Need help with my do while loop problem. :)

[jam51197]

I have been experiencing difficulties finishing my function. To those who have time, please take a look.

void prefix(){
	int a;
	system("cls");
	
	do{
		
		cout << "[1] Start\n[2] Return\n";
		cin >> a;
		switch (a){
		case 1:
		{
				  char exp[STACK_SIZE], prefixExp[STACK_SIZE];
				  int j = 0;
				  cout << "Enter the expression: ";
				  cin.getline(exp, STACK_SIZE);
				  _strrev(exp);
				  strcat_s(exp, "(");
				  npush(')');


				  for (int i = 0; i < strlen(exp); i++){
					  if (isalnum(exp[i]))
						  prefixExp[j++] = exp[i];
					  else if (exp[i] == '+' || exp[i] == '-'){
						  while (stack[top] == '*' || stack[top] == '/' || stack[top] == '^')
							  prefixExp[j++] = npop();
						  npush(exp[i]);
					  }
					  else if (exp[i] == '(')
					  {
						  while (stack[top] != ')')
							  prefixExp[j++] = npop();
						  npop();
					  }
					  else
						  npush(exp[i]);
				  }
				  prefixExp[j] = '\0';
				  _strrev(prefixExp);
				  cout << prefixExp;

		}
			break;
		case 2:
			break;
		default:
			cout << "Invalid Input";
			break;
		}
	} while (a != 2);
}

The line cout << "[1] Start\n[2] Return\n"; keeps executing even though I'm in the "enter the expression" process.

EDIT:

#include <iostream>
#define STACK_SIZE 5
#include<cstring>
#define QUEUE_SIZE 5
#define NSTACK_SIZE 100
using namespace std;
int queueArr[QUEUE_SIZE];
int front = -1;
int rear = -1;	
int stack[STACK_SIZE];
int top = -1;
char nstack[NSTACK_SIZE];

void npush(int x)
{
	nstack[++top] = x;
}

char npop()
{
	return nstack[top--];
}


void push(int value)
{
	if (top == (STACK_SIZE - 1))
	{
		cout << "Stack is full!";
	}
	else
		stack[++top] = value;
}

void pop()
{
	if (top == -1)
	{
		cout << "Stack is empty!";
	}
	else
		cout << "You removed " << stack[top--] << endl;
}

void display()
{
	if (top == -1)
	{
		cout << "Stack is empty!";
	}
	else
	for (int i = top; i >= 0; i--)
		cout << stack[i] << "\t";
}
void enqueue(int x){
	if (front == -1)
		front++;
	if (rear == (QUEUE_SIZE - 1)){
		cout << "Queue is full!";
	}
	else

		queueArr[++rear] = x;
}

void displayq() {
	for (int i = front; i <= rear; i++)
		cout << queueArr[i] << " ";


}
void dequeue(){

	if (front == QUEUE_SIZE){
		cout << "Queue is empty!";
		front = -1;
		rear = -1;
	}
	else
		cout << "You removed " << queueArr[front++] << endl;

}

void stacks(){
	int choice;
	do{
		system("cls");
		int value;
		cout << "[1] Push\n[2] Pop\n[3] Display\n[4] Return\n";
		cout << "Enter your choice: ";
		cin >> choice;

		switch (choice)
		{
		case 1:
			cout << "Enter value: ";
			cin >> value;
			push(value);
			break;
		case 2:
			pop();
			cin.ignore();
			cin.get();
			break;
		case 3:
			display();
			cin.ignore();
			cin.get();
			break;
		case 4: 
			break;
		default:
			cin.ignore();
			cin.get();
			cout << "Invalid Input";

		}
	} while (choice != 4);
}
void queues(){
	int choice, item;
	do{
		system("cls");
		cout << "[1] Enqueue";
		cout << "\n[2] Dequeue";
		cout << "\n[3] Display";
		cout << "\n[4] Return";
		cout << "\nEnter your choice:";
		cin >> choice;
		switch (choice){

		case 1:
			cout << "Enter item to add:";
			cin >> item;
			enqueue(item);
			system("pause");
			break;
		case 2:
			dequeue();
			system("pause");
			break;
		case 3:
			displayq();
			system("pause");
			break;

		case 4:
			break;
		default:
			cout << "Invalid input";
		}
	}
		while (choice != 4);
}
void prefix(){
	
	system("cls");
	int a;
	int j = 0;
	
	do{
		cout << "[1] Start\n[2] Return\n";
	cin >> a;
		if (a == 1){
			char exp[NSTACK_SIZE], prefixExp[NSTACK_SIZE];

			cout << "Enter the expression: ";
			cin.getline(exp, NSTACK_SIZE);
			_strrev(exp);
			strcat_s(exp, "(");
			npush(')');


