stacks postfix aww fooey

This is a discussion on stacks postfix aww fooey within the C++ Programming forums, part of the General Programming Boards category; Hello all I'm having a horrible time with this concept of stacks using postfix and prefix operators. I've looked all ...

  1. #1
    Registered User
    Join Date
    Apr 2003
    Posts
    78

    stacks postfix aww fooey

    Hello all I'm having a horrible time with this concept of stacks using postfix and prefix operators. I've looked all over for some sort of example showing me how to implement this idea and I can't find it anywhere. My professor says that you can use a cin.get(c) operation to utilize this type of operation. The whole idea has me totally bamboozled. So with that being said I beseech you all here in cprogamming.com could somebody anybody just show me a basic example of how this is done using cin.get(c).

  2. #2
    Code Goddess Prelude's Avatar
    Join Date
    Sep 2001
    Posts
    9,796
    Basic example:
    Code:
    #include <cctype>
    #include <iostream>
    
    using namespace std;
    
    int main()
    {
      char stack[50];
      char c;
      int top = 0;
    
      while ( cin.get ( c ) && c != '\n' ) {
        if ( isspace ( c ) )
          continue;
    
        switch ( c ) {
        case '+':
          if ( top != 0 )
            cout.put ( stack[--top] );
    
          stack[top++] = c;
    
          break;
        case '-':
          if ( top != 0 )
            cout.put ( stack[--top] );
    
          stack[top++] = c;
    
          break;
        default:
          cout.put ( c );
    
          break;
        }
      }
    
      while ( top != 0 )
        cout.put ( stack[--top] );
    
      cout<<endl;
    }
    My best code is written with the delete key.

  3. #3
    Registered User
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    Apr 2003
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    78

    very nice

    Now that wuz pretty cool. So if I wanted to implement someting for "parentheses" all I need is another switch statement

  4. #4
    Code Goddess Prelude's Avatar
    Join Date
    Sep 2001
    Posts
    9,796
    >So if I wanted to implement someting for "parentheses" all I need is another switch statement
    Well, yes, but that's not all you need. You also need to work out precedence where parentheses are the highest precedence. That way the postfix expression is correct. The algorithm would go something like this:
    Code:
    #include <cctype>
    #include <iostream>
    
    using namespace std;
    
    int precedence ( char c )
    {
      if ( c == '+' || c == '-' )
        return 0;
      else if ( c == '*' || c == '/' )
        return 1;
      else if ( c == '(' || c == ')' )
        return 2;
    
      return -1;
    }
    
    int main()
    {
      char stack[50];
      char c;
      int top = 0;
    
      while ( cin.get ( c ) && c != '\n' ) {
        if ( isspace ( c ) )
          continue;
    
        if ( precedence ( c ) != -1 ) {
          if ( top != 0 ) {
            if ( c == ')' ) {
              while ( top != 0 && stack[top - 1] != '(' )
                cout.put ( stack[--top] );
    
              --top;
    
              continue;
            }
            else {
              while ( top != 0 && precedence ( stack[top - 1] ) >= precedence ( c ) ) {
                if ( stack[top - 1] == '(' )
                  break;
                else
                  cout.put ( stack[--top] );
              }
            }
          }
    
          stack[top++] = c;
        }
        else
          cout.put ( c );
      }
    
      while ( top != 0 )
        cout.put ( stack[--top] );
    
      cout<<endl;
    }
    The code is pretty fugly (in other words, plagiarizing it will get you a bad grade ), but I'm too busy with other things to clean it up nice and pretty.
    My best code is written with the delete key.

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