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 02282009, 05:26 AM #1Member
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[SOLVED] Evaluating an Airthmetic Expression
Hi All,
I need to write a Java program, which reads and Airthmetic Expression and evaluates it.
I have done some part of the program, using classes, but got stuck in coding the Evaluation method. But let me admit that I do have some idea of evaluating the inputted expression.
NOTE: Its assumed that user always enters valid expression and the expression has only *,/,+ and  operators only.
Evaluation method:
I am reading a valid arithmetic expression and looking for the first occurrence of * or / sign and then from that index I am taking the string on the left side of the sign and string to right side of the sign.
Then in Left String I am looking for the lastindexOf( ) function which will give me the last occurrence of the + or  sign(since from the precedence rule Multiplication/Division is first and then + )
And on the Right side I am looking for indexOf( ) string function which will give me the index of the first occurrence of the operator + ,  , / or * . Based on these two left and right indices I am extracting two operands and one operator(either * or / whichever comes first) and performing the operation between them and will update the string with this operation result.
I will be doing this till my expression has only + and  operators and then at the end I can evaluate the string.
But now what I did, is very long and its not even giving the required result.
I will really appreciate if any one can help me with this.
Thanks,
learning_javaNow
 02282009, 06:25 AM #2Senior Member
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why not make yourself an example and walk through your algorithm step by step to see where you went wrong? or some pseudocode.
 02282009, 06:47 AM #3Member
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 02282009, 10:28 AM #4Senior Member
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you don't need to implement your own stack, but it's one way to solve the problem. there's probably a few good ways to do this. personally, i find recursion to be the simplest.
btw, a google search would get you this link, which is probably similar (but possibly more advanced) than what you're trying to do: Sample Code to Evaluate an Infix Expression
 02282009, 05:20 PM #5Member
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Thank you that was helpful.
 02282009, 05:21 PM #6Member
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But the thing I am asked to do it without recursions, since its more like How to use different methods in String class.
 03012009, 05:54 AM #7Member
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Help with Evaluatig an Airthmetic expression code
Can any body help me with this execution error???
I have written this program, but it shows me an error saying that "} is expected", though I have put all brackets properly.
this is my code:
import java.util.Scanner;
class AirthExp
{
private String e;
AirthExp()
{
e = "";
}
AirthExp(String s)
{
e = s;
}
/*public void set(String s)
{
e = s;
}*/
public void print()
{
System.out.println(e.substring(0, e.length()1));
}
public int evaluate()
{
int n1=0, n2=0;
int op1 = 1, op2 = 1;
for(int i=0; i<e.length(); i++)
{
if(e.charAt(i)=='*' e.charAt(i) =='/' e.charAt(i)=='#')
{
String eleft = e.substring(0,i);
String eright = e.substring(i+1, e.length());
op1 = eleft.lastIndexOf('+');
op2 = eright.lastIndexOf('');
if(op1 > op2)
n1 = Integer.parseInt(eleft.substring(op1+1, i));
else
n2 = Integer.parseInt(eleft.substring(op2+1, i));
for(int k = 0; k < eright.length()1; k++)
{
if(eright.charAt(k)== '+'  eright.charAt(k)== '' eright.charAt(k)== '*'  eright.charAt(k)== '/' eright.charAt(k)== '#')
{
String erightNum = eright.substring(0, k);
n2 = Integer.parseInt(erightNum);
}
}
if(e.charAt(i)=='*')
n1 = n1 * n2;
else if(e.charAt(i) =='/')
n1 = n1 / n2;
}
return n1;
}
}
class AirthExpDemo
{
public static void main(String [] args)
{
Scanner keyin = new Scanner(System.in);
String str;
System.out.println("Please Enter a valid Airthmetic Expression followed by #:");
str = keyin.next();
AirthExp expStr = new AirthExp(str);
expStr.print();
System.out.println("= ");
}
}
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