prefix notation c++, segmentation fault (stack and queue) - c++

Question

I am trying to figure out why I get segmentation fault, and my guess is that it is in my recursive function, which simplifies a prefix notation operation.
For example:
"m + 4 4" Returns: "+ m 8"
During testing I get a segmentation fault signal:
Exited with signal 11 (SIGSEGV)
I believe though that the problem lies in my recusive function "Operate"
string Operate(stack<string> &S, queue<string> &Q)
{
S.push(Q.front());
Q.pop();
std::string::size_type sz;
string result = "";
if (IsOperator(S.top()) == true)
{
S.push(Operate(S, Q));
}
if (Q.empty() == false)
{
S.push(Q.front());
Q.pop();
if (IsOperator(S.top()) == true)
{
S.push(Operate(S, Q));
}
if (S.size() < 3)
return "wrong input";
string arg1 = S.top();
S.pop();
string arg2 = S.top();
S.pop();
string oper = S.top();
S.pop();
if (StringIsDigit(arg1) && StringIsDigit(arg2))
{
int a = stoi(arg1, &sz);
int b = stoi(arg2, &sz);
char o = oper.at(0);
int c = 0;
if (o == '+')
c = b + a;
else if (o == '-')
c = b - a;
else if (o == '*')
c = b * a;
else
return "e";
result = to_string(c);
}
else
result = oper + " " + arg2 + " " + arg1;
}
else
{
result = S.top();
S.pop();
}
return result;
}
or in the function StringIsDigit:
bool StringIsDigit(string arg)
{
bool result = true;
for (int i = 0; i < arg.size() && result == true; i++)
{
if ((arg.size() != 1) && (arg.at(0) == '-') && (arg.at(i + 1) != ' '))
i++;
else
result = isdigit(arg.at(i));
}
return result;
}
Link to the whole program code:
https://pastebin.com/04pfE55N

Answer 1

The answer was quite simple, my error: SEGFAULT was as many pointed out for me error in reading from memory, segmentation fault, wiki.
When did the segfault occur?
It was when my function StringIsDigit() tried to figure out if negative values over 2 characters was an integer. In the "if statement, when checking if the string was indeed an integer, say -100", I continued to read the string until I reached the end of the arg string, but with arg.at(i + 1). Leading to the code trying to access memory outside the string array.Thanks Struthersneil for finding this flaw!
Please look at my old StringIsDigit() to find out the of by one value error I made:
bool StringIsDigit(string arg)
{
bool result = true;
for (int i = 0; i < arg.size() && result == true; i++)
{
if ((arg.size() != 1) && (arg.at(0) == '-') && (arg.at(i + 1) != ' '))
i++;
else
result = isdigit(arg.at(i));
}
return result;
}
The solution
The solution I want to make sure that the string was an integer since my algorithm supports expressions, such as x+3. This means that I need to iterate through the string is call isdigit() on every character in the string array. Though '-' is not an integer, '-' is needed obviously to express a negative integer, so I made a flawed check as you can see in my old StringIsDigit(). Instead of using that conditional if statement, I checked if the first character '-' and the second is not a whitespace ' ', and then I just let the isdigit() function do the rest of the work.
bool StringIsDigit(string arg)
{
bool result = true;
//I only need to check if the first is a '-' char and
//the next char is not ' '.
for (int i = 0; i < arg.size() && result == true; i++)
{
if ((arg.size() != 1) && (arg.at(0) == '-') && (arg.at(1) != ' '))
i++;
else
result = isdigit(arg.at(i));
}
return result;
}