There's a blog post comment on codinghorror.com by Paul Jungwirth which includes a little programming task:
You have the numbers 123456789, in that order. Between each number, you must insert either nothing, a plus sign, or a multiplication sign, so that the resulting expression equals 2001. Write a program that prints all solutions. (There are two.)
Bored, I thought, I'd have a go, but I'll be damned if I can get a result for 2001. I think the code below is sound and I reckon that there are zero solutions that result in 2001. According to my code, there are two solutions for 2002. Am I right or am I wrong?
/**
* Take the numbers 123456789 and form expressions by inserting one of ''
* (empty string), '+' or '*' between each number.
* Find (2) solutions such that the expression evaluates to the number 2001
*/
$input = array(1,2,3,4,5,6,7,8,9);
// an array of strings representing 8 digit, base 3 numbers
$ops = array();
$numOps = sizeof($input)-1; // always 8
$mask = str_repeat('0', $numOps); // mask of 8 zeros for padding
// generate the ops array
$limit = pow(3, $numOps) -1;
for ($i = 0; $i <= $limit; $i++) {
$s = (string) $i;
$s = base_convert($s, 10, 3);
$ops[] = substr($mask, 0, $numOps - strlen($s)) . $s;
}
// for each element in the ops array, generate an expression by inserting
// '', '*' or '+' between the numbers in $input. e.g. element 11111111 will
// result in 1+2+3+4+5+6+7+8+9
$limit = sizeof($ops);
$stringResult = null;
$numericResult = null;
for ($i = 0; $i < $limit; $i++) {
$l = $numOps;
$stringResult = '';
$numericResult = 0;
for ($j = 0; $j <= $l; $j++) {
$stringResult .= (string) $input[$j];
switch (substr($ops[$i], $j, 1)) {
case '0':
break;
case '1':
$stringResult .= '+';
break;
case '2':
$stringResult .= '*';
break;
default :
}
}
// evaluate the expression
// split the expression into smaller ones to be added together
$temp = explode('+', $stringResult);
$additionElems = array();
foreach ($temp as $subExpressions)
{
// split each of those into ones to be multiplied together
$multplicationElems = explode('*', $subExpressions);
$working = 1;
foreach ($multplicationElems as $operand) {
$working *= $operand;
}
$additionElems[] = $working;
}
$numericResult = 0;
foreach($additionElems as $operand)
{
$numericResult += $operand;
}
if ($numericResult == 2001) {
echo "{$stringResult}\n";
}
}
Thankfully, asking “stupid” questions won't cause the demise of your career. In fact, when you phrase them the right way, you'll demonstrate that you're competent and—more importantly—not afraid to ask for whatever you need to get the job done.
People ask silly questions for a number of reasons: They want to know the answer. They want a good laugh. They're bored and want to stir up a discussion.
Further down the same page you linked to.... =)
"Paul Jungwirth wrote:
You have the numbers 123456789, in that order. Between each number, you must insert either nothing, a plus sign, or a multiplication sign, so that the resulting expression equals 2001. Write a program that prints all solutions. (There are two.)
I think you meant 2002, not 2001. :)
(Just correcting for anyone else like me who obsessively tries to solve little "practice" problems like this one, and then hit Google when their result doesn't match the stated answer. ;) Damn, some of those Perl examples are ugly.)"
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