making me some calculator

[ascii]

ive posted on this in the past but i had some time earlier and i basically rewrote this whole program and have some questions. the calculator functions by letting the user enter an expression like:
3+2-15*3

and it will give the result following the order of operations. to make it work i could just add '*' and '/' possibilities in my switch statement, but then it would just work from left to right, it wouldnt follow the order of operations. anyone have advice on how i might go about adding functionality for these two following PEMDAS? thanks!

#include <iostream>
#include <cctype>
using std::cout;
using std::endl;
using std::cin;
void eatspaces(char* str);
int addNumbers(char* str);
int number(char* str, int& index);

int main(void)
{
	char str[15];
	cin.getline(str, 15, '\n');
	eatspaces(str);

	int sum = addNumbers(str);

	cout << "Total is " << sum << endl;

	return 0;
}

void eatspaces(char* str)
{
	int i = 0;
	int j = 0;

	while((*(str + i) = *(str + j++)) != '\0')
		if (*(str + i) != ' ')
			i++;
	return;
}

int number(char* str, int& index)
{
	int number = 0;
	while(isdigit(*(str + index)))
	{
		number *= 10;
		number += (*(str + index)) - '0';
		index++;
	}

	cout << number << endl;
	return number;
}

int addNumbers(char* str)
{
	int index = 0;
	int total = 0;
	
	total += number(str, index);
	for(;;)
	{
		switch(*(str + index))
		{
		case '+':
			index++;
			total += number(str, index);
			break;
		case '-':
			index++;
			total -= number(str, index);
			break;
		case '\0':
			return total;
		default:
			cout << "ERROR AT POSITION " << index << endl << "ABORTING" << endl;
			exit(1);
		}
	}

	return total;
}

[Khaltazar]

You are probably looking to use:

http://www.cplusplus.com/reference/string/string/find/

Have the find in a if statement.

[ascii]

dont really see how thats going to help getting the order of operations to work. please expand ;)

[Khaltazar]

Using if statements:

if (Find "(" && Find ")") // Code
if (Find "^") // Code
if (Find "*" || Find "/") // Code
if (Find "+" || Find "-") // Code

Etc. That is not the syntax. I haven't used find yet but it will work, just read up on it.

If it finds "*" for example then make a code inside the if statement getting the number to the left and to the right of the found position. Then call your MultFunc();

[ascii]

ive used the find function before and it isnt very useful in this situation. there needs to be a switch statement for the program to work.

[Mathhead200]

Your reading the string one character at a time (i.e. from left to right.) Mathimatical expresions are not read that way. "2 + 3 * 5" is read as 3 * 5 first then 2 + that answer.

If you read the string from left to right you need to first check for (), then ^, then * and /, and then + and -. As you find operations replace the value there with the answer.

Example: 2 + 3 * (5 – 1)
After step 1: 2 + 3 * 4
After step 2: 2 + 12
After step 3: 14
Done.

[ascii]

ive thought of that but have had some troubles getting the values replaced, ill try again i guess

[filipe]

A simple solution is to write a recursive descent parser (or a Pratt parser, my favorite). Google them. You'll need to tokenize the input before parsing, though.

[ascii]

heres the latest build of it which now has decimals and im looking into recursive descent parsers :)

#include <iostream>
#include <cctype>
using std::cout;
using std::endl;
using std::cin;
void eatspaces(char* str);
double addNumbers(char* str);
double number(char* str, int& index);
double decimal(char* str, int& index);

int main(void)
{
	char str[15];
	cout << "Calculator: " << endl
		<< "+ - add" << endl
		<< "- - substract" << endl
		<< "* - multiply" << endl
		<< "/ - divide" << endl << endl;
	
	char cont = 'y';
	//continue solving equations until they say to stop
	while (cont == 'y' || cont == 'Y')
	{
		cin.getline(str, 15, '\n');
		eatspaces(str);

		double sum = addNumbers(str);

		cout << "\t = " << sum << endl;
		
		cout << "Continue(Y/N)? ";
		cin >> cont;
		cin.ignore(1, '\n');
	}

	return 0;
}

void eatspaces(char* str)
{
	//remove all spaces from the string entered
	int i = 0;
	int j = 0;

	while((*(str + i) = *(str + j++)) != '\0')
		if (*(str + i) != ' ')
			i++;
	return;
}

double number(char* str, int& index)
{
	//get the value of the next number in the series and increment index to the next position
	double number = 0;
	while(isdigit(*(str + index)))
	{
			number *= 10;
			number += (*(str + index)) - '0';
			index++;
	}
	//if no decimal return the number
	if(*(str + index) != '.')
		return number;
	//if there is a decimal add it to the number and then return it
	else
		index++;
		return (number + decimal(str, index));
}

double decimal(char* str, int& index)
{
	//get the value of a decimal after the number if there is one
	double decimal = 0;
	double multiplier = .1;
	while(isdigit(*(str + index)))
	{
		decimal += (*(str + index) - '0') * multiplier;
		multiplier /= 10;
		index++;
	}

	return decimal;
}

double addNumbers(char* str)
{
	int index = 0;
	double total = 0;
	
	//get the value of the first term

	total += number(str, index);
	for(;;)
	{
		switch(*(str + index))
		{
		//run through each operaton and perform the needed operation to the total value
		case '+':
			index++;
			total += number(str, index);
			break;
		case '-':
			index++;
			total -= number(str, index);
			break;
		case '*':
			index++;
			total *= number(str, index);
			break;
		case '/':
			index++;
			total /= number(str, index);
			break;
		case '\0':
			//if at the end of the string return the total value
			return total;
		
		default:
			//if invalid input, show where the error occured and close the program
			cout << endl << endl;
			cout << "ERROR AT POSITION " << index << endl;
			cout << str << endl;
			for (; index > 0; index--)
				cout << " ";
			cout << "^";

			cout << endl;
			exit(1);
		}
	}

	return total;
}