在日常开发中，常常需要对金额加逗号来显示，我把这件事放在NSString的分类里来做，直接加一个类方法。

+(NSString*)strmethodComma:(NSString*)string
{
    if (!string || [string isEqualToString:@"0"]) {
        return @"0.00";
    }
    
    NSString *sign = nil;
    if ([string hasPrefix:@"-"]||[string hasPrefix:@"+"]) {
        sign = [string substringToIndex:1];
        string = [string substringFromIndex:1];
    }
    
    if (string.length < 4) {
        if (sign) {
            string = [sign stringByAppendingString:string];
        }
        return string;
    }
    
    NSString *pointLast  = nil;
    NSString *pointFront = nil;
    NSInteger location   = [string rangeOfString:@"."].location;
    if (location != NSNotFound) {
        NSString *substring  = [string substringFromIndex:location];
        pointLast = [string substringFromIndex:[string length]-substring.length];
        pointFront = [string substringToIndex:[string length]-substring.length];
    }else {
        pointLast = @"";
        pointFront = string;
    }
    
    int commaNum = ([pointFront length]-1)/3;
    NSMutableArray *arr = [NSMutableArray array];
    for (int i = 0; i < commaNum+1; i++) {
        int index = [pointFront length] - (i+1)*3;
        int leng = 3;
        if(index < 0)
        {
            leng = 3+index;
            index = 0;
        }
        NSRange range = {index,leng};
        NSString *stq = [pointFront substringWithRange:range];
        [arr addObject:stq];
    }
    NSMutableArray *arr2 = [NSMutableArray array];
    for (int i = [arr count]-1; i>=0; i--) {
        
        [arr2 addObject:arr[i]];
    }
    NSString *commaString = [[arr2 componentsJoinedByString:@","] stringByAppendingString:pointLast];
//    NSArray *ary = [commaString componentsSeparatedByString:@","];
    
    if (sign) {
        commaString = [sign stringByAppendingString:commaString];
    }
    return commaString;
    
}