How to Make a Calculator in PHP
Problem:
Create a PHP calculator that can evaluate simple algebraic expressions entered by users in standard algebraic notation, including expressions with parentheses.
Common Approach:
An inefficient but temporary solution involves replacing expressions in a string representation of the expression, such as:
for ($a=1; $a <p><strong>Shunting Yard Algorithm:</strong></p><p>A more efficient approach is to use the Shunting Yard algorithm.</p><p><strong>Implementation:</strong></p><p><strong>Terminal Expressions (Values and Operators):</strong></p><pre class="brush:php;toolbar:false">abstract class TerminalExpression { protected $value = ''; public static function factory($value) { // Create terminal expressions based on the provided value (number, operator, or parenthesis) } abstract public function operate(Stack $stack); } class Number extends TerminalExpression { public function operate(Stack $stack) { return $this->value; } } class Operator extends TerminalExpression { protected $precidence = 0; protected $leftAssoc = true; public function getPrecidence() { return $this->precidence; } public function isLeftAssoc() { return $this->leftAssoc; } } class Addition extends Operator { protected $precidence = 4; } class Subtraction extends Operator { protected $precidence = 4; } class Multiplication extends Operator { protected $precidence = 5; } class Division extends Operator { protected $precidence = 5; } class Parenthesis extends TerminalExpression { protected $precidence = 7; public function isParenthesis() { return true; } }
Stack Implementation:
class Stack { protected $data = []; public function push($element) { array_push($this->data, $element); } public function pop() { return array_pop($this->data); } }
Math Class (Executor):
class Math { protected $variables = []; public function evaluate($string) { $stack = $this->parse($string); return $this->run($stack); } public function parse($string) { // Tokenize expression $tokens = array_map('trim', preg_split('((\d+|\+|-|\(|\)|\*|/)|\s+)', $string, null, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE)); // Parse operators and parentheses using the Shunting Yard algorithm $output = new Stack(); $operators = new Stack(); foreach ($tokens as $token) { $expression = TerminalExpression::factory($token); if ($expression->isOperator()) { $this->parseOperator($expression, $output, $operators); } elseif ($expression->isParenthesis()) { $this->parseParenthesis($expression, $output, $operators); } else { $output->push($expression); } } // Pop remaining operators on stack and push to output while (($op = $operators->pop()) && $op->isOperator()) { if ($op->isParenthesis()) { throw new RuntimeException('Mismatched Parenthesis'); } $output->push($op); } return $output; } public function run(Stack $stack) { // Evaluate stack and return result while (($operator = $stack->pop()) && $operator->isOperator()) { $value = $operator->operate($stack); $stack->push(TerminalExpression::factory($value)); } return $operator ? $operator->render() : $this->render($stack); } protected function extractVariables($token) { if ($token[0] == '$') { $key = substr($token, 1); return isset($this->variables[$key]) ? $this->variables[$key] : 0; } return $token; } // ... }
Using this implementation, you can evaluate expressions as follows:
$math = new Math(); $answer = $math->evaluate('(2 + 3) * 4'); // 20 $answer = $math->evaluate('1 + 2 * ((3 + 4) * 5 + 6)'); // 83 $answer = $math->evaluate('(1 + 2) * (3 + 4) * (5 + 6)'); // 231 $math->registerVariable('a', 4); $answer = $math->evaluate('($a + 3) * 4'); // 28
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