Arithmetic from an input file to an output file? - c++

Question

This is a two part question and I hope I make myself understood. I'll edit as necessary! I'm attempting to write a program that will do calculations in succession from an input file. The text file will look something like this:
int + 25 10
double / 5.5 8.5
...
Each instance starts with a type like int, double, float, etc., then a calculation type (add, subtract, etc.) and then two numbers. I want to be able to read each in succession and output the sum, product, etc. into an output file. For instance, if we use the first example above, the output in the file will be:
int 25 10 = 35
I have code which will do the calculations below:
void doAddition(ifstream &inFile) {
int num1, num2;
inFile >> num1 >> num2;
cout << num1 << num2 << " = "<< (num1+num2) << '\n'; }
The only problem with this is that I don't know how to add the type of variable (I've tried using string but it doesn't seem to work), for instance "int" or "double" so I get:
25 10 = 35
Instead of:
int 25 10 = 35
My second problem as you may see is that I'm currently using "cout" to show the information on screen when I really want to add it to the outfile. Here's some more info:
What I use to move to the next line:
void readToNextLine(ifstream& inFile) {
string t1, t2;
inFile >> t1 >> t2; }
Code in my Main:
ifstream inFile;
//ofstream outFile;
char ch;
int num1, num2;
inFile.open("infile.txt");
//outFile.open("outfile.txt");
if (inFile.is_open()){
inFile >> ch;
while (!inFile.eof())
{
switch (ch)
{
case '+':
doAddition(inFile);
break;
...
As you can see I commented out the ofstream part because I couldn't get it to work properly. Any suggestions? I have about 10 windows and two C++ books open right now just trying to put it all together logically!
Edit: I'm not sure if switches would be the best method for this. I need the program to see "int" and realize it's a word. If I used 4 variable types like int, double, float, and long perhaps I could have it check the first letter of each: i, d, f, l and then once it knows what type it could go into the +, -, etc. checks. It feels like by doing this logically it's just taking more time when I could be using a series of classes, but I'm just not sure where to start with that.

Answer 1

I'm really not understanding all this trouble reading from files. There are too many examples on Stackoverflow and the web. Perhaps it's people not searching or they are demanding an example that matches their exact code.
Try this:
struct Input_Record
{
std::string data_type_as_string;
std::string operation;
std::string value1_as_string;
std::string value2_as_string;
friend std::istream& operator>>(std::istream& inp, Input_Record& r);
};
std::istream& operator>>(std::istream& inp, Input_Record& r)
{
inp >> r.data_type_as_string;
inp >> r.operation;
inp >> r.value1_as_string;
std::getline(inp, r.value2_as_string); // Use getline to eat the line ending.
}
// ...
Input_Record r;
while (input_file >> r)
{
// Do stuff with r
};
If you really want to have some fun, You could use a parent base class and a factory pattern to generically create objects based on the input:
class Binary_Operation // Base class for factory pattern.
{
public:
//! Evaluate the object and return the result as a string
// e.g. numbers as text
virtual std::string evaluate(void) const = 0;
};
class Binary_Integer_Operation : public Binary_Operation
{
public:
std::string evaluate(void) const
{
// Convert values to integers than perform the operation.
// Convert result to string using std::istringstream.
};
};
class Binary_Double_Operation : public Binary_Operation
{
// See Binary_Integer_Operation above.
};
This allows you to do something like:
Binary_Operation * factory_create(const Input_Record& r)
{
Binary_Operation * p_obj = nullptr;
if (r.data_type == "int")
{
p_obj = new Binary_Integer_Operation;
// Initialize fields
}
if (r.data_type == "double")
{
p_obj = new Binary_Double_Operation;
// Initialize fields;
}
return p_obj;
}
Your processing loop would look like:
Input_Record r;
while (input_file >> r)
{
Binary_Operation * p_operation = factory_create(r);
std::string result = p_operation->evaluate();
cout << "result = " << result << "\n";
delete p_operation;
}

Answer 2

Let's start with an example like the one you provided:
int + 25 10
The type for the "type" and arithmetic operator is simple, std::string and char respectively.
std::ifstream in("infile.txt");
std::string type; char op;
if (in >> type >> op)
{
// ...
}
For the other two values, you also have to extract them into strings because you first have to find out the value of type before you can convert them:
if (in >> type >> op >> a >> b) // a and b are strings
Now use a function to check type and convert a and b to the correct types:
void convertTo(std::string const& typeName, std::string const& a, std::string const& b, char op)
{
if (typeName == "int")
{
int a1 = std::stoi(a),
b2 = std::stoi(b);
doOperation(op, a1, b2)
} else if (typeName == "double") {
double a1 = std::stod(a),
b2 = std::stod(b);
doOperation(op, a1, b2);
} else if (typeName == "float") {
// std::stof()
}
}
doOperation() is templated and is implemented like this:
template<typename T>
struct F;
template<> struct F<int> { static const std::string value = "int"; };
template<> struct F<double> { static const std::string value = "double"; };
template<> struct F<float> { static const std::string value = "float"; };
template<typename U, std::string name = F<U>::value>
void doOperation(char op, U a, U b)
{
std::ofstream out("outfile.txt");
switch (op)
{
case '+':
out << name << " " << op << " " << (a + b);
case '-':
out << name << " " << op << " " << (a - b);
case '/':
// ...
// ...
}
}