6.4 Reading Numbers

The getline function of the previous section reads one line from the user, as a string. What if we want to read a number? One straightforward way is to read a string as before, and then immediately convert the string to a number. The standard C library contains a number of functions for doing this. The simplest to use are atoi(), which converts a string to an integer, and atof(), which converts a string to a floating-point number. (Both of these functions are declared in the header <stdlib.h>, so you should #include that header at the top of any file using these functions.) You could read an integer from the user like this:

	#include <stdlib.h>

	char line[256];
	int n;
	printf("Type an integer:\n");
	getline(line, 256);
	n = atoi(line);

Now the variable n contains the number typed by the user. (This assumes that the user did type a valid number, and that getline did not return EOF.)

Reading a floating-point number is similar:

	#include <stdlib.h>

	char line[256];
	double x;
	printf("Type a floating-point number:\n");
	getline(line, 256);
	x = atof(line);

(atof is actually declared as returning type double, but you could also use it with a variable of type float, because in general, C automatically converts between float and double as needed.)

Another way of reading in numbers, which you're likely to see in other books on C, involves the scanf function, but it has several problems, so we won't discuss it for now. (Superficially, scanf seems simple enough, which is why it's often used, especially in textbooks. The trouble is that to perform input reliably using scanf is not nearly as easy as it looks, especially when you're not sure what the user is going to type.)

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