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find_first_not_of problem

CrypticA1 (50)
I've been working on a parser for a while and i seem to be having an error with a certain bit of code "find first not of". The main issue i'm having is that whenever I enter a number bigger than 99 the program only reads the first two numbers.

*code is not complete yet
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string.legnth; //assume it is 5+500
string token_1;    //declaration of token 1 & 2
string token_2;
size_t pos_1 = 0;   //pos_1 necessary to save point on a string

int addition = 0;
double answer = 0;
for (int i = 0; i < input.length(); ++i)
{
	if(isdigit(input[i]) && answer == 0)
	{
		pos_1 = input.find_first_not_of("1234567890");
		token_1 = input.substr(0, pos_1+1);
		cout << "token_1=" << token_1 << endl;
		answer = stoi(token_1);
		token_1 = "";
	}
	else if (isdigit(input[i]) && answer != 0)
	{
		continue;
	}
	else if (input[i] == '+')
	{
		input[i] = 'A';
		pos_1 = input.find_first_not_of("1234567890");
		token_2 = input.substr(i+1, pos_1+1);
		cout << "token_2=" << token_2 << endl;
		answer += stoi(token_2);


What's weird is that when I enter 500+5 it will work for the first number but not for the second number/token. Does anyone have any ideas? Also if you have any questions please ask.











IMPORTANT!!: you don't have to look at the entire code but here it is if you're truly interested.
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//************************************
// C++ addition parser
//************************************
#include <iostream>	//headers needed and ones i put in cause 
#include <cstdlib>	//im paranoid
#include <stdlib.h>
#include <string>
#include <sstream>



using namespace std;

int addition(int[]);	//in progress
void analyze(string);	//analyzingsection
void help();	//help section

int main()
{
	string input;	//string that user enters
	for (;;)//infinite for loop
	{
		cout << "\n Enter formula or type '+_+' for help.\n"; // prompt
		getline(cin, input);	//input string
		

		if (input == "+_+")
			help();	//acces help function

		else
			
		analyze(input);	//access analyzer
	}
system("pause");
return 0;
}

void analyze(string input) //analyzer function
{


string token_1;
string token_2;
size_t pos_1 = 0;
size_t pos_2 = 0;
size_t pos_3 = 0;

int addition = 0;
int subtraction = 0;
int multiplication_product = 1;
double answer = 0;

for (int i = 0; i < input.length(); ++i)
{
	if(isdigit(input[i]) && answer == 0)
	{
		pos_1 = input.find_first_not_of("1234567890");
		token_1 = input.substr(0, pos_1+1);
		cout << "token_1=" << token_1 << endl;
		answer = stoi(token_1);
		token_1 = "";
	}
	else if (isdigit(input[i]) && answer != 0)
	{
		continue;
	}
	else if (input[i] == '+')
	{
		input[i] = 'A';
		pos_1 = input.find_first_not_of("1234567890");
		token_2 = input.substr(i+1, pos_1+1);
		cout << "token_2=" << token_2 << endl;
		answer += stoi(token_2);

	}
	else if (input[i] == 'x')
	{
		input[i] = 'M';
		pos_1 = input.find_first_not_of("1234567890");
		token_2 = input.substr(i + 1, pos_1 + 1);
		cout << "token_2=" << token_2 << endl;
		answer *= stoi(token_2);


	}

	else

		break;
	
}
cout << "\n" << input << "=\t" << answer;
}



void help() // help function
{
	cout << "\naccepted operators\n" << "\nmultiplication = lower case 'x'\n" << "\ndivision = '/' \n" << "\naddition = '+' \n"
		<< "\nexponent = ^" << endl;
	cout << "\nparenthesis in progress\n" << endl;
}


/* NOTES FOR DAYS

//isdigit()
//.at()
//size()
//getline(cin,str)
//void lexicon(char [], int );
//input.find_first_of('('
//isdigit[]
//input.find_first_not_of("1234567890");
/*switch (input.at(i))
{
case isdigit:
pos_1 = input.find('+');
token_1 = input.substr(0, pos_1);
input.erase(0, pos_1 + 1);
cout << token_1;
break;
default:

break;
}


for (int i = 0; i < number_counter; i++)	//where the math happens..this will be moved into an addtion function
{
pos_1 = input.find('+');
pos_2 = input.find(0-9);
token_1 = input.substr(0, pos_1);
input.erase(0, pos_1 + 1);
addition += stoi(token_1);
}
cout << addition;

*/
jlb (4973)
Please show a sample of your input string.

Also be careful with the string.find functions. These functions return a value between zero and std::string::npos with zero being the first character of the string and std::string::npos is std::numeric_limits<std::string::size_type>::max(), the value that represents the largest possible std::string.length(). This usually indicates that the find() was unsuccessful.

So in the following snippet if the first character is not a digit pos_1 will equal zero.
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		pos_1 = input.find_first_not_of("1234567890");
		token_1 = input.substr(0, pos_1+1);


If however there are only digits in the string pos_1 will be equal to std::string::npos and in this case trying to add one to this value will also yield a value of zero.

CrypticA1 (50)
I'm sorry I'm not great with programming terminology. I entered 5+500 for the input. I found that I can get the first token just fine, but the real issue is the second token.

i did find a solution for the problem although I'm not so sure how it works, but here it is
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input[i] = 'A';
		pos_1 = input.find_first_not_of("1234567890");
		token_2 = input.substr(i+1, pos_1+input.length());
		cout << "token_2=" << token_2 << endl;
		answer += stoi(token_2);

I added the length of the string to pos_1, but shouldn't that = the current position of pos_1 + the entire length of the string? I've also added values like 1000 and have seen similar results.

Update: even if i add pos_1 - input.length() it still works. Heck I'm sure i could put pos_1 - unicorn and it would still work...Am I drunk
Last edited on
jlb (4973)
Have you considered using a stringstream to parse the line? Something like:
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void analyze(string input) //analyzer function
{
   stringstream ins(input_string);
   int first_number, second_number;
   char operator;
   int answer;

   ins >> first_number >> operator >> second_number;

   switch(operator)
   {
      case '+':
          // DO your addition operations.
          answer = first_number + second_number;
      case '-':
          // Do your subtraction operations.
...


CrypticA1 (50)
what's a string stream? Also for this parser I want it to be able to read everything the user enters not just 2 values. It does seem like a good basic code though I could probably use it for a function. thanks
TheIdeasMan (6817)
Hi,

I thought this might help:

https://en.wikipedia.org/wiki/Shunting-yard_algorithm



what's a string stream?


Google std::stringstream
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