infix to postfix program

Question

I have written the following infix to postfix program but it's not working. My program takes input but doesn't show any result. Can anyone help find the problem in my program. And also it would be a great help if you tell if my Algorithm for converting infix to postfix is correct or not.

using namespace std;

class Stack
{
private:
    int top;
    char s[mx];
public:
    Stack()
    {
        top=-1;
    }

    void push(char c)
    {
        if(!stackFull())
        s[++top]=c;
    }

    void pop()
    {
        if(!stackEmpty())
        top--;
        else cout<<"Stack is empty"<<endl;
    }

    char topShow()
    {
        if(!stackEmpty())
        return s[top];
    }

    bool stackEmpty()
    {
        if(top==-1)
            return 1;
        else return 0;
    }

    bool stackFull()
    {
        if(top == (mx-1))
            return 1;
        else return 0;
    }
};

class Expression
{
private:
    char entry2;
    int precedence;
    char infix[mx];
    char postfix[mx];
public:

int prec(char symbol)
{
    switch(symbol)
    {
        case '(':return 0; break;
        case '-':return 1; break;
        case '+':return 2; break;
        case '*':return 3; break;
        case '/':return 4; break;
    }
}

void Read()
{
    cout<<"Enter the infix expression: ";cin>>infix;
    for(int i=0;infix[i]!='\0';i++)
    {
        convertToPostfix(infix[i]);
    }
}

void ShowResult()
{
    cout<<"Postfix expression"<<endl;
    for(int j=0;postfix[j]!='\0';j++)
    {
        cout<<postfix[j];
    }
}

void convertToPostfix(char c)
{
    int p=0;
    Stack myStack;
    precedence=prec(c);
    entry2=myStack.topShow();
    if(isdigit(c))
    {
        postfix[++p]=c;
    }

    if(precedence>prec(entry2))
    {
        myStack.push(c);
    }

    if(precedence<prec(entry2))
    {
        switch(c)
        {
            case '(': myStack.push(c); break;
            case ')': while(myStack.topShow()!= '(')
                        {
                            postfix[++p]=myStack.topShow();
                            myStack.pop();
                        };myStack.pop();break;
            case '+':
            case '-':
            case '*':
            case '/': while(prec(myStack.topShow())>=precedence)
                        {
                            postfix[++p]=myStack.topShow();
                            myStack.pop();
                        };break;
        }
    }

}

};

int main()
{
    Expression myExp;
    myExp.Read();
    myExp.ShowResult();
    return 0;
}

Answer 1

Here are some issues I found:

Boolean Functions Return true or false Match return types with return values. The numbers 1 and 0 are not Boolean values.

Precedence table
Add and subtract have same precedence.
Multiply and divide have same precedence.
Multiply and divide have higher precedence than add and subtract.

Stack disappears
Since the stack is declared as a local variable in the function, it will be created fresh when entering the function and destroyed before exiting the function.

Solution: move it to the class as a class member or declare it as static .

Multiple statements per line are not more efficient
Blank lines and newlines do not affect performance, and add negligible time to the build.
However, they make your program more readable which helps when inspecting or debugging. Use them.

And similarly with space before and after operators.
Build the habit now rather than correcting when you get a job.

Call function once and store the value
You call prec(entry2) twice, which is a waste of time. Call it once and save the value in a variable. Similarly with stack.TopShow() .

Use std::vector not an array
The std::vector will grow as necessary and reduce the chance of buffer overflow.
With an array, you must check that your indices are always within range. Also, array capacities don't change; you have to declare a new instance and copy the data over.

The variable mx is not declared
The compiler should catch this one. You use mx as the capacity for an array and comparing for full . However, it is never declared, defined nor initialized. Prefer std::vector and you won't have to deal with these issues.

Input is not validated
You input a letter, but don't validate it.
Try these characters: space, #, @, A, B, etc.

Missing default for switch
Crank up your compiler warnings to maximum. Your switch statements need default s. What precedence do numeric characters ('0'..'9') have?
(You check the precedence of numeric characters.)

Check all paths through your functions and program. Using a debugger (see below) or pen and paper, check your program flow through you functions. Include boundary values and values not within the bounds.

Case statements: break or return
You don't need a break after a return statement. Think about it. Can the program continue executing at the line after a return statement?

Use a debugger or print statements
You can print variables at different points in your program. This is an ancient technique when debuggers are not available.
Learn to use a debugger. Most IDEs come with them. You can single step each statement and print out variable values. Very, very, useful.

