数字之和返回奇数低位小数？

Question

When summing a group of numbers sometimes I end up with some low decimals? Why can it happen when the numbers are parsed as strings? I know there is some %"&! about floats

function parse(){
    foreach($_SESSION['import_csv_posts']['result']['csv'] as $key => $post){
        $amount = $this->parse_amount($post[$this->param['amount']]);
        if($this->param['vat_amount']){
            $amount += $this->parse_amount($post[$this->param['vat_amount']]);
        }

        $this->balance += $amount;
        echo "$amount
";
    }

    echo "
balance = ".$this->balance;
}

function parse_amount($amount){
    $amount = strval($amount);
    if(strstr($amount, '.') && strstr($amount, ',')){
        preg_match('/^\-?\d+([\.,]{1})/', $amount, $match);
        $amount = str_replace($match[1], '', $amount);
    }

    return str_replace(',', '.', $amount);
}

result

-87329.00
-257700.00
-11400.00
-9120.00
-47485.00
-15504.00
122800.00
1836.00
1254.00
200.00
360.00
31680.00
361.60
1979.20
1144.00
7520.00
6249.49
balance = -399.00000000003

Answer 1

The "%"&! about floats" is that floats are simply not precise. There's a certain inaccuracy inherent in how infinite numbers are stored in finite space. Therefore, when doing math with floats, you won't get 100% accurate results.

Your choice is to either round, format numbers to two decimal places upon output, or use strings and the BC Math package, which is slower, but accurate.