C++ vector out of range exception - c++

Question

I'm writing a program which makes calculations based on commands, but i'm having a little trouble with the vector's range.
Basically i am entering integers into a vector, a sequence of integers each on a new line, and for example if i enter the command "sum" it gets the last two elements of the vector (the sequence of integers) sums them, deletes them from the sequence and adds to the sequence the new number.
Example:
1
2
4
5
sum
end (this command ends my program)
Output:
1
2
9
Here is the code:
#include "stdafx.h"
#include <iostream>
#include <string>
#include <sstream>
#include <vector>
#define CONSOLE_LOG(x) std::cout << x
#define COMMAND_END command[0]
#define COMMAND_SUM command[1]
#define COMMAND_SUBTRACT command[2]
#define COMMAND_CONCAT command[3]
#define COMMAND_DISCARD command[4]
#define COMMAND_DISPSEQ command[5]
std::string sum(std::string& x, std::string& y, std::vector<std::string>& vec)
{
int xInt = atoi(x.c_str());
int yInt = atoi(y.c_str());
int result = xInt + yInt;
std::ostringstream os;
std::string strResult;
os << result;
strResult = os.str();
vec.erase(vec.end() - 1);
vec.erase(vec.end() - 1);
return strResult;
}
/*
std::string subtract(std::string& x, std::string& y)
{
}
std::string concat(std::string& x, std::string& y)
{
}
std::string discard(std::string& x, std::string& y)
{
}
*/
void dispseq(std::vector<std::string>& vec)
{
std::vector<std::string>::const_iterator iter;
for (iter = vec.begin(); iter != vec.end(); iter++)
{
std::cout << *iter << std::endl;
}
}
void enterSequence()
{
std::vector<std::string> command =
{
"end",
"sum",
"subtract",
"concat",
"discard",
"dispseq"
};
std::vector<std::string> sequence;
std::string input;
std::string a = sequence[sequence.size() - 1];
std::string b = sequence[sequence.size() - 2];
do
{
std::cin >> input;
sequence.emplace_back(input);
if (input == COMMAND_END)
{
sequence.erase(sequence.end() - 1);
}
if (input == COMMAND_SUM)
{
sequence.emplace_back(sum(a, b, sequence));
}
dispseq(sequence);
} while (input != COMMAND_END);
}
int main()
{
enterSequence();
std::cin.get(); std::cin.get();
return 0;
}
And here is my problem. Of course i need to define the last two elements of the vector, and i do it with
std::string a = sequence[sequence.size() - 1];
std::string b = sequence[sequence.size() - 2];
But it gives me an exception, and tells me i'm out of range. I guess im not supposed to pass a size of a vector, which still doesn't have one.
How can i write it then?

Answer 1

a and b are being initialized to a negative index of your vector. You can fix that by modifying your do-while loop.
std::string a;
std::string b;
do
{
if(sequence.size() >= 2)
{
b = sequence[sequence.size() - 2];
a = sequence[sequence.size() - 1];
}
else if(sequence.size() >= 1)
{
a = sequence[sequence.size() - 1]
}
std::cin >> input;
sequence.emplace_back(input);
if (input == COMMAND_END)
{
sequence.erase(sequence.end() - 1);
}
if (input == COMMAND_SUM)
{
sequence.emplace_back(sum(a, b, sequence));
}
dispseq(sequence);
} while (input != COMMAND_END);