由于項(xiàng)目是和金錢有關(guān)的，要求比較高，不能有精度丟失。用NSString的DoubleValue方法并不準(zhǔn)確，金額較大會出現(xiàn)精度丟失的問題。
這里我推薦直接轉(zhuǎn)換為NSDecimalNumber來處理，保證精度不丟失。

NSDecimalNumber介紹

NSDecimalNumber是NSNumber的子類，封裝了一系列運(yùn)算的接口。方便我們直接調(diào)用api進(jìn)行高精度數(shù)字的加減乘除，冪運(yùn)算，四舍五入等。NSDecimalNumber可以直接通過NSString類型構(gòu)建，不用轉(zhuǎn)為Double類型。

最常用的場景就是輸入框，用戶輸了一個比較大的數(shù)字，需要我們處理，如果此時轉(zhuǎn)為Double，精度無法保證，如果用NSDecimalNumber,就可以直接用NSString構(gòu)建，直接運(yùn)算，防止精度丟失。

不過，NSDecimalNumber，運(yùn)算時間相對要長一點(diǎn)。

具體使用方法，為了以后方便使用，封裝了一個Category，我把源碼放上來。

NSString+calculate.h

#import <Foundation/Foundation.h>

@interface NSString (calculate)
//加
- (NSString *)calculateByAdding:(NSString *)stringNumer;
//減
- (NSString *)calculateBySubtracting:(NSString *)stringNumer;
//乘
- (NSString *)calculateByMultiplying:(NSString *)stringNumer;
//除
- (NSString *)calculateByDividing:(NSString *)stringNumer;
//冪運(yùn)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;
//轉(zhuǎn)成小數(shù)
- (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];
}
//減
- (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];
    
}
//冪運(yùn)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

源碼：https://github.com/enoughpower/NSString-calculate.git
歡迎提意見