创建一个字符串对象：
NSstring * str1 = @"hello world";
NSString * str = [[NSString alloc]initWithString:@"hello world!"];
NSString * str_1 = [[NSString alloc]initWithUTF8String:"hello world"];//把C的字符串转化为OC的字符串
int a = 123;
NSString * str_3 = [[NSString alloc]initWithFormat:@"a = %d %s%@", a, "abcd", @"efg"];//用格式化字符串初始化
//可完毕字符串的拼接以及完毕C的字符串与OC的字符串的转化
NSString * str1 = [NSString stringWithString:@"hello world"];
NSString * str2 = [NSString stringWithUTF8String:"hello world"];
NSString * str3 = [NSString stringWithFormat:@"%s","hello world"];
NSstring * str1 = @"hello";
NSString * str = [[NSString alloc]initWithString:@"hello world!"];
NSString * str_1 = [[NSString alloc]initWithUTF8String:"hello world"];//把C的字符串转化为OC的字符串
int a = 123;
NSString * str_3 = [[NSString alloc]initWithFormat:@"a = %d %s%@", a, "abcd", @"efg"];//用格式化字符串初始化
字符串转化：
NSString * str = @"hello";
const char *p = [str UTF8String];将OC字符串转化为C的字符串
str = @"123";
int a = [str intValue];//将数字串转化成整型数据
[str floatValue];//将数字串转化成float型
[str doubleValue];//将数字串转化成double型
NSString * str = @"hello";
const char *p = [str UTF8String];将OC字符串转化为C的字符串
str = @"123";
int a = [str intValue];//将数字串转化成整型数据
[str floatValue];//将数字串转化成float型
[str doubleValue];//将数字串转化成double型
经常使用的NSString ：
NSLog(@"%ld",[str1 length]); //求字符串长度
NSLog(@"%c", [str1 characterAtIndex:1]);//获取字符串中的字符
BOOL ret = [str1 isEqualTo:str2];//比較两个字符串是否相等,相等返回YES(1),不相等返回NO(0) (BOOL YES(1) NO(0))
long ret1 = [str1 compare:str2];//比較两个字符串的大小,str1大于 返回1 相等返回0 小于返回-1
long ret2 = [str1 caseInsensitiveCompare:str2];//不区分大写和小写比較字符串大小
NSString * ptr = [str2 uppercaseString];//将字符串中的全部小写字符转换成大写 不改变原来的字符串
ptr = [str1 lowercaseString];//将字符串中的全部大写字符转换成小写 不改变原来的字符串
ptr = [str3 capitalizedString];//将字符串中出现的第一个字母转换成大写，其余字母小写
NSString * str4 = @"hello world";
NSRange range = [str4 rangeOfString:@"wr"];//查找子串，找不到返回NSNotFound 找到返回location和length
if (range.location != NSNotFound) {
NSLog(@"%ld %ld",range.location, range.length);
}
NSString * str5 = @"helloworld";
NSString * ptr1 = [str5 substringToIndex:4];//字符串抽取 从下标0開始到4 不包含4
NSString * ptr2 = [str5 substringFromIndex:5];//从下标5開始抽取到字符串结束 包含5
NSRange range1 = {4,3};//结构体初始化
NSString * ptr3 = [str5 substringWithRange:range1];//在range指定范围内抽取
NSString * ptr4 = [str5 substringWithRange:NSMakeRange(4,3)];//NSMakeRange能够生成一个结构体
NSString * str = @"www.baidu.com";
BOOL ret = [str hasPrefix:@"www"];//推断字符串是否以指定字符串开头
NSString * str1 = @"1.txt";
BOOL ret1 = [str hasSuffix:@".txt"];//推断字符串是否以指定的字符串结尾
NSMutableString可变字符串(动态添加和降低的) 继承于NSString能够使用NSString的全部方法
NSMutableString * str = [[NSMutableString alloc]initWithString:@"hello"];//将不可变的字符串转换为可变的字符串
[str insertString:@"123" atIndex:1];//在指定下标(不要越界)位置插入NSString类型字符串
[str appendString:@"123"];//在字符串末尾追加字符串
[str deleteCharactersInRange:NSMakeRange(0, 2)];//从指定下标删除length个字符
[str setString:@"baidu"];//改动字符串亦称对该可变字符串赋值
[str replaceCharactersInRange:NSMakeRange(3, 1) withString:@"ios"];//将指定下标位置的length个字符替换为指定的字符串