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Chapter 4: Making Decisions

Chapter 4: Making Decisions

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Chapter 4: Making Decisions

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  1. Chapter 4: Making Decisions

  2. 4.1 Relational Operators • Relational operators allow you to compare numeric values and determine if one is greater than, less than, equal to, or not equal to another. • Chars are also considered numeric values.

  3. Programs You’ve Written • Gather input from the user. • Perform one or more calculations. • Display the results on the screen.

  4. Computers • Good at performing calculations. • Adept to comparing values to determine if one is greater than, less than, or equal to, the other. • The computer can assist with: • Examining sales figures • Determining profit and loss • Checking a number to ensure it is within an acceptable range • Validating the input given by the user

  5. Table 4-3: Relational Expressionsand Their True or False Values In C++, relational expressions represent true states with the number 1 and false states with the number 0.

  6. Table 4-5: Examples of if Statements Here is the general format of the if statement: If (expression) statement;

  7. Be Careful With Semicolons • Semicolons do not mark the end of a line, but the end of a complete C++ statement. • The if statement isn’t complete without the conditionally executed statement that comes after it. • Therefore, you do not put a semicolon after the if (expression) portion of an if statement. if (expression) No semicolon goes here statement; Semicolon goes here

  8. Program 4-3 // This program demonstrates how a misplacedsemicolon // prematurely terminates an if statement. #include <iostream.h> void main(void) { int x = 0, y = 10; cout << “x is " << x << " and y is " << y << endl; if (x > y); // misplaced semicolon! cout << “x is greater than y\n"; // Always executed } Program Output X is 0 and Y is 10 X is greater than Y

  9. Comparing Floating Point Numbers • You should be careful when using the equality operator (==) to compare floating point values. Because of round-off errors. • A number that should be mathematically equal to another might not be. • To prevent errors like this, stick with greater-than and less-than comparisons with floating point numbers. Refer to Program 4-4

  10. 4.4 Expanding the if Statement • The if statement can conditionally execute a block of statement enclosed in braces. if (expression) { statement; statement; // Place as many statements here as necessary. }

  11. Program 4-7 Continued cout.precision(1); cout.setf(ios::showpoint | ios::fixed); cout << "Your average is " << average << endl; if (highScore) { cout << "Congratulations!\n"; cout << "That's a high score.\n"; cout << "You deserve a pat on the back!\n"; } }

  12. Don’t Forget the Braces! • If you intend to execute a block of statements with an if statement, don’t forget the braces. • Without the braces, the if statement only executes the very next statement.

  13. 4.5 The if/else Statement • The if/else statement will execute one group of statements if the expression is true, or another group of statements if the expression is false. • The if/else statement is an expansion of the if statement. Here is the format: if (expression) statement or block of statements; else statement or block of statements;

  14. Program 4-9 // This program uses the modulus operator to determine // if a number is odd or even. If the number is evenly // dividedby 2, it is an even number. A remainder // indicates it isodd. #include <iostream.h> void main(void) { int number; cout << "Enter an integer and I will tell you if it\n"; cout << "is odd or even. "; cin >> number; if (number % 2 == 0) cout << number << " is even.\n"; else cout << number << " is odd.\n"; }

  15. 4-9 Program Output with Example Input Enter an integer and I will tell you if itis odd or even. 17 [Enter] 17 is odd.

  16. 4.6 The if/elseif Construct • The if/else if statement is a chain of if statements. The perform their tests, one after the other, until one of them is found to be true. If (expression) statement or block of statements; else if (expression) statement or block of statements; // put as many else it’s as needed here else if (expression) statement or block of statements;

  17. Program 4-11 // This program uses an if/else if statement // to assign aletter grade (A, B, C, D, or F) // to a numeric test score. #include <iostream.h> void main(void) { int testScore; char grade; cout << "Enter your numeric test score and I will\n"; cout << "tell you the letter grade you earned: "; cin >> testScore;

  18. if (testScore < 60) grade = 'F'; else if (testScore < 70) grade = 'D'; else if (testScore < 80) grade = 'C'; else if (testScore < 90) grade = 'B'; else if (testScore <= 100) grade = 'A'; cout << "Your grade is " << grade << ".\n"; } 4-11 Program Continued

  19. 4-11 Program Output with Example Input Enter your test score and I will tell you the letter grade you earned: 88 [Enter] Your grade is B.

  20. Program 4-13 //This program uses an if/else if statement to //assign a letter grade ( A, B, C, D, or F ) //to a numeric test score. #include<iostream.h> void main(void) { int testScore; cout << "Enter your test score and I will tell you\n"; cout << "the letter grade you earned: "; cin >> testScore; if (testScore < 60) { cout << "Your grade is F.\n"; cout << "This is a failing grade. Better see your "; cout << "instructor.\n"; } else if (testScore < 70) { cout << "Your grade is D.\n"; cout << "This is below average. You should get "; cout << "tutoring.\n"; }

  21. else if (testScore < 80) { cout << "Your grade is C.\n"; cout << "This is average.\n"; } else if(testScore < 90) { cout << "Your grade is B.\n"; cout << "This is an above average grade.\n"; } else if (testScore <= 100) { cout << "Your grade is A.\n"; cout << "This is a superior grade. Good work!\n"; } } 4-13 Program Continued

  22. 4-13 Program Output with Example Input Enter your test score and I will tell youthe letter grade you earned: 94 [Enter] Your grade is A. This is a superior grade. Good work!

  23. 4.7 Using a Trailing else • A trailing else, placed at the end of an if/else if statement, provides default action when none of the if’s have true expressions.

  24. Table 4-6: Lists C++’s Logical Operators

  25. cout << "Have you graduated from college "; cout << "in the past two years? "; cin >> recentGrad; if (employed == 'Y‘&& recentGrad == 'Y') // && Operator { cout << "You qualify for the special "; cout << "interest rate.\n"; } else { cout << "You must be employed and have \n"; cout << "graduated from college in the\n"; cout << "past two years to qualify.\n"; } } 4-18 Program Continued

  26. Table 4-9: Shows a Truth Table for the 1 Operator

  27. Program 4-20 //This program asks the user for his annual income and //the number of years he has been employed at his current job. //The ! operator reverses the logic of the expression in the if/else //statement. #include <iostream.h> void main(void) { float income; int years; cout << "What is your annual income? "; cin >> income; cout << "How many years have you worked at " << "your current job? "; cin >> years; if (!(income >= 35000 || years > 5)) // Uses the ! Logical operator { cout << "You must earn at least $35,000 or have\n"; cout << "been employed for more than 5 years.\n"; } else cout << "You qualify.\n"; }

  28. Table 4-10: Precedence of Logical Operators ! && ||