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Chapter 7 Conditional Statements

Chapter 7 Conditional Statements. Spring 2014. Chapter 7 Conditional Statements. 7.1 Conditional Expressions. Conditions - compare the values of variables, constants and literals using one or more relational operators

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Chapter 7 Conditional Statements

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  1. Chapter 7 Conditional Statements Spring 2014

  2. Chapter 7Conditional Statements

  3. 7.1 Conditional Expressions • Conditions - compare the values of variables, constants and literals using one or more relational operators • Binary operators- a variable, constant, or literal must appear on each side of the operator • The comparison determines whether the expression is true or false

  4. 7.1.1 Relational Operators • Single equal sign (=) is an assignmentDouble equal sign (==) tests for equality

  5. 7.1.1 Relational Operators • constint CONST_INT_EXP = 9; • int int_exp1 = 0, int_exp2 = 5; • floatfloat_exp = 9.0; • char char_exp = 'a'; • bool result; • result = int_exp1 == 0;// true • result = int_exp2 >= int_exp1;// true • result = int_exp1 > CONST_INT_EXP;// false • result = float_exp == CONST_INT_EXP;// true • result = char_exp <= int_exp1;// false result = int_exp1 != int_exp2;// true • result = char_exp == 'a';// true

  6. 7.1.1 Relational Operators • // ----- ILLEGAL OR MALFORMED CONDITIONS ---- • result = int_exp1 < int_exp2 < float_exp; • // Malformed condition. May or may notcompile // depending on compiler used. Even if the// compiler will compile this condition, it // should NEVER be written in this manner. • result = char_exp == "a"; • // Illegal. Attempting to compare a character// to a string literal.

  7. 7.1.2 Logical Operators • Logical operators - combine multiple relational operators into a larger composite condition • Operators: || (OR) - binary operator • False value only if the conditions on each side of the operator are false

  8. 7.1.2 Logical Operators • Operators: (continued) && (AND) - binary operator • Results in a true value only if the condition on both sides of the operator are true ! (NOT) - unary operator • Reverses logic of the single condition

  9. 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

  10. 7.1.2 Logical Operators • Logical NOT truth table • Order of precedence - && operator evaluated before the || operator • The ! operator - highest level of precedence of all logical operators and is higher than the relational operators

  11. 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

  12. 7.1.2 Logical Operators • Various logical operators • int int_exp1 = 0, int_exp2 = 5; • float float_exp = 9.0; • charchar_exp = 'a'; • constintCONST_INT_EXP = 9; • bool result; • result = int_exp1 < int_exp2 && float_exp == 9.0; // true • result = int_exp1 > CONST_INT_EXP || float_exp == 9.0;// true • result = !(float_exp == 9.0 || int_exp1 > CONST_INT_EXP);// false • // Short-Circuit Evaluation • result = float_exp == 9.0 || int_exp1 > CONST_INT_EXP; // true T T T T F T T F F T T T Not Evaluated

  13. 7.2 The if Statement • if statement - uses conditions to determine a specific action • Syntax:if ( <condition> ) • <action>

  14. 7.2 The if Statement • <condition> -any valid expression, either built from relational and logical operators or from evaluation of a single variable • zero is false while any non-zero value is considered true <action> -any valid C++ statement • multiple statements must be enclosed in curly braces { }

  15. 7.2 The if Statement • if statement - uses conditions to determine a specific action • // Example 1 • if ( test >= 80 && test < 90 ) • cout << "You have earned a B" << endl;// Action block • // Example 2 • if( test >= 90 ) • {// Start of the action block • cout << "You have earned an A" << endl; • cout << "Excellent work!" << endl; • }// End of the action block • // Example 3 • if( test >= 70 && test < 80 ) • {// Start of the action block • cout << "You have earned a C" << endl; • }// End of the action block

  16. 7.2.1 The else Statement • else statement - optional part of if statement • Can’t stand alone • Must be associated with an if • if( <condition> ) • <action 1> • else • <action 2>

  17. 7.2.1 The else Statement • else • no condition or expression associated with it • relies on results of the condition associated with the if • executes action(s) only if the condition is false • action can contain one or more statements • if more than one statement, the action must be enclosed in curly braces

  18. 7.2.1 The else Statement • else Example • if ( grade >= 60 ) • pass = true; • else • { • pass =false; • cout << "Hope you do better next time" << endl; • }

  19. 7.2.1 The Nested if • Nested if - embedding another ifin action block of the else • if( avg >= 90 ) • cout << "A" << endl; • else • if ( avg >= 80 ) • cout << "B" << endl;

  20. 7.2.1 The Nested if • Nested if indentation can cause the code to become difficult to read • if ( <condition 1> ) • <action 1> • else if ( <condition 2> ) • <action 2> • else if ( <condition 3> ) • <action 3> • . . . • else // Optional • <last action>

  21. 7.2.1 The Nested if • Inefficient if statement • 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;

  22. 7.2.1 The Nested if • Using else if statements is much more efficient than using separate if statements • else if statement • 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;

  23. 7.2.1 The Nested if • Why is else if more efficient? • else if only evaluates if condition until it finds true • Inefficient if evaluates every if condition • 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; • 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;

  24. 7.2.1 The Nested if • Flow of an if statement

  25. 7.2.1 The Nested if • Nested control statement - has another control statement in its action block • Map most nested if statement with nearest unmatched else • 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; • }

  26. 7.3 Variable Scope • Scope of a variable – determines: • What code can access or change the variable • How long the variable exists or lives

  27. 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

  28. 7.3 Variable Scope • Local scope – variables or constants declared within braces

  29. 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.

  30. 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.

  31. 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.

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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

  37. 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; • }

  38. 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; • }

  39. 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; • }

  40. 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:

  41. 7.5 Conditional Operator • Conditional operator- considered a ternary operator, meaning it has three operands • Syntax: <condition> ? <true expression> : <false expression>

  42. 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

  43. 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;

  44. 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)

  45. 7.7 C – The Differences • Previous versions of C did not have a Boolean data type • There isn’t a predefined true or false • All relational operators return either a zero for false or a non-zero value, usually one, for true

  46. 7.7 C – The Differences • C programmers often create their own Boolean data type as shown below • The C99 version of the ANSI Standard includes a Boolean data type • Not currently supported by Visual Studio • #defineBOOLint • #defineTRUE 1 • #defineFALSE 0 • intmain(void) • { BOOL done = FALSE; • return0; • }

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