This chapter is from the book
This chapter is from the book
This chapter is from the book
3.6 Input
The standard library offers istreams for input. Like ostreams, istreams deal with character string representations of built-in types and can easily be extended to cope with user-defined types.
The operator >> (''get from'') is used as an input operator; cin is the standard input stream. The type of the right-hand operand of >> determines what input is accepted and what is the target of the input operation. For example,
void f()
{
int i;
cin >> i; // read an integer into i
double d;
cin >> d; // read a double-precision, floating-point number into d
}
reads a number, such as 1234, from the standard input into the integer variable i and a floating-point number, such as 12.34e5, into the double-precision, floating-point variable d.
Here is an example that performs inch-to-centimeter and centimeter-to-inch conversions. You input a number followed by a character indicating the unit: centimeters or inches. The program then outputs the corresponding value in the other unit:
int main()
{
const float factor = 2.54; // 1 inch equals 2.54 cm
float x, in, cm;
char ch = 0;
cout << "enter length: ";
cin >> x; // read a floating-point number
cin >> ch; // read a suffix
switch (ch) {
case 'i': // inch
in = x;
cm = x*factor;
break;
case 'c': // cm
in = x/factor;
cm = x;
break;
default:
in = cm = 0;
break;
}
cout << in << " in = " << cm << " cm\n";
}
The switch-statement tests a value against a set of constants. The break-statements are used to exit the switch-statement. The case constants must be distinct. If the value tested does not match any of them, the default is chosen. The programmer need not provide a default.
Often, we want to read a sequence of characters. A convenient way of doing that is to read into a string. For example:
int main()
{
string str;
cout << "Please enter your name\n";
cin >> str;
cout << "Hello, " << str << "!\n";
}
If you type in
Eric
the response is
Hello, Eric!
By default, a whitespace character such as a space terminates the read, so if you enter
Eric Bloodaxe
pretending to be the ill-fated king of York, the response is still
Hello, Eric!
You can read a whole line using the getline() function. For example:
int main()
{
string str;
cout << "Please enter your name\n";
getline(cin,str) ;
cout << "Hello, " << str << "!\n";
}
With this program, the input
Eric Bloodaxe
yields the desired output:
Hello, Eric Bloodaxe!
The standard strings have the nice property of expanding to hold what you put in them, so if you enter a couple of megabytes of semicolons, the program will echo pages of semicolons back at you—unless your machine or operating system runs out of some critical resource first.