Getting the Length of a String
The length of the string in a string object can be obtained by accessing the length method of the string object:
1 2 3 | NSString *string1 = @"This string is Immutable"; int len = [string1 length]; NSLog (@"String length is %i", len); |
Searching for a Substring
A common requirement when working with strings is to identify whether a particular sequence of characters appears within a string. This can be achieved using the rangeOfString method. This method returns a structure of type NSRange. The NSRange structure contains a location value providing the index into the string of the matched substring and a length value indicating the length of the match.
1 2 3 4 5 | NSString *string1 = @"The quick brown fox jumped"; NSRange match; match = [string1 rangeOfString: @"brown fox"]; NSLog (@"match found at index %i", match.location); NSLog (@"match length = %i", match.length); |
The NSLog call will display the location and length of the match. Note that the location is an index into the string where the match started and that the index considers the first position in a string to be 0 and not 1. As such, the location in our example will be 10 and the length will be 9.
In the event that no match is found, the rangeOfString method will set the location member of the NSRange structure to NSNotFound. For example:
1 2 3 4 5 6 7 | NSString *string1 = @"The quick brown fox jumped"; NSRange match; match = [string1 rangeOfString: @"brown dog"]; if (match.location == NSNotFound) NSLog (@"Match not found"); else NSLog (@"match found at index %i", match.location); |
Replacing Parts of a String
Sections of a mutable string may be replaced by other character sequences using the replaceCharactersInRange method. This method directly modifies the string object on which the method is called so only works on mutable string objects.
This method requires two arguments. The first argument is an NSRange structure consisting of the location of the first character and the total number of characters to be replaced. The second argument is the replacement string. An NSRange structure can be created by calling NSMakeRange and passing though the location and length values as arguments. For example, to replace the word “fox” with “squirrel” in our sample mutable string object we would write the following Objective-C code:
1 2 3 | NSMutableString *string1 = [NSMutableString stringWithString: @"The quick brown fox jumped"]; [string1 replaceCharactersInRange: NSMakeRange(16, 3) withString: @"squirrel"]; NSLog (@"string1 = %@", string1); |
As you may have noted from the above example, the replacement string does not have to be the same length as the range being replaced. The string object and replacement method will resize the string automatically.
String Search and Replace
Previously we have covered how to perform a search in a string and how to replace a subsection of a string using the rangeOfString and replaceCharactersInRange methods respectively. The fact that both of these methods use the NSRange structure enables us to combine the two methods to perform a search and replace. In the following example, we use rangeOfString to provide us with an NSRange structure for the substring to be replace and then pass this through to replaceCharactersInRange to perform the replacement:
1 2 | NSMutableString *string1 = [NSMutableString stringWithString: @"The quick brown fox jumped"]; [string1 replaceCharactersInRange: [string1 rangeOfString: @"brown fox"] withString: @"black dog"]; |
When executed, string1 will contain the string “The quick black dog jumped”.
Deleting Sections of a String
Similar techniques to those described above can be used to delete a subsection of a string using the deleteCharactersInRange method. As with the preceding examples, this method accepts an NSRange structure as an argument and can be combined with the rangeOfString method to perform a search and delete:
1 2 | NSMutableString *string1 = [NSMutableString stringWithString: @"The quick brown fox jumped"]; [string1 deleteCharactersInRange: [string1 rangeOfString: @"jumped"]]; |
Extracting a Subsection of a String
A subsection of a string can be extracted using the substringWithRange method. The range is specified using an NSRange structure and the extracted substring is returned in the form of an NSString object:
1 2 3 4 | NSMutableString *string1 = [NSMutableString stringWithString: @"The quick brown fox jumped"]; NSString *string2; string2 = [string1 substringWithRange: NSMakeRange (4, 5)]; NSLog (@"string2 = %@", string2); |
When executed, the above code will output the substring assigned to string2 (“quick”).
Alternatively, a substring may be extracted from a given index until the end of the string using the subStringFromIndex method. For example:
1 2 3 | NSMutableString *string1 = [NSMutableString stringWithString: @"The quick brown fox jumped"]; NSString *string2; string2 = [string1 substringFromIndex: 4]; |
Similarly, the subStringToIndex may be used to extract a substring from the beginning of the source string up until a specified character index into the string.
Inserting Text into a String
The insertString method inserts a substring into a string object and takes as arguments the NSString object from which the new string is to inserted and the index location into the target string where the insertion is to be performed:
1 2 | NSMutableString *string1 = [NSMutableString stringWithString: @"The quick brown fox jumped"]; [string1 insertString: @"agile, " atIndex: 4]; |
Appending Text to the End of a String
Text can be appended to the end of an existing string object using the appendString method. This method directly modifies the string object on which the method is called and as such is only available for mutable string objects.
