CODEWITHSUNDEEP
c++stack
appleLady
appleLady

Reputation: 1

evaluate postfix

My code suppose to calculate the values of postfix expression. But I am stuck at "result" which i dont know how to write the code. I wrote : result = operand1 operation.push operand2 and logically will give an error. I used 2 stack.

int main() 
{
    string input;
    cout << "Enter a postfix expression: " << endl;
    getline(cin, input);

    double operand1, operand2, result;
    stack<double> number;
    stack<char>operation;

    int i=0;
    while (i < input.length()) 
    {
        if (input[i] == '+' || input[i] == '-' || input[i] == '*' || input[i] == '/' || input[i] == '^') 
            operation.push(input[i]);
        else 
            number.push(input[i]);
        i++;
    }

    operand2 = number.top( );
    number.pop( );
    operand1 = number.top( );
    number.pop( );
    result = operand1 operation.push(input[i]) operand2
    cin.ignore(numeric_limits<streamsize>::max(), '\n');
    return 0;
}

Can anyone suggest a better solution? Thank you

Upvotes: 0

Views: 1372

Answers (2)

AndersK
AndersK

Reputation: 36082

first of all you only need one stack

then the expression

result = operand1 operation.push operand2

doesn't look like any postfix I know, instead I was expecting something like

operand1 operand2 operator

so you push operands on the stack and whenever you find a operator, you pop the two topmost items on the stack and push the result.

e.g.

infix 10 * ( 12 + 15 ) -> postfix 12 15 + 10 *

then when evaluating it (pseudo code)

push -> 12 [operand]
push -> 15 [operand]
pop -> +   [operator]
push result 12+15 [operand]
push 10 [operand]
pop -> *  [operator]
push result 27*10 [operand]

Upvotes: 0

pajton
pajton

Reputation: 16226

You need to make a switch on the operator and compute the result yourself:

char op = operation.top();
switch( op ) {
    case '+': result = operand1 + operand2; break;
    case '-': result = operand1 - operand2; break;
    case '*': result = operand1 * operand2; break;
    case '/': result = operand1 / operand2; break;
    case '^': result = pow(operand1, operand2) ; break;
}

Upvotes: 1

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