How to mathematically evaluate a string like “2-1” to produce “1”?

by

I know this question is old, but I came across it last night while searching for something that wasn’t quite related, and every single answer here is bad. Not just bad, very bad. The examples I give here will be from a class that I created back in 2005 and spent the past few hours updating for PHP5 because of this question. Other systems do exist, and were around before this question was posted, so it baffles me why every answer here tells you to use eval, when the caution from PHP is:

The eval() language construct is very dangerous because it allows execution of arbitrary PHP code. Its use thus is discouraged. If you have carefully verified that there is no other option than to use this construct, pay special attention not to pass any user provided data into it without properly validating it beforehand.

Before I jump in to the example, the places to get the class I will be using is on either PHPClasses or GitHub. Both the eos.class.php and stack.class.php are required, but can be combined in to the same file.

The reason for using a class like this is that it includes and infix to postfix(RPN) parser, and then an RPN Solver. With these, you never have to use the eval function and open your system up to vulnerabilities. Once you have the classes, the following code is all that is needed to solve a simple (to more complex) equation such as your 2-1 example.

require_once "eos.class.php";
$equation = "2-1";
$eq = new eqEOS();
$result = $eq->solveIF($equation);

That’s it! You can use a parser like this for most equations, however complicated and nested without ever having to resort to the ‘evil eval‘.

Because I really don’t want this only only to have my class in it, here are some other options. I am just familiar with my own since I’ve been using it for 8 years. ^^

Wolfram|Alpha API
Sage
A fairly bad parser
phpdicecalc

Not quite sure what happened to others that I had found previously – came across another one on GitHub before as well, unfortunately I didn’t bookmark it, but it was related to large float operations that included a parser as well.

Anyways, I wanted to make sure an answer to solving equations in PHP on here wasn’t pointing all future searchers to eval as this was at the top of a google search. ^^

More Related Contents:

  1. calculate math expression from a string using eval
  2. How to evaluate formula passed as string in PHP?
  3. Process mathematical equations in php
  4. Increment number php every next number
  5. The 3 different equals
  6. When is eval evil in php?
  7. What is the use of the @ symbol in PHP?
  8. Stacking Multiple Ternary Operators in PHP
  9. ‘AND’ vs ‘&&’ as operator
  10. Reference assignment operator in PHP, =&
  11. What is the “->” PHP operator called? [closed]
  12. Display numbers with ordinal suffix in PHP
  13. Test if number is odd or even
  14. What is (the ‘Spaceship’ Operator) in PHP 7? [duplicate]
  15. instantiate a class from a variable in PHP?
  16. What is the meaning of three dots (…) in PHP?
  17. Combinations, Dispositions and Permutations in PHP
  18. Calculating distance between zip codes in PHP
  19. Double not (!!) operator in PHP
  20. Round to max thousand, hundred etc in PHP
  21. PHP: Create an array for a range
  22. Execute PHP code in a string [duplicate]
  23. Max file number can php upload at same time
  24. How does PHP compare strings with comparison operators?
  25. PHP float calculation error when subtracting
  26. php random x digit number
  27. ‘At’ symbol before variable name in PHP: @$_POST
  28. php: number only hash?
  29. Incrementing numbers starting from 0000 in php
  30. Convert number into xx.xx million format? [closed]

Leave a Comment