Calculators | Inverse Polish expression

1. Principle of implementation:

Description: In my original inverse Polish calculator, postfix was used to denote inverse polish expressions. But after discussion with Buptpatriot, want to implement directly, the following is the method of direct implementation:

First, define the priority level:

function level ($OP) {
	switch ($op) {case
		' + ': Case
		'-':
			return 1;
		Case ' * ': Case
		'/':
			return 2;
		Case ' ^ ':  
			return 3;
	}
}

After that, the input sequence is scanned, the input string is read, and the numbers (possibly floating-point, negative numbers) and operators are extracted.

Collect floating-point numbers:

function Readin ($expr) {
	$expr = Str_split ($expr);
	$len  = count ($expr);
	$infix = Array ();
	$opts  = Array (' + ', '-', ' * ', '/', ' (', ') ');
	$num = "";
	for ($i = 0; $i < $len; $i + +) {
		if (In_array ($expr [$i], $opts) | | ($i = = $len-1)) {
			if ($num = Floatval ($num)) {
				Array_push ($infix, $num);
				$num = "";
			}
			if ($expr [$i] = = '-' && ($expr [$i-1] = = NULL | | $expr [$i-1] = = ' (') 
				&& (Is_numeric ($expr [$i +1]) | | $ex pr[$i +1] = = ' (')} {
				$expr [$i] = ' ^ ';//unary negotion
			}
			Array_push ($infix, $expr [$i]);
		}
		else {
			$num. = $expr [$i];
		}
	}
	Var_dump ($infix);
	return $infix;
}

For the negative sign (-) do the processing:

if ($expr [$i] = = '-' && ($expr [$i-1] = = NULL | | $expr [$i-1] = = ' (') 
				&& (Is_numeric ($expr [$i +1]) | | $ex pr[$i +1] = = ' (')
				} {$expr [$i] = ' ^ '; 
			}

At this point, the negative sign '-' turns into ' ^ '

1) If it is a number, it is deposited in the stack postfix;

2) If it is an operator:

2.1) is ' (', then deposited in stack stack;

2.2) is ' + ', '-', ' * ', '/', ', then check if stack stack is empty:

2.2.1) stack is empty: it is stored in the stack

2.2.2) stack is not empty, perform the following actions:

POPs an element op from a stack

If the element is not ' (' and the priority of the popup element is greater than the priority of the current scan element, the pop-up element is deposited into the postfix stack, and a 2 element, a, B, is popped from the postfix, and the result of a-op is computed and deposited into the postfix;

Otherwise, the popup element op is re-pressed into the stack stack

Finally, the currently scanned operator is in the stack stack

2.3) is ' ^ ': Perform the following actions:

1 elements A is popped from Postfix, and (0-a) is deposited in postfix;

2.4) is ') ': Perform the following actions:

The pop-up element is stored in the postfix stack, and a 2 element, a, B, is popped from the postfix, and the result of a Op b. is calculated and deposited into the postfix;

Until the popup element is ' (';

Draw a table to see: 4/((3-1) * * = 1 of the calculation process

Current	Stack	Postfix
4	Empty	4
/	/	4
(	/(	4
(	/((	4
3	/((	43
-	/((-	43
1	/((-	431
)	/(	42
*	/(*	42
2	/(*	422
）	/	44
	Empty	1 (Result)

2.php Code:

 

 

 
3. Operation Result:


Input: 4/((3-1) * *)

Output:







Contains the result of the negative symbol:

Input:-( -1.2+1.8)/(1/3.0)

Output: