Nsstring cannot be modified string-objective-C

Summary of common nsstring Methods

+ (ID) stringwithcontentsoffile: path encoding: ENC err	Create a new string and set it to the content of the file specified by path. Use the character encoding ENC to return an error on err.
+ (ID) stringwithcontentsofurl: URL encoding: ENC err	Create a new string and set it to the content pointed to by the URL. Use the character encoding ENC to return an error on err.
+ (ID) String	Create a new Null String
+ (ID) stringwithstring: nsstring	Create a new string and set its content to nsstring
-(ID) initwithstring: nsstring	Set the string content to nsstring content
-(ID) initwithcontentsoffile: path encoding: ENC error: Err	SetStringSet to path to specify the file content, use character encoding ENC, and return an error on err
-(ID) initwithcontentsofurl: URL encoding: ENC error: Err	Sets the string to the content pointed to by the URL, uses the character encoding ENC, and returns an error on err
-(Unsigned INT) Length	Returns the number of characters in a string.
-(Unichar) characteratindex: I	Returns the Unicode Character of index I.
-(Nsstring *) substringfromindex: I	Returns the substring from index I to the end.
-(Nsstring *) substringwithrange: Range	Returns a substring based on a specified range.
-(Nsstring *) substringtoindex: I	Returns the substring from the string to the index I.
-(Nscomparator *) caseinsensitivecompare: nsstring	Compare the two string sizes, case insensitive
-(Nscomparator *) compare: nsstring	Compare two string sizes
-(Bool) hasprefix: nsstring	Test whether the string starts with nsstring
-(Bool) hassuffix: nsstring	Test whether the string ends with nsstring
-(Bool) isw.tostring: nsstring	Test whether two strings are equal
-(Nsstring *) capitalizedstring	Returns a string. the first letter of each word in the string is uppercase, And the other letters are lowercase.
-(Nsstring *) lowercasestring	Returns a string converted to lowercase.
-(Nsstring *) uppercasestring	Returns a string converted to uppercase.
-(Const char *) utf8string	Returns a UTF-8 encoded string.
-(Double) doublevalue	Returns a string converted to the double type.
-(Float) floatvalue	Returns a string converted to the float type.
-(Nsinteger) integervalue	Returns a new string converted to the nsinteger type.
-(INT) intvalue	Returns the string converted to int.