- Let's first study the following problem:
- Suppose we enter 1 (one) digit on the key board
- Write a Java program that prints out the value of the input digit + 1
Example:
- If we enter 0, the program must print 1
- If we enter 1, the program must print 2
- And so on.
- Let's run the
/home/cs255001/demo/atoi/JavaInput_3.java
program to get some idea on what to do.
Here are some output when you run /home/cs255001/demo/atoi/JavaInput_3.java using 1 digit inputs:
Enter input: 0 String = 0 ASCII codes (in decimal) for the string: 48
Enter input: 1 String = 1 ASCII codes (in decimal) for the string: 49
Enter input: 2 String = 2 ASCII codes (in decimal) for the string: 50
Enter input: 3 String = 3 ASCII codes (in decimal) for the string: 51Notice that:
Input typed ASCII code (in decimal) ------------- ------------------------- 0 ----> 48 1 ----> 49 2 ----> 50 3 ----> 51 4 ----> 52 5 ----> 53 6 ----> 54 7 ----> 55 8 ----> 56 9 ----> 57This is because:
- The ASCII code for 0 is the binary number 01100001 which is equal to the decimal number 48(10).
- The ASCII code for 1 is the binary number 01100010 which is equal to the decimal number 49(10).
- The ASCII code for 2 is the binary number 01100011 which is equal to the decimal number 50(10).
- And so on.
Try it yourself. Type any number here:
The input corresponds to these binary numbers:
Convert the binary number to decimal values and you will find the above mapping.
- A mapping bewteen
2 sets of values can
easily (but clumpsily) implemented
as follows:
if ( input = input1 ) output = output1; else if ( input = input2 ) output = output2; else ....
-
Here is a Java program that
reads in one ASCII digit code,
converts it using the above
"naive" mapping algorithm into
an int (2's complement code)
so that we can perform
an addition (+1) to
the value:
// // File: /home/cs255001/demo/atoi/JavaInput_5.java // // Read in 1 character and add 1 to the input // import java.util.Scanner; public class JavaInput_4 { public static void main(String[] args) throws Exception { Scanner in = new Scanner(System.in); String s = null; int x = 0; while ( true ) { System.out.print("Enter input: "); s = in.next( ); // Read a String from key board if ( s.charAt(0) == '0' ) x = 0; else if ( s.charAt(0) == '1' ) x = 1; else if ( s.charAt(0) == '2' ) x = 2; else if ( s.charAt(0) == '3' ) x = 3; else if ( s.charAt(0) == '4' ) x = 4; else if ( s.charAt(0) == '5' ) x = 5; else if ( s.charAt(0) == '6' ) x = 6; else if ( s.charAt(0) == '7' ) x = 7; else if ( s.charAt(0) == '8' ) x = 8; else if ( s.charAt(0) == '9' ) x = 9; else { System.out.println("Input '"+s.charAt(0)+"' is NOT a digit !\n"); continue; } System.out.println("integer value x = " + x); x = x + 1; // Show that we can compute with x ! System.out.println("x + 1 = " + x + "\n"); } }
- Example Program:
(Demo above code)
- Prog file: /home/cs255001/demo/atoi/JavaInput_5.java
How to run the program:
- To compile: javac JavaInput_5.java
- To run: java JavaInput_5
- Notice that
the ASCII codes for
the digits are
consecutive:
character ASCII code (in decimal) --------- ----------- '0' 48 '1' 49 '2' 50 '3' 51 '4' 52 '5' 53 '6' 54 '7' 55 '8' 56 '9' 57
We can also perform the mapping using this mapping function:
input - '0' (≡ input - 48)Because:
Note: '0' ≡ 48 !!! Input typed ASCII code (in decimal) mapped input value (in decimal) ------------- ------------------------- ------------------------- '0' ----> 48 48 - 48 = 0 '1' ----> 49 49 - 48 = 1 '2' ----> 50 50 - 48 = 2 '3' ----> 51 51 - 48 = 3 '4' ----> 52 52 - 48 = 4 '5' ----> 53 53 - 48 = 5 '6' ----> 54 54 - 48 = 6 '7' ----> 55 55 - 48 = 7 '8' ----> 56 56 - 48 = 8 '9' ----> 57 57 - 48 = 9Here is a Java program that reads in one ASCII digit code, converts it using the above mapping function into an int (2's complement code) so that we can perform an addition (+1) to the value:
// // File: /home/cs255001/demo/atoi/JavaInput_4.java // // Read in 1 character and add 1 to the input // import java.util.Scanner; public class JavaInput_4 { public static void main(String[] args) throws Exception { Scanner in = new Scanner(System.in); String s = null; while ( true ) { System.out.print("Enter input: "); s = in.next( ); // Read a String from key board if ( s.length() > 1 ) { System.out.println("Error: you must type 1 digit !"); System.out.println("Try again\n"); continue; } System.out.println("ASCII codes (in decimal) = " + (int) s.charAt(0) ); int x = (int) s.charAt(0); x = x - '0'; // Convert ASCII repr to 2's complement System.out.println("integer value x = " + x); x = x + 1; // Now we can add !!! System.out.println("After x+1, x = " + x); } } }
- Example Program:
(Demo above code)
- Prog file: /home/cs255001/demo/atoi/JavaInput_4.java
How to run the program:
- To compile: javac JavaInput_4.java
- To run: java JavaInput_4
- Experiment:
- Try enter the input ":" and see if you understand why this program returns the value 10.
- In the above example program,
we took advantage of the
fact that:
- The ASCII code values
for digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
increase by 1
- Therefore, we can use the subtraction:
inputDigit - '0'to obtain the 2s complement representation for the value that is represented by a digit
- The ASCII code values
for digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
increase by 1
- If the digit are
not in sequence, we will have to
use the T color="red">general way to write
a map:
int x = (int) s.charAt(0); // Convert ASCII repr to 2's complement if ( s.charAt(0) == '0' ) // This test if ASCII code = 48 x = 0; // This assigns 0000.00 to x !! else if ( s.charAt(0) == '1' ) x = 1; else if ( s.charAt(0) == '2' ) x = 2; else if ( s.charAt(0) == '3' ) x = 3; else if ( s.charAt(0) == '4' ) x = 4; else if ( s.charAt(0) == '5' ) x = 5; else if ( s.charAt(0) == '6' ) x = 6; else if ( s.charAt(0) == '7' ) x = 7; else if ( s.charAt(0) == '8' ) x = 8; else if ( s.charAt(0) == '9' ) x = 9;This is the most general way to convert an ASCII code to its corresponding 2s complement value !!