Handling a two character operator in a switch statement

Hi!
I am setting up a program where the user enters two fractions and an operator (arithmetic or relational) and the program performs the operation and displays the result. I am having trouble figuring out how to handle the two character relational operators. Does anyone have any ideas.
I tried:
char oper1, oper2;
cin >> oper1 >> oper2;
but the program hangs when there isn't a second operator.
Then I tried:
string oper;
getline(cin, oper);
but my switch statement doesn't like that because oper is now a string.
I was thinking about creating an enum with all the operators and using that in an if else, then using the eum type in my switch, but there has to be a better, shorter way. But, I am stumped.

This is the code I have:

Code:

#include <string>
using namespace std;

#include "fractionType.h"

int main()
{
	fractionType x;
	fractionType y;
	fractionType z;

	string operStr;

	cout << "\nThis program performs aritnmetic and relational operations"
		 << "\non two fractions that you enter.";

	const string MENU =  "\nThe operations that you can use are:"
		 "\n Addition ------------------->  use '+'"
		 "\n Subtraction ---------------->  use '-'"
		 "\n Multiplication ------------->  use '*'"
		 "\n Division ------------------->  use '/'"
		 "\n Equal To ------------------->  use '=='"
		 "\n Not Equal To --------------->  use '!='"
		 "\n Less Than ------------------>  use '<'"
		 "\n Greater Than --------------->  use '>'"
		 "\n Less Than or Equal To ------>  use '<='"
		 "\n Greater Than or Equal To --->  use '>='"
		 "\n Or to QUIT -----------------> enter 'Q'"
		 "\nEnter the required symbol when prompted."
		 "\nEnter the first fraction in the form a/b: ";

	cout << MENU;
	cin >> x;
	cout << "\nEnter the second fraction in the form a/b: ";
	cin >> y;
	cout << "\nEnter the operation symbol: ";
	getline(cin, operStr);
	cin.ignore();

	while (oper != "Q" || oper != "q")
	{
		if
		cout << endl;
		switch(oper)
			{
			case '==':
				{
				if (x == y)
					cout << x << " is Equal to " << y << endl;
				else 
					cout << x << " is Not Equal to " << y << endl;
				break;
				}//end case '=='
			case '!=':
				{
				if (x != y)
					cout << x << " is Not Equal to " << y << endl;
				else 
					cout << x << " is Equal to " << y << endl;
				break;
				}//end case '!='
			case '<=':
				{
				if (x <= y)
					cout << x << " is Less Than or Equal to " << y << endl;
				else 
					cout << x << " is Not Less Than or Equal to " << y << endl;
				break;
				}//end case '<='
			case '>=':
				{
				if (x >= y)
					cout << x << " is Greater Than or Equal to " << y << endl;
				else 
					cout << x << " is Not Greater Than or Equal to " << y << endl;
				break;
				}//end case '>='
			case '+':
				{
					z = x + y;
					cout << "The sum of " << x << " + " << y << " = " << z << endl;
					break;
				}//end case '+'
			case '-':
				{
					z = x - y;
					cout << "The difference of " << x << " - " << y << " = " << z << endl;
					break;
				}//end case '-'
			case '*':
				{
					z = x * y;
					cout << "The product of " << x << " * " << y << " = " << z << endl;
					break;
				}//end case '*'
			case '/':
				{
					z = x / y;
					cout << "The result of " << x << " / " << y << " = " << z << endl;
					break;
				}//end case '/'
			default:
                cerr << "\nInvalid operation choice, please try again!";
			}//end switch
		cout << MENU;
		cin >> x;
		cout << "\nEnter the second fraction in the form a/b: ";
		cin >> y;
		cout << "\nEnter the operation symbol: ";
		getline(cin, oper);
		cin.ignore();
	}//end while
}//end main

Thanks for any input.

