coin_change function seem to be working only for certain values

user_17263840 0
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I wrote a function to calculate the different coins I will received based on the amount enter. However, the function seem to be working for certain amount only.

Example when I enter $5.1, I should be have 5($1) and 1(1cent). But my function is returning me with 5($1), 1(5cent) and 4(1cent) instead.

I cant seem to figure out what is wrong with my function.

function coin_change($amount) {
    $coinDenominations = array(
        '1$'  => 1.00,
        '50c' => 0.50,
        '20c'    => 0.20,
        '10c'   => 0.10,
        '5c'    => 0.05,
        '1c'    => 0.01
    );
    $change = [];
    foreach($coinDenominations as $denomiation => $value){
        $count = floor($amount / $value);
        $change[$denomiation] = $count;
        
        if($amount > 0){
            $amount -= ($count * $value);
        }else{
            return $change;
        }
    }
    return $change;
}

echo "Enter amount: $amount <br/>";
var_dump(coin_change($amount));
php
1 Answers

Your logic is fine. Your problem comes from limited precision of floating point numbers in the floor function. When calculating floor(amount / $value), floor(0.99999999) will return 0.

You can fix that with round. So your function would become:

function coin_change($amount) {
    
    $coinDenominations = array(
        '1$'  => 1.00,
        '50c' => 0.50,
        '20c'    => 0.20,
        '10c'   => 0.10,
        '5c'    => 0.05,
        '1c'    => 0.01
    );
    
    $change = [];
    
    foreach ($coinDenominations as $denomination => $value) {
        
        // Round will make sure that 0.99999 = 1
        // Then floor(1) = 1
        $count = floor(round($amount / $value, 2));
        
        $change[$denomination] = $count;
        
        $amount -= $count * $value;
        $amount = round($amount, 2); // Fixes imprecision for operation above

        if ($amount == 0.00) {
            return $change;
        }
    }
    return $change;
}

$amount = 5.1;
var_dump(coin_change($amount));
user_65128570
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