兰州理工大学研究生VC++课件3

COMPARING VALUES
• Unless you want to make decisions on a whim, you need a mechanism for comparing things. • This involves some new operators called relational operators.
• This is illustrated in Figure.
• A simple example of an if statement is:
if('A' == letter)
cout << "The first capital, alphabetically speaking.";
• You could extend this example to change the value of letter if it contains the value 'A':
• • • • • • • •
// Ex03-01.cpp // A nested if demonstration #include <iostream> using std::cin; using std::cout; using std::endl; int main() { char letter(0); // Store input in here cout << endl << "Enter a letter: "; // Prompt for the input cin >> letter; // then read a character if (letter >= 'A') // Test for 'A' or larger { if (letter <= 'Z') // Test for 'Z' or smaller { cout << endl << "You entered a capital letter." << endl; return 0; } } if (letter >= 'a') // Test for 'a' or larger { if (letter <= 'z') // Test for 'z' or smaller { cout << endl << "You entered a lowercase letter." << endl; return 0; } } cout << endl << "You did not enter a letter." << endl; return 0; }
• Suppose you have created integer variables i and j with the values 10 and –5, respectively.
• • • • • • The expressions, i>j i != j j > -8 i <= j + 15 all return the value true.
• }
• There is a standard library function to convert letters to uppercase, so you don’t normally need to program this yourself. • It has the name toupper() and appears in the ctype standard header file. • You will see more about standard library facilities when you getຫໍສະໝຸດ Baiduto look at how functions are written.
if(letter <= 'Z') // Test for 'Z' or smaller { cout << endl << "You entered a capital letter." << endl; letter += 'a' - 'A'; // Convert to lowercase return 0;
• Each operator compares the values of two operands and returns one of the two possible values of type bool: true if the comparison is true, or false if it is not. • You can see how this works by having a look at a few simple examples. • The operands can be variables, literals, or expressions.
• • • • Take a look at the following expressions: -1 < y true j < (10 - i) false 2.0*x >= (3 + y) true
The if Statement
• The basic if statement allows your program to execute a single statement — or a block of statements enclosed within braces — if a given condition expression evaluates to true, or to skip the statement or block of statements if the condition evaluates to false.
3.Decision and Loops
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WHAT YOU WILL LEARN IN THIS CHAPTER:
• How to compare data values • How to alter the sequence of program execution based on the result • How to apply logical operators and expressions • How to deal with multiple choice situations • How to write and use loops in your programs
• Let’s consider some slightly more complicated comparisons with variables defined by the statements: • int i = -10, j = 20; • double x = 1.5, y = -0.25E-10;
The Extended if Statement
• • • • • • • • // Ex3_02.cpp // Using the extended if #include <iostream> using std::cin; using std::cout; using std::endl; int main() {
long number(0L); // Store input here cout << endl << "Enter an integer number less than 2 billion: "; cin >> number; if(number % 2L) // Test remainder after division by 2 cout << endl // Here if remainder 1 << "Your number is odd." << endl; else cout << endl // Here if remainder 0 << "Your number is even." << endl; return 0;
• You could easily arrange to change uppercase to lowercase by adding just one extra statement to the if, checking for uppercase: • if(letter >= 'A') // Test for 'A' or larger
if('A' == letter) { cout << "The first capital, alphabetically speaking."; letter = 'a'; } The block of statements that is controlled by the if statement is delimited by the curly braces.
• Because all information in your computer is ultimately represented by numerical , comparing numerical values is the essence of all decision making.
• You have six operators for comparing two values:
Using Nested ifs
• The if statement that follows the input checks whether the character entered is 'A' or larger. • The ASCII codes for lowercase letters (97 to 122) are greater than those for uppercase letters (65 to 90).
• • • • • • • • •
Assume that you have defined the following variables: char first = 'A', last = 'Z'; Here are some examples of comparisons using these: first == 65 first < last 'E' <= first first != last All four expressions compare ASCII code values. The first expression returns true because first was initialized with 'A', which is the equivalent of decimal 65. • The second expression checks whether the value of first, which is 'A', is less than the value of last, which is 'Z'.
Nested if Statements
• The statement to be executed when the condition in an if statement is true can also be an if. • This arrangement is called a nested if. • The condition for the inner if is only tested if the condition for the outer if is true. • An if that is nested inside another can also contain a nested if. • You can generally continue nesting ifs one inside the other like this for as long as you still know what youare doing.