Split String with math expression - c++

Question

How to split the string to two-parts after I assign the operation to math operator? For example 4567*6789 I want to split string into three part
First:4567 Operation:* Second:6789
Input is from textfile
char operation;
while (getline(ifs, line)){
stringstream ss(line.c_str());
char str;
//get string from stringstream
//delimiter here + - * / to split string to two part
while (ss >> str) {
if (ispunct(str)) {
operation = str;
}
}
}

Answer 1

Maybe, just maybe, by thinking this out, we can come up with a solution.
We know that operator>> will stop processing when encounter a character that is not a digit. So we can use this fact.
int multiplier = 0;
ss >> multiplier;
The next characters are not digits, so they could be an operator character.
What happens if we read in a character:
char operation = '?';
ss >> operation;
Oh, I forgot to mention that the operator>> will skip spaces by default.
Lastly, we can input the second number:
int multiplicand = 0;
ss >> multiplicand;
To confirm, let's print out what we have read in:
std::cout << "First Number: " << multiplier << "\n";
std::cout << "Operation : " << operation << "\n";
std::cout << "Second Number: " << multiplicand << "\n";
Using a debugger here will help show what is happening, as each statement is executed, one at at time.
Edit 1: More complicated
You can always get more complicated and use a parser, lexer or write your own. A good method of implementation is to use a state machine.
For example, you would read a single character, then decide what to do with it depending on the state. For example, if the character is a digit, you may want to build a number. For a character (other than white space), convert it to a token and store it somewhere.
There are parse trees and other data structures which can ease the operation of parsing. There are parsing libraries out there too, such as boost::spirit, yacc, bison, flex and lex.

Answer 2

One way is:
char opr;
int firstNumber, SecondNumber;
ss>>firstNumber>>opr>>SecondNumber;
instead of:
while (ss >> str) {
if (ispunct(str)) {
operation = str;
}
}
Or using regex for complex expersions. Here is an example of using regex in math expersions.

Answer 3

If you have a string at hand, you could simply split the string into left and right at the operator position as follows:
char* linePtr = strdup("4567*6789"); // strdup to preserve original value
char* op = strpbrk(linePtr, "+-*");
if (op) {
string opStr(op,1);
*op = 0x0;
string lhs(linePtr);
string rhs(op+1);
cout << lhs << " " << opStr << " " << rhs;
}

Answer 4

A simple solution would be to use sscanf:
int left, right;
char o;
if (sscanf("4567*6789", "%d%c%d", &left, &o, &right) == 3) {
// scan valid...
cout << left << " " << o << " " << right;
}

Answer 5

My proposual is to create to functions:
std::size_t delimiter_pos(const std::string line)
{
std::size_t found = std::string::npos;
(found = line.find('+')) != std::string::npos ||
(found = line.find('-')) != std::string::npos ||
(found = line.find('*')) != std::string::npos ||
(found = line.find('/')) != std::string::npos;
return found;
}
And second function that calculate operands:
void parse(const std::string line)
{
std::string line;
std::size_t pos = delimiter_pos(line);
if (pos != std::string::npos)
{
std::string first = line.substr(0, pos);
char operation = line[pos];
std::string second = line.substr(pos + 1, line.size() - (pos + 1));
}
}
I hope my examples helped you