I'm currently writing a program that takes in a function such as ( 7x+19 ) and I need to evaluate the function that user inputs at range of numbers. I've got the loop figured out to loop through my desired numbers, but I don't know what to call the function when the user cin>>function. Should it be a char, string, int, double? Because later I'll need to call function and mult. it by lets say 3, or 5, or 12, etc? So what should I do to accomplish what I need to get done, I'm kind of stuck on this one. I'm kind of a beginner.

Depends, is the function hard coded or can the user enter any function at the command line?

Do you want to multiply numbers or do you want to replace the x in your inputfunction with a value?

## All 8 Replies

Depends, is the function hard coded or can the user enter any function at the command line?

Do you want to multiply numbers or do you want to replace the x in your inputfunction with a value?

yea, I need to exchange x for the the number. such as 3, 4, 5 etc. Ill run a while loop to get my x,y coord.

yea, I need to exchange x for the the number. such as 3, 4, 5 etc. Ill run a while loop to get my x,y coord.

You probably need to post some code and provide an example of a run of the program, even though you haven't written the program yet. You need to decide what kind of functions are legal (polynomials, exponents, logarithms, etc.). Have the user enter the function, read it in as a string, then parse the string in such a way that you can tell what kind of function it is (e.g. polynomial, whether it's a legal function that you can handle, then parse the string further to extract the relevant numbers (i.e. coefficients and exponents in a polynomial). But I think you'll have to read the function in as a string from the user.

The big thing is to decide what input your program needs to be able to handle, what assumptions it can make as far as good input, then go from there, in my opinion.

Sounds like your trying to parse a function. You'll have to enter the function as a string (either char * or string class), then use a loop to figuire out its contents. Depending on how robust/useful you want it to be, this could be a bit challenging for a beginner. Here's a start:

``````#include <string>
#include <iostream>
using namespace std;
int main()
{
string sFormula = "";
strin  sBuffer = "";
bool numStream(false);
int iVal = {0};
int iCount(0);
cout << "Enter formula: ";
cin >> sFormula
for (int i(0); i < strlen(sFormula); ++i)
{
if (isdigit(sFormula[i]))
{
numStream = true;
sBuffer += sFormula[i];
}
if (numStream && !isdigit(sFormula[i])
{
numStream = false;
iVal[iCount++] =  atoi(sBuffer);
}
}
return 0;
}``````

This will parse the formula and extract all the numbers into an array. This isn't really useful for solving anything, but it's a start. I would sugguest creating a class for holding/handling formulas and their operations. As you can see, it's a bit more complex an issue than it seems

As VernonDozier said you have to read your input as a string. But then you have to parse your inputstring and that's a whole world on it's own! But if you're adventurous enough to try it, don't hesitate! Start with http://en.wikipedia.org/wiki/Parsing

Here's an actual working math parser. It only works with addition and subtraction though. You've got me interested now, so I'm gonna write a (much more useful) class for this problem.

``````#include <string.h>
#include <iostream>
using namespace std;
int main()
{
string sFormula = "";
string  sBuffer;
char szFormula;
bool numStream(false);
int iVal = {0};
int iReturnVal(0);
int iCount(0);
char cLastOperator(0);
cout << "Enter formula: ";
cin.getline(szFormula, 54);
for (unsigned int i(0); i < strlen(szFormula); ++i)
{
if (isdigit(szFormula[i]))
{
numStream = true;
sBuffer += szFormula[i];
}
if (numStream && (!isdigit(szFormula[i]) || i == strlen(szFormula) - 1))
{
numStream = false;
iVal[iCount++] =  atoi(sBuffer.c_str());
if (cLastOperator == '+')
{
iReturnVal += iVal + iVal;
iVal = 0;
iVal = 0;
iCount = 0;
cLastOperator = 0;
}
else if (cLastOperator == '-')
{
iReturnVal -= iVal + iVal;
iVal = 0;
iVal = 0;
iCount = 0;
cLastOperator = 0;
}
sBuffer = "";
}
if (!numStream && (szFormula[i] == '+' || szFormula[i] == '-' || szFormula[i] == '*'))
{
cLastOperator = szFormula[i];
}
}
cout << szFormula << " = " << iReturnVal << endl;
cin.get();
return 0;
}``````

It works like this:
Input a stream of numbers and operands (- or +), all seperated by a space. Ie 2 + 6 + 8 - 11
And it will output the result. It won't throw any exceptions, so if you see eratic results you probably entered the equation wrong.

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