Practical C Programming, 3rd Edition

So far you've learned how to compute expressions and output the results. You need to have your programs read numbers as well. The output class object std::cout uses the operator << to write numbers. The input object std::cin uses the operator >> to read them. For example:

std::cin >> price >> number_on_hand;

This code reads two numbers: price and number_on_hand . The input to this program should be two numbers, separated by whitespace. For example, if you type:

32 5

price gets the value 32 and number_on_hand gets 5 .

In Example 5-3, we use std::cin to get a number from the user , then we double it.

Example 5-3. double/double.cpp

#include <iostream> int value; // a value to double int main( ) { std::cout << "Enter a value: "; std::cin >> value; std::cout << "Twice " << value << " is " << value * 2 << '\n'; return (0); }

Notice that there is no \n at the end of Enter a value: . This is because we do not want the computer to print a newline after the prompt. For example, a sample run of the program might look like this:

Enter a value: 12 Twice 12 is 24

If we replaced Enter a value: with Enter a value:\n the result would be:

Enter a value: 12 Twice 12 is 24

Question 5-1: Example 5-4 is designed to compute the area of a triangle, given its width and height. For some strange reason, the compiler refuses to believe that we declared the variable width . The declaration is right there on line two, just after the definition of height. Why isn't the compiler seeing it?

Example 5-4. comment/comment.cpp

#include <iostream> int height; /* the height of the triangle int width; /* the width of the triangle */ int area; /* area of the triangle (computed) */ int main( ) { std::cout << "Enter width height? "; std::cin >> width >> height; area = (width * height) / 2; std::cout << "The area is " << area << '\n'; return (0); }

The general form of a std::cin statement is:

std::cin >> variable ;

This works for all types of simple variables such as int , float , char , and wchar_t .

Reading strings is a little more difficult. To read a string, use the statement:

std::getline(std::cin, string );

For example:

std::string name; // The name of a person std::getline(std::cin, name);

We discuss the std::getline function in Chapter 16.

When reading a string, the std::cin class considers anything up to the end-of-line part of the string. Example 5-5 reads a line from the keyboard and reports the line's length.

Example 5-5. len/len.cpp

#include <string> #include <iostream> std::string line; // A line of data int main( ) { std::cout << "Enter a line:"; std::getline(std::cin, line); std::cout << "The length of the line is: " << line.length( ) << '\n'; return (0); }

When we run this program we get:

Enter a line: test The length of the line is: 4