PostfixEvaluation.cpp

/*
  Evaluation Of postfix Expression in C++ 
  Input Postfix expression must be in a desired format. 
  Operands must be integers and there should be space in between two operands.
  Only '+'  ,  '-'  , '*' and '/'  operators are expected. 
*/
#include<iostream>
#include<stack>
#include<string>

using namespace std;

// Function to evaluate Postfix expression and return output
int EvaluatePostfix(string expression);

// Function to perform an operation and return output. 
int PerformOperation(char operation, int operand1, int operand2);

// Function to verify whether a character is operator symbol or not. 
bool IsOperator(char C);

// Function to verify whether a character is numeric digit. 
bool IsNumericDigit(char C);

int main() 
{
	string expression; 
	cout<<"Enter Postfix Expression \n";
	getline(cin,expression);
	int result = EvaluatePostfix(expression);
	cout<<"Output = "<<result<<"\n";
}

// Function to evaluate Postfix expression and return output
int EvaluatePostfix(string expression)
{
	// Declaring a Stack from Standard template library in C++. 
	stack<int> S;

	for(int i = 0;i< expression.length();i++) {

		// Scanning each character from left. 
		// If character is a delimitter, move on. 
		if(expression[i] == ' ' || expression[i] == ',') continue; 

		// If character is operator, pop two elements from stack, perform operation and push the result back. 
		else if(IsOperator(expression[i])) {
			// Pop two operands. 
			int operand2 = S.top(); S.pop();
			int operand1 = S.top(); S.pop();
			// Perform operation
			int result = PerformOperation(expression[i], operand1, operand2);
			//Push back result of operation on stack. 
			S.push(result);
		}
		else if(IsNumericDigit(expression[i])){
			// Extract the numeric operand from the string
			// Keep incrementing i as long as you are getting a numeric digit. 
			int operand = 0; 
			while(i<expression.length() && IsNumericDigit(expression[i])) {
				// For a number with more than one digits, as we are scanning from left to right. 
				// Everytime , we get a digit towards right, we can multiply current total in operand by 10 
				// and add the new digit. 
				operand = (operand*10) + (expression[i] - '0'); 
				i++;
			}
			// Finally, you will come out of while loop with i set to a non-numeric character or end of string
			// decrement i because it will be incremented in increment section of loop once again. 
			// We do not want to skip the non-numeric character by incrementing i twice. 
			i--;

			// Push operand on stack. 
			S.push(operand);
		}
	}
	// If expression is in correct format, Stack will finally have one element. This will be the output. 
	return S.top();
}

// Function to verify whether a character is numeric digit. 
bool IsNumericDigit(char C) 
{
	if(C >= '0' && C <= '9') return true;
	return false;
}

// Function to verify whether a character is operator symbol or not. 
bool IsOperator(char C)
{
	if(C == '+' || C == '-' || C == '*' || C == '/')
		return true;

	return false;
}

// Function to perform an operation and return output. 
int PerformOperation(char operation, int operand1, int operand2)
{
	if(operation == '+') return operand1 +operand2;
	else if(operation == '-') return operand1 - operand2;
	else if(operation == '*') return operand1 * operand2;
	else if(operation == '/') return operand1 / operand2;

	else cout<<"Unexpected Error \n";
	return -1; 
}

char in C++ evaluates to the decimal equivalent.
'0' is 48
'1' is 49
'2' is 50
.
.
.
if expression[i] = '2' which evaluates to 50
so (expression[i] - '0') in this case will be (50 - 48) = 2

The reason segmentation fault happens is that you don't put space between each operand.
For example, if you type: "11+", it's buggy. It must be "1 1+" or "1 1 +".
By applying this logic, it can count up multi digits numbers.

Here is my implemention, I have fixed the while loop error:
#include
using namespace std;
#include
#include

bool IsOperator(char x) {
if (x == '+' || x == '-' || x == '*') {
return true;
}
return false;
}

bool IsOperand(char x) {
// cout<<"x"<<x<<endl;
if (x <= '9' && x >= '0') return true;
return false;

}

int PerformOperation(char c, int x, int y) {
if (c == '+') return(x + y);
else if (c == '-') return(x - y);
else if (c == '*') return(x * y);
else cout << "unexpected error" << endl;
return -1;
}

void Print(stack S) {
while (!S.empty()) {
cout << S.top() << endl;
S.pop();
}
cout << "" << endl;
}

int EvaluatePostfix(string expression) {
stack S;

for (int i = 0; i < expression.length(); i++) {
    // cout<<IsOperator(expression[i])<<endl;
   //  cout<<IsOperand(expression[i])<<endl;
    if (expression[i] == ' ' || expression[i] == ',') continue;
    else if (IsOperator(expression[i])) {
        int operand2 = S.top(); S.pop();
        if (S.empty()) { cout << "Error: check your postfix" << endl; return -1; }
        int operand1 = S.top(); S.pop();
        int result = PerformOperation(expression[i], operand1, operand2);
        S.push(result);
        cout << "进入operatorS is:" << endl;
        Print(S);
    }

    else if (IsOperand(expression[i])) {
        int operand = 0;
        while (IsOperand(expression[i]) && i < expression.length()) {
            operand = operand * 10 + (expression[i] - '0');
            i++;
            cout << i << endl;
        }
        i--;  //比如35 6+ 在遇到5后i++=2 在这次程序结束后还要自动+1 会跳过对i=2的判断  所以需要-1
        S.push(operand);
        cout << "进入operand S is:" << endl;
        Print(S);
    }


    //  cout << i << endl;
}
return S.top();

}

int main()
{
cout << "input postfix" << endl;
string expression;
// cin>>expression;
getline(cin, expression);
cout << "expression:" << expression << endl;
int result = EvaluatePostfix(expression);
cout << "result is " << result << endl;

}