PHP Calculator: Undefined Overflow when answer is 0

Question

I am creating a calculator in JS, but the calculations are made in PHP. The calculator must be able to process more than 1 operator (ex. 1+2*3-4/5) without using eval() or similar tricks.

After searching a lot, I ended up with this:

if (isset($_POST)) {
 $equation = $_POST["textview"];
}
$stored = $equation;
$components = preg_split('~([*/%+-])~', $stored, NULL, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE);
while (($index = array_search('*', $components)) !== false) {
 array_splice($components, $index - 1, 3, $components[$index - 1] * $components[$index + 1]);
}
while (($index = array_search('/', $components)) !== false) {
 array_splice($components, $index - 1, 3, $components[$index - 1] / $components[$index + 1]);
}
while (($index = array_search('%', $components)) !== false) {
 array_splice($components, $index - 1, 3, fmod($components[$index - 1], $components[$index + 1]));
}
while (($index = array_search('+', $components)) !== false) {
 array_splice($components, $index - 1, 3, $components[$index - 1] + $components[$index + 1]);
}
while (($index = array_search('-', $components)) !== false) {
 array_splice($components, $index - 1, 3, $components[$index - 1] - $components[$index + 1]);
}
echo current($components);

It seems to work perfectly, except for one problem: when the result of the calculation is 0, it gives me an error, and doesn't end the "while" loop

Notice: Undefined offset: -1 in C:\xampp\htdocs\***************\component\calculation.php on line 26
Notice: Undefined offset: 1 in C:\xampp\htdocs\****************\component\calculation.php on line 26

In this case, line 26 would be the subtraction (did a 1-1 operation), but it happens with every other calculation that should return a 0.

I have no idea why it happens and how to solve it, so if someone could help me, please, it would be great.

• aynber: Possible duplicate of "Notice: Undefined variable", "Notice: Undefined index", and "Notice: Undefined offset" using PHP
• Barmar: When $index is 0, $components[$index - 1] will try to access $components[-1].
• rcoelho14: @Barmar what's the way to fix it, then? I really have no idea :|
• aynber: See 3v4l.org/IhW94 At the end of the first loop, you've replaced the array of 0=>1, 1=> '-', 2=>1 with 0=>0, so it breaks. For some odd reason, array_search('-', [0]) returns 0, which is not false.
• Barmar: When I try this with $stored = "1-1"; I get an infinite loop, not that error.

Answer 1

The problem is that array_search() is performing loose comparison. When an element of $components is a number, it will convert the search string to a number before comparing. A string that doesn't look like a number is converted to 0, so array_search("-", [0]) returns 0 rather than false (try var_dump('-' == 0, '-' === 0)).

array_search has an optional strict parameter that makes it do strict comparison (like === rather than ==). Adding this to all the array_search calls fixes the problem.

$stored = "1-1";
$components = preg_split('~([*/%+-])~', $stored, NULL, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE);
while (($index = array_search('*', $components, true)) !== false) {
 array_splice($components, $index - 1, 3, $components[$index - 1] * $components[$index + 1]);
}
while (($index = array_search('/', $components, true)) !== false) {
 array_splice($components, $index - 1, 3, $components[$index - 1] / $components[$index + 1]);
}
while (($index = array_search('%', $components, true)) !== false) {
 array_splice($components, $index - 1, 3, fmod($components[$index - 1], $components[$index + 1]));
}
while (($index = array_search('+', $components, true)) !== false) {
 array_splice($components, $index - 1, 3, $components[$index - 1] + $components[$index + 1]);
}
while (($index = array_search('-', $components, true)) !== false) {
 array_splice($components, $index - 1, 3, $components[$index - 1] - $components[$index + 1]);
}
echo current($components);

• rcoelho14: Thanks, it works very well! I actually didn't know about that "strict" parameter. Still don't know very well how the array_search works