Help! Program to evaluate expression...

I need help in writing a program that will evaluate an expression from a file and output if the expression is a valid C++ expression or not.

EXAMPLE This is read from input file:
Input:
a
ab
(a+b)
(a)

This is ouput to screen after each read and test till end of file.

Output:
a
** Is a valid expression **
ab
** Is N O T a valid expression **
(a+b)
** Is a valid expression **
(a)
** Is N O T a valid expression **

As far as I know the rule for a valid expression in c++ is that all the operators must be paranthized. This is my program below but it doesn't work properly can someone point out mistakes?

------------------------------------------------------------------------------------
#include <fstream.h>
#include <iostream.h>
#include <stdlib.h>
#include <ctype.h>
#define in_file "input.dat"

void FindAnExpression(int[]);
ifstream ins;
ofstream outs;
char line[20];


int main()
{
int test = 0;
ins.open(in_file);
if(ins.fail())
{
cout <<"*** ERROR: Cannot open "<< in_file
<<"for input." << endl;
exit(1);
}

do
{
ins.getline(line,20);
cout << line;
test= FindAnExpression(line);
if (test==1)
cout << "This is a valid Expression.";
else
cout << "This is NOT a valid Expression.";
}while(!ins.eof())

return 0;
}

FindAnExpression(int[]);
{
int index = 0;
if isalpha(line[index])
{
index++
if isalpha(line[index])
return 1;
else if (line[index]='(')
{
FindAnExpression
if ((line[index]='+') || (line[index]='-') || (line[index]='*') || (line[index]='/'))
FindAnExpression
if (line[index]=')'
return 1;
}
return 0;
}
}

bump

are you sure your allowed to assign an integer to a function?
oh, and don't bump your thread!

First problem is that you cannot even text your program since
you have
void FindAnExpression(int[])
and you want return from that function so
int FindAnExpression(int[])

then

Quote:

---

As far as I know the rule for a valid expression in c++ is that all the operators must be paranthized

---

Are you testing arithmetic, logic or ALL C++ expression ?

Quote:

---

ab
** Is N O T a valid expression **

---

but a is. Why ?
Are you using only one letter variables names?

Is a*b a valid expression ?

ok, this is what you did wrong:

#1: you forgot 'void' before you started ur function
#2: FindAnExperession(line) is illigal cuz line is a char and your prototype says FindAnExpression(int[])
#3: forgot semicolon after while statement after do statement
#4: your making your functions 'void' when they return values!

Quote:

---

Originally posted by Xterria
ok, this is what you did wrong:

#1: you forgot 'void' before you started ur function
#2: FindAnExperession(line) is illigal cuz line is a char and your prototype says FindAnExpression(int[])
#3: forgot semicolon after while statement after do statement
#4: your making your functions 'void' when they return values!

---

Thank you, Mr. Compiler.

Ok to answer your questions, I fixed those problems above and my program runs not but it goes in an endless loop! As far as the rules, it follows that all expressions are paranthized except when their is only one letter..

a
**is valid

(a)
**not valid

a+b
**not valid

(a+b)
**is valid

Here is my updated program, what is wrong with the recursion?
-----------------------------------------------------------------------------------
#include <fstream.h>
#include <iostream.h>
#include <stdlib.h>
#include <ctype.h>
#define in_file "input.dat"

int FindAnExpression(char[]);
ifstream ins;
ofstream outs;
char line[80];


int main()
{
int test = 0;
ins.open(in_file);
if(ins.fail())
{
cout <<"*** ERROR: Cannot open "<< in_file
<<"for input." << endl;
exit(1);
}

do
{
ins.getline(line,20);
cout << line;
test= FindAnExpression(line);
if (test==1)
cout << "This is a valid Expression.";
else
cout << "This is NOT a valid Expression.";
}while(!ins.eof());

return 0;
}

int FindAnExpression(char[])
{
int index = 0;
if (isalpha(line[index]))
{
index++;
if (isalpha(line[index]))
return 1;
else if (line[index]='(')
{
FindAnExpression;
if ((line[index]='+') || (line[index]='-') || (line[index]='*') || (line[index]='/'))
FindAnExpression;
if (line[index]=')')
return 1;
}
return 0;
}
}

At first look I see 2 problems.

First : int FindAnExpression(char[]);
which you call without arg in your recursion.


Second :every time you call FindAnExpression you get a new instance of index so you do not parse your expression.
Try with a global variable index or
try static int index.