1 2 3 | NSMutableString *string1 = [NSMutableString stringWithString: @"The quick brown fox jumped"]; [string1 appendString: @" over the lazy dog"]; NSLog (@"string1 = %@", string1); |
Comparing Strings
String objects cannot be compared using the equality (==) operator. The reason for this is that any attempt to perform a comparison this way will simply compare whether the two string objects are located at the same memory location. Let’s take a look at this via an example:
1 2 3 4 5 6 | NSString *string1 = @"My String"; NSString *string2 = @"My String"; if (string1 == string2) NSLog (@"Strings match"); else NSLog (@"Strings do not match"); |
In the above code excerpt, string1 and string2 are pointers to two different string objects both of which contain the same character strings. If we compare them using the equality operator, however, we will get a “Strings do not match” result. This is because the if (string1 == string2) test is asking whether the pointers point to the same memory location. Since string1 and string2 point to entirely different objects the answer, obviously, is no.
We can now take this a step further and change the code so that both string1 and string2 point to the same string object:
1 2 3 4 5 6 7 | NSString *string1 = @"My String"; NSString *string2; string2 = string1; if (string1 == string2) NSLog (@"Strings match"); else NSLog (@"Strings do not match"); |
Now when we run the code, we get a “Strings match” result because both variables are pointing to the same object in memory.
To truly compare the actual strings contained within two string objects we must use the isEqualToString method:
1 2 3 4 5 6 | NSString *string1 = @"My String"; NSString *string2 = @"My String 2"; if ([string1 isEqualToString: string2]) NSLog (@"Strings match"); else NSLog (@"Strings do not match"); |
Another option is to use the compare method (to perform a case sensitive comparison) or the caseInsenstiveCompare NSString methods. These are more advanced comparison methods that can be useful when sorting strings into order.
Checking for String Prefixes and Suffixes
A string object can be tested to identify whether the string begins or ends with a particular sequence of characters (otherwise known as prefixes and suffixes). This is achieved using the hasPrefix and hasSuffix methods respectively, both of which return boolean values based on whether a match is found or not.
1 2 3 4 5 6 7 8 9 | NSString *string1 = @"The quick brown fox jumped"; BOOL result; result = [string1 hasPrefix: @"The"]; if (result) NSLog (@"String begins with The"); result = [string1 hasSuffix: @"dog"]; if (result) NSLog (@"String ends with dog"); |
Converting to Upper or Lower Case
The Foundation NSString classes provide a variety of methods for modifying different aspects of case within a string. Note that each of these methods returns a new string object reflecting the change, leaving the original string object unchanged.
1 | capitalizedString |
Returns a copy of the specified string with the first letter of each word capitalized and all other characters in lower case:
1 2 3 | NSString *string1 = @"The quicK brOwn fox jumpeD"; NSString *string2; string2 = [string1 capitalizedString]; |
The above code will return a string object containing the string “The Quick Brown Fox Jumped” and assign it to the string2 variable. The string object referenced by string1 remains unmodified.
1 | lowercaseString |
Returns a copy of the specified string with all characters in lower case:
1 2 3 | NSString *string1 = @"The quicK brOwn fox jumpeD"; NSString *string2; string2 = [string1 lowercaseString]; |
The above code will return a string object containing the string “the quick brown fox jumped” and assign it to the string2 variable. The string object referenced by string1 remains unmodified.
1 | uppercaseString |
Returns a copy of the specified string with all characters in upper case:
1 2 3 | NSString *string1 = @"The quicK brOwn fox jumpeD"; NSString *string2; string2 = [string1 uppercaseString]; |
The above code will return a string object containing the string “THE QUICK BROWN FOX JUMPED” and assign it to the string2 variable. The string object referenced by string1 remains unmodified.
Converting Strings to Numbers
String objects can be converted to a variety of number types:
Convert String to int
1 2 3 | NSString *string1 = @"10"; int myInt = [string1 intValue]; NSLog (@"%i", myInt); |
Convert String to double
1 2 3 | NSString *string1 = @"10.1092"; double myDouble = [string1 doubleValue]; NSLog (@"%f", myDouble); |
Convert String to float
1 2 3 | NSString *string1 = @"10.1092"; float myFloat = [string1 floatValue]; NSLog (@"%f", myFloat); |
Convert String to NSInteger
1 2 3 | NSString *string1 = @"10"; NSInteger myInteger = [string1 integerValue]; NSLog (@"%li", myInteger); |
Converting a String Object to ASCII
The string contained within a string object can be extracted and converted to an ASCII C style character string using the UTF8String method. For example:
1 2 3 | NSString *string1 = @"The quick browen fox"; const char *utfString = [string1 UTF8String]; printf ("Converted string = %s\n", utfString); |
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