字符串操作(排序，最大最小值)

1.字符串升降序

  // ascending:YES升序  NO降序   
 NSSortDescriptor *descriptor = [NSSortDescriptor sortDescriptorWithKey:nil ascending:YES];
    NSArray *descriptors = [NSArray arrayWithObject:descriptor];
    NSArray *myDataArray = [NSArray arrayWithObjects:@"account3rd", @"login_type", nil];
    NSArray *resultArray = [myDataArray sortedArrayUsingDescriptors:descriptors];
    NSLog(@"%@", resultArray);
    
    NSString * resultStr = [[NSString alloc] init];
    for (NSString * str in resultArray) {
         resultStr = [resultStr stringByAppendingString:str];
    }
    NSString * lastStr = [NSString stringWithFormat:@"%@%@", APPKEY, resultStr];
    NSLog(@"===%@===", lastStr);
最后打印(
    account3rd,
    "login_type"
)

2.字符串值比较

NSArray *array = [NSArray arrayWithObjects:@"2.0", @"2.3", @"3.0", @"4.0", @"10", nil];
    //sum求和
    CGFloat sum = [[array valueForKeyPath:@"@sum.floatValue"] floatValue];
    //avg求平均值
    CGFloat avg = [[array valueForKeyPath:@"@avg.floatValue"] floatValue];
    //求最大值
    CGFloat max =[[array valueForKeyPath:@"@max.floatValue"] floatValue];
    //求最小值
    CGFloat min =[[array valueForKeyPath:@"@min.floatValue"] floatValue];
    NSLog(@"sum = %f\navg = %f\nmax = %f\nmin = %f",sum,avg,max,min);
最后打印
sum = 21.299999
avg = 4.260000
max = 10.000000
min = 2.000000