Polish notation

polishnotation.php

<?php
/**
 * Polish notation
 * @param $expression - Space seperated operators and param indices
 * @param $param - two or more params for the calculation
 */
function pn(string $expression, mixed ...$param) {
    $operators = "+-*/%"; $delimiter = " ";

    $expression = explode($delimiter, $expression);
    $stack = [];
    do {
        $itm=array_pop($expression);
        if( !strstr($operators, $itm) ) { // treat everything not an operator as operand
            if( !ctype_digit( $itm ) ) {
                throw new \InvalidArgumentException( "Operand index not integer: $itm\n" );

            }
            if(! (0 <= $itm and $itm < count( $param )) ) {
                throw new \OutOfBoundsException ( "Operand index out of range: $itm\n" );

            }

            if( !is_numeric( $param[$itm] ) ) {
                throw new \InvalidArgumentException( "Operand not numeric: $param[$itm]\n" );

            }

            array_push($stack, $param[$itm]);
            continue;
        }
        // we have an operator
        switch($itm) {
            case '+':  array_push( $stack, array_pop($stack) + array_pop($stack) ); break;
            case '-':  array_push( $stack, array_pop($stack) - array_pop($stack) ); break;
            case '*':  array_push( $stack, array_pop($stack) * array_pop($stack) ); break;
            case '/':  array_push( $stack, array_pop($stack) / array_pop($stack) ); break;
            case '%':  array_push( $stack, array_pop($stack) % array_pop($stack) ); break;
            //default: throw new \InvalidArgumentException( "Unknown operator $itm" );
        }
    } while ($expression);

    return array_pop($stack);
}


// Parameters are indexed from 0 upwards
try{
    echo pn("/ + 3 0 / 2 1", 30, 40, 50, 69), "\n";
} catch( Exception $e) {echo $e->getMessage();}
try{
    echo pn("/ + 3 0 / 2 1", 30, 40, 50), "\n";
} catch( Exception $e) {echo $e->getMessage();}
try{
    echo pn("/ + 3 3 / 2 1.1", 30, 40, 50, "a"), "\n";
} catch( Exception $e) {echo $e->getMessage();}
try{
    echo pn("/ + 3 3 / 2 1", 30, 40, 50, "a"), "\n";
} catch( Exception $e) {echo $e->getMessage();}
// If parameters are in an array already use unpacking
$operants = [3, 4, 5, 6];
$expresion = "/ + 0 1 / 2 3";
echo pn($expresion, ...$operants), "\n";

for a rpn remove the array_reverse()

for a rpn use array_shift() in line 13

or use a foreach($expression as $itm);

FSR
Make this into a class to

1. be able to (not) reverse
2. be able to specify a delimiter other than ' '
3. checking of a parameters and better exception handling.
4. custom operators (add swap, dup(icate), drop