In the function vector in_to_post(vector infix) (the vector=(1 + 2) * 3 ^ 4 ) is being converted to an output vector which is of the form (Postfix: 1 2 + 3 4 ^ *) and i am getting an error saying invalid use of void expression . I have commented the lines where i am getting this error.
char get_optr(Token t) {
// your code here
return t.optr;
}
vector<Token> in_to_post(vector<Token> infix)
{
vector<Token> output;
// your code here
stack<Token>b;
for(int i=0;i<infix.size();i++)
{
if(is_opnd(infix[i]))
{
output.push_back (infix[i]);
}
else if(get_optr(infix[i])=='(')
{
b.push(infix[i]);
}
else if(get_optr(infix[i])==')')
{
while(b.empty()==false)
{
//char i=get_optr(b.pop());
if(get_optr(b.pop())=='(') // I am getting error here
{
break;
}
else
{
output.push_back(b.pop()); //error
}
}
}
else if(is_opnd(infix[i])==false)
{
// char i=infix[i];
//char j=b.top();
if((priority(infix[i])>priority(b.top()))||get_optr(infix[i])=='('||get_optr(infix[i])==')')
{
b.push(infix[i]);
}
}
}
while(b.empty()==false)
{
output.push_back(b.pop()); //error
}
return output;
}
Answer
The issue is that pop on a std::stack simply pops the top element off the stack, but doesn’t return it. This function has a void return type, and if you try to use it in a context that requires a value, you get an error
output.push_back(b.pop()); //error
Instead, you can use top to get top element of the std::stack
output.push_back(b.top()); // ok
Note that this will simply return a reference to the top element without popping it. You can call pop after top to pop it.