由于项目是和金钱有关的，要求比较高，不能有精度丢失。用NSString的DoubleValue方法并不准确，金额较大会出现精度丢失的问题。
这里我推荐直接转换为NSDecimalNumber来处理，保证精度不丢失。

NSDecimalNumber介绍

NSDecimalNumber是NSNumber的子类，封装了一系列运算的接口。方便我们直接调用api进行高精度数字的加减乘除，幂运算，四舍五入等。NSDecimalNumber可以直接通过NSString类型构建，不用转为Double类型。

最常用的场景就是输入框，用户输了一个比较大的数字，需要我们处理，如果此时转为Double，精度无法保证，如果用NSDecimalNumber,就可以直接用NSString构建，直接运算，防止精度丢失。

不过，NSDecimalNumber，运算时间相对要长一点。

具体使用方法，为了以后方便使用，封装了一个Category，我把源码放上来。

NSString+calculate.h

#import <Foundation/Foundation.h>

@interface NSString (calculate)
//加
- (NSString *)calculateByAdding:(NSString *)stringNumer;
//减
- (NSString *)calculateBySubtracting:(NSString *)stringNumer;
//乘
- (NSString *)calculateByMultiplying:(NSString *)stringNumer;
//除
- (NSString *)calculateByDividing:(NSString *)stringNumer;
//幂运算
- (NSString *)calculateByRaising:(NSUInteger)power;
//四舍五入
- (NSString *)calculateByRounding:(NSUInteger)scale;
//是否相等
- (BOOL)calculateIsEqual:(NSString *)stringNumer;
//是否大于
- (BOOL)calculateIsGreaterThan:(NSString *)stringNumer;
//是否小于
- (BOOL)calculateIsLessThan:(NSString *)stringNumer;
//转成小数
- (double)calculateDoubleValue;

@end

NSString+calculate.m

#import "NSString+calculate.h"

@implementation NSString (calculate)
//加
- (NSString *)calculateByAdding:(NSString *)stringNumer
{
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:self];
    NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithString:stringNumer];
    NSDecimalNumber *addingNum = [num1 decimalNumberByAdding:num2];
    return [addingNum stringValue];
}
//减
- (NSString *)calculateBySubtracting:(NSString *)stringNumer
{
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:self];
    NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithString:stringNumer];
    NSDecimalNumber *subtractingNum = [num1 decimalNumberBySubtracting:num2];
    return [subtractingNum stringValue];
}
//乘
- (NSString *)calculateByMultiplying:(NSString *)stringNumer
{
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:self];
    NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithString:stringNumer];
    NSDecimalNumber *multiplyingNum = [num1 decimalNumberByMultiplyingBy:num2];
    return [multiplyingNum stringValue];
}
//除
- (NSString *)calculateByDividing:(NSString *)stringNumer
{
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:self];
    NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithString:stringNumer];
    NSDecimalNumber *dividingNum = [num1 decimalNumberByDividingBy:num2];
    return [dividingNum stringValue];
    
}
//幂运算
- (NSString *)calculateByRaising:(NSUInteger)power
{
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:self];
    NSDecimalNumber *raisingNum = [num1 decimalNumberByRaisingToPower:power];
    return [raisingNum stringValue];
    
}
//四舍五入
- (NSString *)calculateByRounding:(NSUInteger)scale
{
    NSDecimalNumberHandler * handler = [[NSDecimalNumberHandler alloc] initWithRoundingMode:NSRoundPlain scale:scale raiseOnExactness:NO raiseOnOverflow:YES raiseOnUnderflow:YES raiseOnDivideByZero:YES];
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:self];
    NSDecimalNumber *roundingNum = [num1 decimalNumberByRoundingAccordingToBehavior:handler];
    return [roundingNum stringValue];
}
//是否相等
- (BOOL)calculateIsEqual:(NSString *)stringNumer
{
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:self];
    NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithString:stringNumer];
    NSComparisonResult result = [num1 compare:num2];
    if (result == NSOrderedSame) {
        return YES;
    }
    return NO;
}
//是否大于
- (BOOL)calculateIsGreaterThan:(NSString *)stringNumer
{
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:self];
    NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithString:stringNumer];
    NSComparisonResult result = [num1 compare:num2];
    if (result == NSOrderedDescending) {
        return YES;
    }
    return NO;

}
//是否小于
- (BOOL)calculateIsLessThan:(NSString *)stringNumer
{
    NSDecimalNumber *num1 = [NSDecimalNumber decimalNumberWithString:self];
    NSDecimalNumber *num2 = [NSDecimalNumber decimalNumberWithString:stringNumer];
    NSComparisonResult result = [num1 compare:num2];
    if (result == NSOrderedAscending) {
        return YES;
    }
    return NO;

}

- (double)calculateDoubleValue
{
    NSDecimalNumber *num = [NSDecimalNumber decimalNumberWithString:self];
    return [num doubleValue];
}

@end

源码：https://github.com/enoughpower/NSString-calculate.git
欢迎提意见