			for (int i = 0; i < strlen(exp); i++){
				if (isalnum(exp[i]))
					prefixExp[j++] = exp[i];
				else if (exp[i] == '+' || exp[i] == '-'){
					while (nstack[top] == '*' || nstack[top] == '/' || nstack[top] == '^')
						prefixExp[j++] = npop();
					npush(exp[i]);
				}
				else if (exp[i] == '(')
				{
					while (nstack[top] != ')')
						prefixExp[j++] = npop();
					npop();
				}
				else
					npush(exp[i]);
			}
			prefixExp[j] = '\0';
			_strrev(prefixExp);
			cout << "Prefix Notation: " << prefixExp << endl;
		}
	} 
	while (a != 2);
}
void postfix(){
	system("cls");
	int a;
	do{
		cout << "[1] Start\n[2] Return\n";
		cin >> a;
		switch (a){
		case 1:
		{
				  char exp[NSTACK_SIZE], postExp[NSTACK_SIZE];
				  int j = 0;
				  cout << "Enter the expression: ";
				  cin.getline(exp, NSTACK_SIZE);

				  strcat_s(exp, ")");
				  npush('(');


				  for (int i = 0; i < strlen(exp); i++){
					  if (isalnum(exp[i]))
						  postExp[j++] = exp[i];
					  else if (exp[i] == '+' || exp[i] == '-'){
						  while (nstack[top] == '+' || nstack[top] == '-' || nstack[top] == '*' || nstack[top] == '/' || nstack[top] == '^')
							  postExp[j++] = npop();
						  npush(exp[i]);
					  }
					  else if (exp[i] == '*' || exp[i] == '/'){
						  while (nstack[top] == '*' || nstack[top] == '/' || nstack[top] == '^')
							  postExp[j++] = npop();
						  npush(exp[i]);
					  }
					  else if (exp[i] == '^'){
						  while (nstack[top] == '^')
							  postExp[j++] = npop();
						  npush(exp[i]);
					  }
					  else if (exp[i] == ')')
					  {
						  while (nstack[top] != '(')
							  postExp[j++] = npop();
						  npop();
					  }
					  else
						  npush(exp[i]);
				  }
				  postExp[j] = '\0';

				  cout << postExp;
		}
			break;
		case 2:
			break;
		default:
			cout << "Invalid Input";
		}
	} while (a != 2); 
}

void notation(){
	int i;
	system("cls");
	do{
		cout << "[1] Infix to Prefix\n[2] Infix to Postfix\n[3] Return\n";
		cin >> i;
		switch (i){
		case 1:
			prefix();
			break;
		case 2:
			postfix();
			break;
		case 3:
			break;
		default:
			cout << "Invalid Input";
		}
	} while (i != 3);
}

int main()
{
	system("cls");
	int i;
	while (1){
		cout << "[1] Stacks\n[2] Queues\n[3] Notation\n[4] Exit\n";
		cin >> i;
		switch (i){
		case 1:
			stacks();
			break;
		case 2:
			queues();
			break;
		case 3:
			notation();
			break;
		case 4:
			exit(1);
			break;
		default:
			cout << "Invalid Input.";
			break;
		}
	}
	system("pause");
	return 0;
}

Notice when you enter number 3, it will show a menu consisting of infix to postfix and infix to prefix. Both of those programs have an error in their respective functions. They will both repeat the line cout <<"[1] Start\n[2] Return\n";

[TarikNeaj]

Its a do-while loop. First It prints out

cout << "[1] Start\n[2] Return\n"; Then it takes input cin >> a;.

Now. It enters the switch statement. Say case 1 happens. Now all the stuff inside of case 1 happens, and then it comes back to the switch statement and goes down to the end

	}
	} while (a != 2);
}

Then it sees the while statement, saying, run this as long as while is not 2. And then it remembers, ooohhh... a is equal to 1, becuase the user entered 1, not 2, so I will keep going!!!

Now, it goes all the way back to the beginning of the do-while loop. And what does it see first? You guessed it.

This cout << "[1] Start\n[2] Return\n";.

You will need to move this statement outside of the do-while loop if you dont want it to repeat. Also, I might have gotten your question wrong, but this is how I took it from the information you've given. If I did get your question wrong, please elaborate on what is wrong and be more specific.

[jam51197]

I don't know how to write my question properly. I'll just post the whole source code and I guess you'll understand what I mean when you run it. Sorry for the hard time, and a million thank yous.

[TarikNeaj]

Sorry but, you'll have to ask a specific question, I cant go through all of the code like that. Whats going wrong? What are you inputting, what output are you expecting, what output are you actually getting? What line of code? What function, etc.

[jam51197]

When I run my code, and I reach the "[1] Start\n[2] Return\n"; part, if I press 1, this line should execute

cout << "Enter the expression: ";
				  cin.getline(exp, NSTACK_SIZE);

instead, I get an output of:

[1] Start
[2] Return
Enter the expression: [1] Start
[2] Return


What I'm expecting is:
[1] Start
[2] Return
<cls>
Enter the expression: <user input here>
Prefix Notation: <the prefix of what inputted>

[TarikNeaj]

Much better question. Its because you're switching between cin and getline. cin lives a buffer or some shit.

Add a cin.ignore() after the cin >> a; in both prefix and postfix.

cin >> a;
cin.ignore();

Also, in the postfix, add an endl after you print out the postExp.
cout << postExp << endl;

[jam51197]

Thank you very much!! Solved my problem. Have a great day man, thanks! :)