Okay...
Two or more characters, like '==' isn't a char. It's a char* ('==' isn't even valid, because '' indicates a char, but this isn't a char, it's char*, so it should be "==").
Switch expressions must be constant. You can't use a char* expression in there. A char ('+' for example) works fine, because it's basically an int.
When comparing to a char*, you should use string functions or a string class. Never do a == "==".
The easiest solution to your problem would be to input to a string (cin >> std::string), then compare that string with IF statements to find what the user typed.

What should you do?
- Input to a std::string (cin to a std::string).
- Change your while loop argument to a an IF statement checking for "Q".
- Remove your switch and replace with IF/ELSE checking against strings ("").

These are my suggestions.

Thanks! Didn't even think of using the if else for the different operator options using a string.
That was my last hurdle and this program is finished!
Thanks again!

Not a problem. Hope it works for you.

Originally Posted by Elysia

'==' isn't even valid

This compiles and runs as expected ( g++ )

Code:

#include <iostream> 
#include <iomanip> 

using namespace std;

int main() {

    int ch[] = { '==', '!=', '>=' };
    
    for ( int i = 0; i < 3; ++i ) {
        switch(ch[i]) {
            case '==' : 
                cout << hex << "0x" << setw(4) << setfill('0') << ch[i] << ": ==" << endl; 
                break;
            case '!=' : 
                cout << hex << "0x" << setw(4) << setfill('0') << ch[i] << ": !=" << endl; 
                break;
            case '>=' : 
                cout << hex << "0x" << setw(4) << setfill('0') << ch[i] << ": >=" << endl; 
                break;
        }
    }
}

But then I don't think that this is very useful for the OP.
Just wanted to point out that "multibyte character constants" are not illegal.
Kurt

Originally Posted by C++ Standard Final Draft

An ordinary character literal that contains more than one c-char is a multicharacter literal. A multicharacter literal has type int and implementation-defined value.

It's legal, but not portable.

I can't believe such a thing exists in the standard either.

Code:

char c = '!=';

Gives a truncation warning in VC++ (ie, it discards the first '!' and initalizes the variable only with the last char value ('=').

Originally Posted by Elysia

I can't believe such a thing exists in the standard either.

Code:

char c = '!=';

Gives a truncation warning in VC++ (ie, it discards the first '!' and initalizes the variable only with the last char value ('=').

That is because the the constant doesn't fit into a char. Try an int instead.
Kurt

Which is why I said '==' isn't legal (it's basically two bytes, but char is one byte).

Which is why I said '==' isn't legal (it's basically two bytes, but char is one byte).

Such a character literal appears to be legal, at least according to section 2.13.2 of the 2003 edition of the C++ Standard:

Code:

character-literal:
    'c-char-sequence'
    L'c-char-sequence'

c-char-sequence:
    c-char
    c-char-sequence c-char

c-char:
    any member of the source character set except
        the single-quote ', backslash \, or new-line character
    escape-sequence
    universal-character-name

Well, I suppose it is legal since it's possible to do

Code:

int c = '==';

Which makes me think if the above code couldn't be redone with and int instead.

UPDATE:
It actually works with some manipulation:

UPDATE2:
New, safer code below.

cin >> (char*)&oper;

Now that's just a stack overflow waiting to happen.

True enough. But so again, so is inputting to a variable without checking for errors a bad thing.
The point wasn't to take errors and bad things into consideration, but to make it work.
(I left out error checking on purpose.)

UPDATE:
Safer version:

Code:

int main()
{
	double x, y, z;
	string operation;
	string sx, sy;
	int oper = 0;

	cout << "\nThis program performs aritnmetic and relational operations"
		 << "\non two fractions that you enter.";

	const string MENU =  "\nThe operations that you can use are:"
		 "\n Addition ------------------->  use '+'"
		 "\n Subtraction ---------------->  use '-'"
		 "\n Multiplication ------------->  use '*'"
		 "\n Division ------------------->  use '/'"
		 "\n Equal To ------------------->  use '=='"
		 "\n Not Equal To --------------->  use '!='"
		 "\n Less Than ------------------>  use '<'"
		 "\n Greater Than --------------->  use '>'"
		 "\n Less Than or Equal To ------>  use '<='"
		 "\n Greater Than or Equal To --->  use '>='"
		 "\n Or to QUIT -----------------> enter 'Q'"
		 "\nEnter the required symbol when prompted.";

