5.2 Basic console I/O 5.2.1 Writing to console For non-GUI Java programs, you use System.out.println() to output something to the command line console. For C#, you use System.Console.WriteLine() . WriteLine is a static method of the Console class, which is found in the System namespace (see section 5.3). Both methods are similar “ you can pass in a string, a numeric value, or a combination of strings and numeric values (using the + concatenation operator). This example should make things clear. 1: using System; 2: 3: class TestClass{ 4: public static void Main(){ 5: Console.WriteLine 6: ("Words\n" + "and " + 99 + " numbers."); 7: Console.WriteLine 8: ("Interesting {0} of WriteLine","usage"); 9: Console.WriteLine 10: ("{0} plus {1} gives {2}", 10, 20, 10+20); 11: 12: TestClass c = new TestClass(); 13: Console.WriteLine (c); 14: } 15: } This generates the following output. c:\expt>test Words and 99 numbers. Interesting usage of WriteLine 10 plus 20 gives 30 TestClass Like Java -
Like Java's System.out.println() , System.Console.WriteLine() adds a line feed automatically after the line is printed out. You can also use special characters such as \n inside strings to insert a line break. -
In addition to WriteLine() , the Console class also has a Write() method. Write() does not insert a newline character at the end of the printed statement as WriteLine() does. Console.Write() and Console.WriteLine() are similar to System.out.println() and System.out.print() , respectively. -
Like Java, the string concatenation operator (+) in C# works from left to right. When only one of the two operands of the string concatenation operator is a string, the other operand will be implicitly cast into a string before concatenation occurs. Study the following WriteLine statements and their results. Console.WriteLine("1"+2); // Output: "12" Console.WriteLine("1"+"2"); // Output: "12" Console.WriteLine(1+"2"); // Output: "12" Console.WriteLine(1+2+"3"); // Output: "33", since 1+2 is // evaluated first. Console.WriteLine("1"+2+3); // Output: "123", since "1"+2 // is evaluated first. -
When you try to print out an object reference (line 13), the ToString() method of that class is called. In this case, the class type ( TestClass ) is displayed. For Java, when you try to print out the value of an object reference variable, the toString() method of the object is invoked automatically to return a String for printing. For C#, the ToString() method of the object is also automatically invoked when you do the same thing. Your C# class can override ToString() (using the method signature public override string ToString(); ) to return something more meaningful if you want. By default, Java's java.lang.Object 's toString() method returns a hash value of the object (in the form typename@hashcode ). C#'s System.Object 's ToString() method (which has been inherited by TestClass in this case) returns the class type. Unlike Java -
Take note of lines 7 “ 10. Here is some legacy stuff borrowed from C. The format is similar to traditional C's printf statement in which {0} will be matched to the first element after the comma, {1} will correspond to the second, and so forth. Study lines 9 “ 10: 9: Console.WriteLine 10: ("{0} plus {1} gives {2}", 10, 20, 10+20); Here {0} is matched with the first value after the string ( 10 ), {1} is matched with the second ( 20 ), and {2} will be matched with the third, ( 10 + 20 ), resulting in the output: 10 plus 20 gives 30 -
You can also use variables instead of values. Assuming that a and b are int variables, this statement: Console.WriteLine ("{0} plus {1} gives {2}", a, b, a+b); and the following: Console.WriteLine (a + " plus " + b + " gives " + (a+b)); do exactly the same thing. -
Java converts will definitely find using the + operator much more intuitive “ choosing either way is largely a personal preference. However, if you are printing out floating-point values, using {} to print out variable values has one big advantage. You can specify the number of digits you want to be shown after the decimal point with the appropriate rounding using # es. [5] Examine the code fragment below and its output. [5] To do this in Java would require use of the DecimalFormat class. float f = 3.87769f; Console.WriteLine ("{0}, {1:#}, {2:#.#},{3:#.##}",f,f,f,f); Output: 3.87769, 4, 3.9, 3.88 One limitation of using # es is that trailing zeros are not displayed: float f = 3.8f; Console.WriteLine ("{0}, {1:#}, {2:#.#}, {3:#.##}",f,f,f,f); Output: 3.8, 4, 3.8, 3.8 5.2.2 Reading from console Java has I/O classes to support the reading of user inputs from the keyboard [6] for console applications. In C#, this is even easier with System.Console.ReadLine() . [6] Most Java developers will do something like this to read user input via the command line console: BufferedReader r = new BufferedReader (new InputStreamReader(System.in)); String s = r.readLine(); Examine this example: 1: using System; 2: 3: class TestClass{ 4: public static void Main(){ 5: Console.Write("Enter your name: "); 6: string name = Console.ReadLine (); 7: Console.WriteLine("Hi " + name + "!"); 8: } 9: } Output: c:\expt>test Enter your name: Mok Hi Mok! 5.2.3 Converting a string to an int You can use the System.Convert class for common type conversions. Useful methods in the Convert class include ToInt32() , ToBoolean() , and ToDouble() . An example of how a string is converted to an int is shown here. 1: using System; 2: 3: class TestClass{ 4: public static void Main(){ 5: 6: Console.Write("Enter a number: "); 7: string userInput = Console.ReadLine(); 8: 9: try{ 10: int number = Convert.ToInt32 (userInput); 11: Console.WriteLine(number+10); 12: } 13: catch (System.FormatException){ 14: // exception handler 15: } 16: 17: } 18: } Output: c:\expt>test Enter a number: 9 19 |
