Figuring Out What's Going on This Code (Recursive Descent Case Study)

Question

I am reading a Data Structures and Algorithms book and there is a case study on recursion, specifically a parser that uses recursive descent. I'm a bit new to C++ (I'm learning it on the side and as I go with Stanley B. Lippman's C++ Primer 5th edition).

I'm a bit stuck on code and am trying to make sense of it. Is what I've written at the bottom (bulleted points) accurate descriptions of what is happening in the functions? I would post the header, but it's a bit too long, you can probably find it online if you search "Data Structures and Algorithms in C++ by Adam Drozdek - interpreter.h".

double Statement::findValue(char* id) {
    IdNode tmp(id);
    list<IdNode>::iterator i = find(idList.begin(), idList.end(), tmp);
    if (i != idList.end())
        return i->value;
    else
        issueError("Unknown variable");
    return 0;
}

void Statement::processNode(char* id, double e) {
    IdNode tmp(id, e);
    list<IdNode>::iterator i = find(idList.begin(), idList.end(), tmp);
    if (i != idList.end())
        i->value = e;
    else
        idList.push_front(tmp);
}

findValue()

• Looks for a value for a certain variable
• Uses an iterator i so that it can traverse the list
• Looks for tmp using find()
• If i doesn't equal the value at the end of the list, return it
• Other wise, the variable cannot be found

processNode()

• Processes nodes by using an iterator i
• Looks for a variable that matches tmp
• Finds the variable and sets it's value to the value of e
• Other wise, store variable on to idList to be evaluated later

Answer 1

Define a function that accepts a pointer to char (likely it's an array) and returns a double:

double Statement::findValue(char* id) {

Create a temporary (will die at 'return') of type IdNode based on that pointer:

IdNode tmp(id);

Use a (std:: I guess, but it could be any function with the same features) function that looks for tmp inside the container idList. The result is an interator i, which must be of the same type of that one used by the container:

list<IdNode>::iterator i = find(idList.begin(), idList.end(), tmp);

Check if something is found. idList.end() means "one past end", beyond the last item in the container:

if (i != idList.end())

Return the member value (which is part of IdNode) for the item found. If value is not a double then convert to it.

    return i->value;

Otherwise, call issueError function.

else
    issueError("Unknown variable");

Exit function, returning a double with value = 0:

return 0;
}

---

Same, but: this function accepts two parameters and returns nothing:

void Statement::processNode(char* id, double e) {

Same, but: IdNode constructor uses two parameters:

IdNode tmp(id, e);

Same

list<IdNode>::iterator i = find(idList.begin(), idList.end(), tmp);

Same

if (i != idList.end())

Now, modify the item found. Just update value member:

    i->value = e;

Otherwise, add tmp, insert it at the very begining of idList container.

else
    idList.push_front(tmp);

Exit function:

}