	do
	{
		cout << MENU;
		cout << "\nEnter the first fraction in the form a/b: ";
		cin >> sx;
		cout << "\nEnter the second fraction in the form a/b: ";
		cin >> sy;
		cout << "\nEnter the operation symbol: ";
		cin >> operation;
		cout << endl;
		
		if (atoi( sx.c_str() ) == 0 && sx.c_str()[0] != '0' || atoi( sy.c_str() ) == 0 && sy.c_str()[0] != '0')
		{
			cerr << "Invalid number entered. Please try again.\n\n";
			continue;
		}
		x = strtod(sx.c_str(), NULL);
		y = strtod(sy.c_str(), NULL);
		const char* temp = operation.c_str();
		oper = (int)temp[0] + (temp[1] << 8);

		switch (oper)
		{
			case '==':
			{
				if (x == y)
					cout << x << " is Equal to " << y << endl;
				else 
					cout << x << " is Not Equal to " << y << endl;
				break;
			}
			case '!=':
			{
				if (x != y)
					cout << x << " is Not Equal to " << y << endl;
				else 
					cout << x << " is Equal to " << y << endl;
				break;
			}
			case '<=':
			{
				if (x <= y)
					cout << x << " is Less Than or Equal to " << y << endl;
				else 
					cout << x << " is Not Less Than or Equal to " << y << endl;
				break;
			}
			case '>=':
			{
				if (x >= y)
					cout << x << " is Greater Than or Equal to " << y << endl;
				else 
					cout << x << " is Not Greater Than or Equal to " << y << endl;
				break;
			}
			case '+':
			{
					z = x + y;
					cout << "The sum of " << x << " + " << y << " = " << z << endl;
					break;
			}
			case '-':
			{
					z = x - y;
					cout << "The difference of " << x << " - " << y << " = " << z << endl;
					break;
			}
			case '*':
			{
					z = x * y;
					cout << "The product of " << x << " * " << y << " = " << z << endl;
					break;
			}
			case '/':
			{
					z = x / y;
					cout << "The result of " << x << " / " << y << " = " << z << endl;
					break;
			}
			default:
                cerr << "\nInvalid operation choice, please try again!";
		}
	}
	while (oper != 'Q' || oper != 'q');
	return 0;
}

I would recommend, for portability and reliability that you use if-statements after selecting onthe first char.

Code:

switch(ch)
{
    case '=':
        if (nextch == '=')
            ... // do equal check.
        else 
            ... // error - not valid operator
        break;
    case '!':
        if (nextch == '=')
           ... // do "not equal"
        else
           ... // error - not valid. 
...
}

It's a little bit extra code, but you don't rely on any complicated multibyte characters [which, by the way are dependant on the byte order in the architecture].

--
Mats

OR could do like this (my favorite)

Code:

typedef void (OpFncPtr)(double, double, double&);

void OpAdd(double x, double y, double& z)
{
	z = x + y;
	cout << "The sum of " << x << " + " << y << " = " << z << endl;
}

void OpSub(double x, double y, double& z)
{
	z = x - y;
	cout << "The difference of " << x << " - " << y << " = " << z << endl;
}

void OpMul(double x, double y, double& z)
{
	z = x * y;
	cout << "The product of " << x << " * " << y << " = " << z << endl;
}

void OpDiv(double x, double y, double& z)
{
	z = x / y;
	cout << "The result of " << x << " / " << y << " = " << z << endl;
}

void OpEqual(double x, double y, double&)
{
	if (x == y)
		cout << x << " is Equal to " << y << endl;
	else 
		cout << x << " is Not Equal to " << y << endl;
}

