Comparing Strings in Visual C#.NET

Generator USS Code 39 in Visual C#.NET Comparing Strings

Handling input from the command line Managing formatted output Working efficiently with strings using the StringBuilder In addition to the examples shown in the rest of this chapter, take a look at the StringCaseChanging and VariousStringTechniques examples on the Web site

It s very common to need to compare two strings For example, did the user input the expected value Or maybe you have a list of strings and need to alphabetize them If all you need to know is whether two strings are equal (same length and same characters in the same order), you can use the == operator (or its inverse, !=, or not equal):

string a = string b = if(a == b) if(a != b) programming ; Programming ; // True if you don t consider case, false otherwise // False if you don t consider case, true otherwise

But comparing two strings for anything but equality or inequality is another matter It doesn t work to say

if(a < b)

So if you need to ask, Is string A greater than string B or Is string A less than string B , you need another approach

Numerous operations treat a string as a single object for example, the Compare() method Compare(), with the following properties, compares two strings as though they were numbers: If the left-hand string is greater than the right string, Compare(left, right) returns 1 If the left-hand string is less than the right string, it returns 1 If the two strings are equal, it returns 0 The algorithm works as follows when written in notational C# (that is, C# without all the details, also known as pseudocode):

compare(string s1, string s2) { // Loop through each character of the strings until // a character in one string is greater than the // corresponding character in the other string foreach character in the shorter string if (s1 s character > s2 s character when treated as a number) return 1 if (s2 s character < s1 s character) return -1 // Okay, every letter matches, but if the string s1 is longer, // then it s greater if s1 has more characters left return 1 // If s2 is longer, it s greater if s2 has more characters left return -1 // If every character matches and the two strings are the same // length, then they are equal return 0 }

Thus, abcd is greater than abbd , and abcde is greater than abcd More often than not, you don t care whether one string is greater than the other, but only whether the two strings are equal You do want to know which string is bigger when performing a sort The Compare() operation returns 0 when two strings are identical The following test program uses the equality feature of Compare() to perform a certain operation when the program encounters a particular string or strings BuildASentence prompts the user to enter lines of text Each line is concatenated to the previous line to build a single sentence This program exits if the user enters the word EXIT, exit, QUIT, or quit:

// BuildASentence -- The following program constructs sentences // by concatenating user input until the user enters one of the // termination characters This program shows when you need to look for // string equality using System; namespace BuildASentence { public class Program { public static void Main(string[] args) { ConsoleWriteLine( Each line you enter will be + added to a sentence until you + enter EXIT or QUIT ); // Ask the user for input; continue concatenating // the phrases input until the user enters exit or // quit (start with an empty sentence) string sentence = ; for (; ; ) {