iOS 浮点数精确计算

有时候我们需要精确计算小数，比如精确计算金额，或者需要高精度计算一些数值等，使用系统默认的Float，Double，CGFloat等类型计算是不准确的。

我们需要使用系统提供的类NSDecimalNumber进行计算。

    // 精确小数点计算
NSString *num1 = @"1.99999999999";
NSString *num2 = @"0.00000000001";
NSString *num3 = @"0.00000001234";
NSString *num4 = @"1.23456789876";

// 创建NSDecimalNumber对象
    NSDecimalNumber *number1 = [NSDecimalNumber decimalNumberWithString:num1];
    NSDecimalNumber *number2 = [NSDecimalNumber decimalNumberWithString:num2];
    NSDecimalNumber *number3 = [NSDecimalNumber decimalNumberWithString:num3];
    NSDecimalNumber *number4 = [NSDecimalNumber decimalNumberWithString:num4];

// 减
    NSDecimalNumber *subNum = [number1 decimalNumberBySubtracting:number2];
NSLog(@"%@ - %@ = %@", number1.stringValue, number2.stringValue, subNum.stringValue);

// 加
    NSDecimalNumber *sumNum = [number2 decimalNumberByAdding:number3];
NSLog(@"%@ + %@ = %@", number2.stringValue, number3.stringValue, sumNum.stringValue);

// 乘
    NSDecimalNumber *multiNum = [number4 decimalNumberByMultiplyingBy:number2];
NSLog(@"%@ * %@ = %@", number4.stringValue, number2.stringValue, multiNum.stringValue);

// 除
    NSDecimalNumber *dividNum = [number4 decimalNumberByDividingBy:number2];
NSLog(@"%@ / %@ = %@", number4.stringValue, number2.stringValue, dividNum.stringValue);

得到的结果

2015-12-10 23:30:10.106 test[3245:79623] 
1.99999999999 - 0.00000000001 = 1.99999999998

2015-12-10 23:30:10.107 test[3245:79623] 
0.00000000001 + 0.00000001234 = 0.00000001235

2015-12-10 23:30:10.107 test[3245:79623] 
1.23456789876 * 0.00000000001 = 0.0000000000123456789876

2015-12-10 23:30:22.250 test[3245:79623]
 1.23456789876 / 0.00000000001 = 123456789876