Answer 2

class infixToPostfix{
    public static void postfix(String str){
        Stack<Character> stk = new Stack<Character>();
        for(Character c : str.toCharArray()){
            // If operands appears just print it
            if(c >= 'A' && c <= 'Z' || c >= 'a' && c <= 'z'){
                System.out.print(c);
            }else{ 
                // Open paranthesis push is 
                if(c == '('){
                    stk.push(c);
                //Close paranthesis pop until close paranthesis
                }else if( c == ')'){
                    while(stk.peek() != '(')
                        System.out.print(stk.pop());
                    stk.pop();
                // check the precedence of operator with the top of stack
                }else if(c == '+' || c == '-'){
                    if(!stk.isEmpty()){
                        char top = stk.peek();
                        if(top == '*' || top == '/' || top == '+' || top == '-'){
                            System.out.print(stk.pop());    
                        }
                    }   
                    stk.push(c);
                }else{
                    if(!stk.isEmpty()){
                        char top = stk.peek();
                        if(top == '/' || top == '*'){
                            System.out.print(stk.pop());
                        }
                    }   
                    stk.push(c);
                }
            }
        }
        //Print all the remaining operands
        while(!stk.isEmpty()) System.out.print(stk.pop());
        System.out.println();
    }
    public static void main(String args[]){
        String str = "A+B-(c+d*Z+t)/e";
        postfix(str);
    }
}

Answer 3

using stack and map u can solve the problem
1) create a map having operator as key and some integer to set priority. operator with same precedence will have same value something like:

map<char,int>oprMap;
oprMap['^'] = 3;
oprMap['*'] = 2;
oprMap['/'] = 2;
oprMap['+'] = 1;
oprMap['-'] = 1;

2) iterate through given expression call these checks
1) if current element
i) is operand add it to result
ii) not operand do following check
a. while not (stack is empty and element is open bracket and found operator with higher precedence. add top of the stack to the result and pop()
b.push current element to stack

iii) if open brackets push to stack
iv) if closed brackets pop until get closed bracket and add it to result

3) while stack is not empty pop() and add top element to the result.

{       
        stack<char>S;
        for (int i = 0; i < n; i++)  {
            if(isOperand(exps[i])) {
                res = res + exps[i];
            } else if(isOperator(exps[i])){

                while(!(S.empty() && isOpenParanthesis(S.top()) && isHeigherPrecedence(S.top(),exps[i])){
                    res = res+S.top();
                    S.pop();
                }  
                S.push(exps[i]);
            } else if(isOpenParanthesis(exps[i])) {
                S.push(exps[i]);
            } else if(isClosingParanthesis(exps[i])) {
                while(!S.empty() && !isOpenParanthesis(S.top())) {
                    res = res+S.top();
                    S.pop();
                }
                S.pop();
            }
        }

        while(!S.empty()) {
            res = res + S.top();
            S.pop();
        }
    }
}

Answer 4

#include<bits/stdc++.h>
using namespace std;

// This isHigher function checks the priority of character a over b.

 bool isHigher(char a,char b)
{
if(a=='+' || a=='-')
return false;

else if((a=='*' && b=='*') || (a=='*' && b=='/') || (a=='/' && b=='*') ||          
 (a=='/' && b == '/')|| (a=='^' && b=='^')||(a=='*' && b=='^') || (a=='/' && 
  b=='^'))
    return false;

    return true;
}

int main(){

    string s;
    cin>>s;
    s = s + ")";
 //Vector postfix contains the postfix expression.
    vector<char>postfix;
    stack<char>mid;
    mid.push('(');
    for(int i=0;i<s.length();i++)
    {
        if(s[i] == '(')
            mid.push(s[i]);

        else if(s[i] == '+' || s[i] == '^' || s[i] == '-' || s[i] == '*'|| 
          s[i] == '/')
        {
            if(mid.top() == '(')
                mid.push(s[i]);

            else {

                if(isHigher(s[i],mid.top()))
                    mid.push(s[i]);

                else
                {

                 while(mid.top()!='(')
                 {

                    if(!isHigher(s[i],mid.top()))
                        {
                            postfix.push_back(mid.top());
                            mid.pop();  
                        }

                    else
                        break;

                 }

                 mid.push(s[i]);

                }
            }
        }
        else if(s[i] == ')')
        {

            while(mid.top() != '(')
            {
                            postfix.push_back(mid.top());
                            mid.pop();      
            }

            mid.pop();

        }

        else
            postfix.push_back(s[i]);

    }

    for(int i=0;i<postfix.size();i++)
        cout<<postfix[i];

 return 0;

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}