第七章 Values and Collections

1.平台相关的数据类型

These types, like NSInteger and NSUInteger, are defined differently depending on the target architecture. When building for a 32-bit environment (such as for iOS), they are 32-bit signed and unsigned integers respectively; when building for a 64-bit environment (such as for the modern OS X runtime) they are 64-bit signed and unsigned integers respectively.

　　尽量使用这种平台相关的数据类型，而不是写int,double,float等基本类型，防止在不同的系统平台上，基本数据类型溢出的问题

2.使用int还是NSInteger

For local variables, such as a counter in a loop, it’s fine to use the basic C types if you know that the value is within the standard limits.

　　写for循环的时候，用int i = 0; i < n; i++ 吧，因为你知道i会不会溢出

3.NSNumber

　　使用NSNumber来表示基本数据类型

NSNumber *magicNumber = [[NSNumber alloc] initWithInt:42];

NSNumber *unsignedNumber = [[NSNumber alloc] initWithUnsignedInt:42u];

NSNumber *longNumber = [[NSNumber alloc] initWithLong:42l];

NSNumber *boolNumber = [[NSNumber alloc] initWithBOOL:YES];

NSNumber *simpleFloat = [NSNumber numberWithFloat:3.14f];

NSNumber *betterDouble = [NSNumber numberWithDouble:3.1415926535];

NSNumber *someChar = [NSNumber numberWithChar:‘T‘];

　　使用NSNumber来表示基本数据类型之更简洁的方法

NSNumber *magicNumber = @42;

NSNumber *unsignedNumber = @42u;

NSNumber *longNumber = @42l;

NSNumber *boolNumber = @YES;

NSNumber *simpleFloat = @3.14f;

NSNumber *betterDouble = @3.1415926535;

NSNumber *someChar = @‘T‘;

　　从NSNumber里面取出基本数据类型

int scalarMagic = [magicNumber intValue];

unsigned int scalarUnsigned = [unsignedNumber unsignedIntValue];

long scalarLong = [longNumber longValue];

BOOL scalarBool = [boolNumber boolValue];

float scalarSimpleFloat = [simpleFloat floatValue];

double scalarBetterDouble = [betterDouble doubleValue];

char scalarChar = [someChar charValue];

　　NSNumber和NSInteger之间的转化

NSInteger anInteger = 64;

NSUInteger anUnsignedInteger = 100;

NSNumber *firstInteger = [[NSNumber alloc] initWithInteger:anInteger];

NSNumber *secondInteger = [NSNumber numberWithUnsignedInteger:anUnsignedInteger];

NSInteger integerCheck = [firstInteger integerValue];

NSUInteger unsignedCheck = [secondInteger unsignedIntegerValue];

4.集合类型的元素必须是OC对象

NSArray,NSSet and NSDictionary are used to manage groups of objects, which means any item you wish to add to a collection must be an instance of an Objective-C class.

If you need to add a scalar value, you must first create a suitable NSNumber or NSValue instance to represent it.

　　集合类型的元素必须是OC对象，而不能是标量值

　　所谓标量值，是指int,double等基本数据类型和NSInteger,NSUInteger,BOOL等平台相关数据类型。标量值，一般声明的时候不加星号，比如NSInteger a = 1;矢量值，一般声明的时候需要加星号，比如NSNumber *b = @1;

5.集合快速初始化的时候有个坑

If you do need to represent a nil value in one of the collection classes, you should use the NSNull singleton class

　　Collection类，如NSArray,NSSet,NSDictionary等都是nil-terminated，所以使用类似下例的方式初始化的时候，注意一定要检查元素是否是nil啊

//事件上报 Norcy
NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys: _resultModel.keyWord?_resultModel.keyWord:@"", @"searchKeyword", nil];　　//这里要检查是否是nil

[QLReportCtlMgr reportStrID:@"video_jce_show_search_result_page" params:dic];

　　不然中间哪个元素是nil的时候，后面的元素就添加不上去了，那真的要表示nil的时候怎么办呢，除了上例的NSString使用了@""之外，还可以用NSNull这个类来代替nil

NSArray *array = @[ @"string", @42, [NSNull null], @YES];

　　由于NSNull是一个单例，[NSNull null]返回的永远是同一个对象，所以可以使用以下方法判断Collection里面哪个是空的

for (id object in array)
{
    if (object == [NSNull null])
        NSLog(@"Found a null object");
}

6.快速枚举

数组之快速枚举

for (id eachObject in array)
{
    NSLog(@"Object: %@", eachObject);
}

字典之快速枚举（注意是key哦）

for (NSString *eachKey in dictionary)
{
    id object = dictionary[eachKey];
    NSLog(@"Object: %@ for key: %@", object, eachKey);
}

高大上的NSEnumerator

for (id obj in [array objectEnumerator])  //正向
{
    NSLog(@"%@", obj);
}

for (id obj in [array reverseObjectEnumerator]) //反向
{
    NSLog(@"%@", obj);
}

while (id obj = [enumerator nextObject])    //nextObject+while
{
    NSLog(@"%@", obj);
}

如果，你离不开for循环的下标

int index = 0;
for (id obj in [array objectEnumerator])  //正向
{
    NSLog(@"%@", obj);
    index++;
}