Ex_4.10_calculator_getline.rst

========================================
Exercise 4.10 - Calculator using getline
========================================

Question
========

An alternate organization uses getline to read an entire input line; this makes
getch and ungetch unnecessary. Revise the calculator to use this approach.

.. literalinclude:: ../../languages/cprogs/Ex_4.10_calculator_getline.c
   :language: c
   :tab-width: 4

.. runcode:: ../../languages/cprogs/Ex_4.10_calculator_getline.c
   :language: c
   :codesite: ideone

Explanation
===========

This program uses `mgetline` to get the characters and operands from the input
and and proceeds with the RPN calculator logic.

This is the main part of the program.


::

	/* getop: get next operator or numeric operand */

	int getop(char s[])
	{
		int c,i;

		if(line[li] == '\0')
			if(mgetline(line,MAXLINE) == 0)
				return EOF;
			else
				li =0;
		
		while((s[0] = c = line[li++]) == ' ' || c == '\t')
			;
		
		s[1] = '\0';
		
		if(!isdigit(c) && c!= '.')
			return c;
		
		i = 0;
		
		if(isdigit(c))
			while(isdigit(s[++i] = c = line[li++]))
				;
		if( c == '.')
			while(isdigit(s[++i] = c = line[li++]))
				;
		
		s[i] = '\0';

		li--;

		return NUMBER;
	}


From the mgetline function, it takes the input in the line character array, and
if if the line is `\0` only, then we define that as EOF and return `EOF`. Then
we assign to `c` the value present at `line` and look for various conditions
like, if line is a space or tab character, we simply skip it. If we encouter c
which is not a digit or not a `.` character, we return `c` immediately.  At the
end if it is valid number, we return a NUMBER, which is then pushed onto the
stack of the RPN calculator.

An example execution will look like this.

::

	10 10 +
	        20
	10.1 20.2 +
	        30.3



.. seealso::

   * :c-suggest-improve:`Ex_4.10_calculator_getline.c`
   * :c-better-explain:`Ex_4.10_calculator_getline.rst`