void OpNEqual(double x, double y, double&)
{
	if (x != y)
		cout << x << " is Not Equal to " << y << endl;
	else 
		cout << x << " is Equal to " << y << endl;
}

void OpLesserThan(double x, double y, double&)
{
	if (x < y)
		cout << x << " is Lesser Than " << y << endl;
	else 
		cout << x << " is Not Lesser Than " << y << endl;
}

void OpGreaterThan(double x, double y, double&)
{
	if (x < y)
		cout << x << " is Greater Than " << y << endl;
	else 
		cout << x << " is Not Greater Than " << y << endl;
}

void OpLessEqualThan(double x, double y, double&)
{
	if (x <= y)
		cout << x << " is Less Than or Equal to " << y << endl;
	else 
		cout << x << " is Not Less Than or Equal to " << y << endl;
}

void OpGreatEqualThan(double x, double y, double&)
{
	if (x >= y)
		cout << x << " is Greater Than or Equal to " << y << endl;
	else 
		cout << x << " is Not Greater Than or Equal to " << y << endl;
}

int main()
{
	double x, y, z;
	string operation;
	string sx, sy;
	int oper = 0;
	
	// Create and fill function table
	const int TableSize = ('>' + ('=' << 8));
	OpFncPtr* FunctionTable[TableSize];
	ZeroMemory(&FunctionTable, sizeof(FunctionTable));
	FunctionTable['+'] = &OpAdd;
	FunctionTable['-'] = &OpSub;
	FunctionTable['*'] = &OpMul;
	FunctionTable['/'] = &OpDiv;
	FunctionTable['=' + ('=' << 8)] = &OpEqual;
	FunctionTable['!' + ('=' << 8)] = &OpNEqual;
	FunctionTable['<'] = &OpLesserThan;
	FunctionTable['<'] = &OpGreaterThan;
	FunctionTable['<' + ('=' << 8)] = &OpLessEqualThan;
	FunctionTable['>' + ('=' << 8)] = &OpGreatEqualThan;

	cout << "\nThis program performs aritnmetic and relational operations"
		 << "\non two fractions that you enter.";

	const string MENU =  "\nThe operations that you can use are:"
		 "\n Addition ------------------->  use '+'"
		 "\n Subtraction ---------------->  use '-'"
		 "\n Multiplication ------------->  use '*'"
		 "\n Division ------------------->  use '/'"
		 "\n Equal To ------------------->  use '=='"
		 "\n Not Equal To --------------->  use '!='"
		 "\n Less Than ------------------>  use '<'"
		 "\n Greater Than --------------->  use '>'"
		 "\n Less Than or Equal To ------>  use '<='"
		 "\n Greater Than or Equal To --->  use '>='"
		 "\n Or to QUIT -----------------> enter 'Q'"
		 "\nEnter the required symbol when prompted.";

	do
	{
		cout << MENU;
		cout << "\nEnter the first fraction in the form a/b: ";
		cin >> sx;
		cout << "\nEnter the second fraction in the form a/b: ";
		cin >> sy;
		cout << "\nEnter the operation symbol: ";
		cin >> operation;
		cout << endl;
		
		if (atoi( sx.c_str() ) == 0 && sx.c_str()[0] != '0' || atoi( sy.c_str() ) == 0 && sy.c_str()[0] != '0')
		{
			cerr << "Invalid number entered. Please try again.\n\n";
			continue;
		}
		x = strtod(sx.c_str(), NULL);
		y = strtod(sy.c_str(), NULL);
		const char* temp = operation.c_str();
		oper = (int)temp[0] + (temp[1] << 8);

		if (FunctionTable[oper] == NULL)
		{
			cerr << "\nInvalid operation choice, please try again!";
			continue;
		}
		try
		{
			FunctionTable[oper](x, y, z);
		}
		catch(...)
		{
			cerr << "\nInvalid operation choice, please try again!";
			continue;
		}
	}
	while (oper != 'Q' || oper != 'q');
	return 0;
}