Reading char from file and writing to another file - c++

Question

I am quite new in c++ and programming so sorry in advance in my question repeats. I have a text file of 3 lines:
7
00000000000000000000000*0000
0 0 0 R 0
What I need to do is read 2nd line and write it into an array as char. But I must not include 3rd line because it will go to a completely different matrix. My code so far :
ifstream input;
input.open("input1.txt");
input >> start;
char a=0;
string line;
while (getline(input, line))
{
a=0;
istringstream iss(line);
int length = line.size();
for (int i=0; i<length; i++)
{
iss >> a;
A[i] = a;
cout << A[i] << " " << i << endl;
}
}
input.close();
However, with this code it always starts new array for 3rd line. What am I doing wrong? What is the easiest way to fix it? Thank you.
-----------------------------Update--------------------------------------
I have modified the code but it still does not work properly. I am getting this kind of result : 5)-└ instead of correct one. My current code:
void Read(int &numLines, int &start, vector<char>&A, char B[][5])
{
ifstream input;
input.open("input.txt");
input >> start;
input.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
string line;
if(getline(input, line))
{
for(char temp: line)
{
A.push_back(temp);
}
}
input.close();
}
A here is a vector I want to write 2nd line to, char by char
Start is just an integer in which I am storing 1st line (7)
Thank you very much for advices

Answer 1

Mixing >> and std::getline is non-trivial. For example, after input >> start; the end of line marker is left in the stream in case it's still needed. In your case it isn't, and it is picked off by the subsequent call to getline, resulting in a read of an empty line.
This is what's complicating your read of line and forcing the while loop and test for empty lines.
Step through your program with your development environment's debugger and you'll see what I'm talking about. Get used to using the debugger. It's possibly the best programming productivity tool you'll ever encounter.
The easiest way to fix it is to place
input.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
after
input >> start;
to eat up the end of the line (and anything else that might be on that line. This needs the addition of #include<limits> to get std::numeric_limits<std::streamsize>::max.
Then you can remove the while loop and replace it with
if (getline(input, line))
No loop, not chance of consuming multiple lines from the file. And the logic for reading and processing the third line can follow.
Side note: instead of that for loop, consider
int i = 0;
while (iss >> a)
{
A[i] = a;
cout << A[i] << " " << i << endl;
i++;
}
This will march through iss until it hits the end of the line. You can also throw iss out entirely and just read the characters directly out of line.
int i = 0;
for(char temp: line)
{
A[i] = temp;
}
And A should probably be a vector if it isn't already to reduce the chances of buffer overruns.
for(char temp: line)
{
A.push_back(temp);
}

Answer 2

I would go with something like this:
std::string start;
std::string Astring;
ifstream input;
input.open("input.txt");
input >> start;
input >> Astring;
// If you really want a char array
char * A = new char[Astring.size()];
for (unsigned int i = 0; i < Astring.size(); i++) {
A[i] = Astring[i];
}
// Don't forget to delete after use
delete[] A;
Moreover, if you just need the char array as an input to something else later, you can call Astring.c_str() instead of that for loop, which returns a C-style char array.