if ( avg >= 60 && avg < 70 ) cout <<"D" << endl; if ( avg < 60 ) cout << "F" << endl;

  20. 7.2.1 Multiple-alternative chain of if • Mutual exclusion: result can be only one of the choices if ( avg >= 90 ) cout << "A" << endl; elseif ( avg >= 80 ) cout << "B" << endl; elseif( avg >= 70 ) cout << "C" << endl; elseif ( avg >= 60 ) cout << "D" << endl; else cout << "F" << endl; 90 80 70 60

  21. 7.2.1 Multiple alternative if chain • Notes: • Each condition is tried until one is satisfied (i.e., value is true) • The matching consequence is executed • The decision process terminates the

  22. 7.2.1 The Nested if construct • Nested the consequence is itself a decision. if ( gpa >= 3.75 ) if ( credits > 25 ) if( money < 30000 ) { scholarship = 5000; cout <<"Way to go!" << endl; } else scholarship = 2000; else scholarship = 1000; else { scholarship = 0; cout << "You're on your own."<< endl; }

  23. 7.3 Variable Scope • Scope of a variable – determines: • What code can access or change the variable • How long the variable exists or lives

  24. 7.3 Variable Scope • Below, var_a and var_b defined within the scope of the block • Both accessible within the block where defined • Final line generates an error message - var_b is not defined { intvar_a = 5, var_b = 10; var_a++; cout <<"var_a: "<< var_a << endl; } cout <<"var_b: "<< var_b;// Error: undeclared // identifiervar_b

  25. 7.3 Variable Scope • Local scope – variables or constants declared within braces

  26. 7.3 Variable Scope • Constant PI and variable global_area - physically declared outside of function - placed at the global level #include<iostream> using std::cout; using std::endl; #include<cmath>// Needed for pow constfloat PI = 3.141592F;// global scope floatglobal_area = 0; // global scope intmain() {floatradius = 5;// local scope global_area =static_cast<float>( PI* pow( radius, 2 ) ); cout << global_area <<" sq. in."<< endl; return0; } // Output 78.5398 sq. in.

  27. 7.3 Variable Scope • Any code within the file can access PI or global_area #include<iostream> using std::cout; using std::endl; #include<cmath>// Needed for pow constfloat PI = 3.141592F;// global scope floatglobal_area = 0; // global scope intmain() {floatradius = 5; // local scope global_area =static_cast<float>(PI* pow(radius, 2)); cout << global_area <<" sq. in."<< endl; return0; } // Output 78.5398 sq. in.

  28. 7.3 Variable Scope • Global variables - automatically initialized to 0 Avoid global variables (i.e., global_area) #include<iostream> using std::cout; using std::endl; #include<cmath>// Needed for pow constfloat PI = 3.141592F;// global scope floatglobal_area = 0; // global scope intmain() {floatradius = 5; // local scope global_area =static_cast<float>(PI* pow(radius, 2)); cout << global_area <<" sq. in."<< endl; return0; } // Output 78.5398 sq. in.

  29. 7.4 The switch Statement • switch statement - another form of conditional statement • Also called a selection statement • Checks only for equality and only for one variable

  30. 7.4 The switch Statement • Works well for checking a variable for limited set of values • Only works with ordinal data types • Ordinal data types - can be translated into an integer to provide a finite, known, number set • Examples include int, bool, char, andlong

  31. 7.4 The switch Statement • General form of the switch statement: • switch( <variable> ) • { // Required • case <literal or const 1>: • <action 1> • break; • case <literal or const 2>: • <action 2> • break; • ... • default: // Optional • <default action> • }// Required When first line is encountered, value of the variable determined Execution jumps to the case which corresponds to the value of the variable being examined Execution continues until either a break statement is encountered or to the end of switch

  32. 7.4 The switch Statement • break statement - stops execution of the control structure prematurely • Stops multiple case statements from being executed • Many believe poor programming to use outside the context of the switch statement

  33. 7.4 The switch Statement • default statement - executed if value of the variable doesn’t match any of previous cases • Type of catch all or “case else” • Technically can use the default case in any position • Should physically be the last one in the switch statement

  34. 7.4 The switch Statement intmenu_item = 0; ... switch ( menu_item ) { case1: // Using literal values cout << "You have chosen option 1." << endl; break; case2: cout << "You have chosen option 2." << endl; break; case3: cout << "You have chosen option 3." << endl; break; default: cout << "Invalid menu option." << endl; }

  35. 7.4 The switch Statement constshortGREEN = 0; constshortYELLOW = 1; constshortRED = 2; shortlight_color = GREEN; switch ( light_color ) { caseGREEN: // Using constants cout <<"Go!"<< endl; break; caseYELLOW:// Let fall through caseRED: cout <<"Stop!"; cout <<"Proceed when light is green."<< endl; break; default: cout <<"Stop!"; cout <<"Power is out!"<< endl; }

  36. 7.4 The switch Statement char letter_grade; cout <<"Enter letter grade: "; cin >> letter_grade; switch ( letter_grade ) { case'A': // Using character literal values cout <<"Excellent!"<< endl; break; case'B': cout <<"Above average." << endl; break; case'C': cout <<"Average."<< endl; break; case'D': cout <<"Below average."<< endl; break; case'F': cout <<"Failed!"<< endl; break; default: cout <<"Invalid letter grade."<< endl; }

  37. 7.4 The switch Statement • One of the most common uses of switch statement is in menu driven programs Student Grade Program - Main Menu - 1. Enter name 2. Enter test scores 3. Display test scores 9. Exit Please enter your choice from the list above:

  38. 7.5 Conditional Operator • Conditional operator- considered a ternary operator, meaning it has three operands • Syntax: <condition> ? <true expression> : <false expression>

  39. 7.5 Conditional Operator • One of the expressions is returned based upon the evaluation of the condition int a = 5, b = 0; intlarger = a > b ? a : b; cout << larger << endl; // Output 5

  40. 7.5 Conditional Operator • Equivalent if statement to code on previous page int a = 5, b = 0; intlarger; if ( a > b ) larger = a; else larger = b;

  41. 7.5 Conditional Operator • More challenging conditional operator example short hour = 9, minute = 10, second = 5; cout << (hour < 10 ?"0" :"") << hour <<":" << (minute < 10 ?"0":"")<< minute <<":" << (second < 10 ?"0":"")<< second << endl; // Output 09:10:05 Empty quotes above tell cout to print nothing if the condition is false (i.e. hour is 10 or greater)

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