NSString类中提供了这样三个方法用于获取子字符串：
– substringFromIndex://取字符串长度从0开始，当index=str.length时字符串为空""– substringWithRange://– substringToIndex：
NSString *str = @"12345";
1. // NSString *subString0 = [str substringFromIndex:-1]; //会越界
NSString *subString1 = [str substringFromIndex:0]; //@"12345"
NSString *subString2 = [str substringFromIndex:1]; //@"2345"
NSString *subString3 = [str substringFromIndex:4]; //@"5"
NSString *subString4 = [str substringFromIndex:5]; //@""
// NSString *subString5 = [str substringFromIndex:6]; //越界
2.// NSString *subString10 = [str substringToIndex:-1]; //越界
NSString *subString11 = [str substringToIndex:0]; //@""
NSString *subString12 = [str substringToIndex:1]; //@"1”
NSString *subString13 = [str substringToIndex:4]; //@"1234“
NSString *subString14 = [str substringToIndex:5]; //@"12345”
// NSString *subString15 = [str substringToIndex:6]; //越界
3.NSRange range1 = NSMakeRange(-1, 5);
NSRange range2 = NSMakeRange(0, 6);
NSRange range3 = NSMakeRange(0, 5);
NSRange range4 = NSMakeRange(1, 4);
NSRange range5 = NSMakeRange(0, 0);
NSRange range6 = NSMakeRange(4, 1);
//NSString *subString31 = [str substringWithRange:range1]; //越界
//NSString *subString32 = [str substringWithRange:range2]; //越界
NSString *subString33 = [str substringWithRange:range3]; //@"12345“
NSString *subString34 = [str substringWithRange:range4]; //@"2345”
NSString *subString35 = [str substringWithRange:range5]; //@""
NSString *subString36 = [str substringWithRange:range6]; //@"5”