Find and Print data from txt file - c++

Question

I have an error message for the string class. Based on examples I have found through trying to solve this, I believe I am using the class correctly.
Below is the code :
int main()
{
string allData, gridNum;
ifstream gridData;
gridData.open ("/Users/Neo/Documents/UNi/Year_3/Grid Data Analysis Program/gridData.txt");
if (gridData.is_open())
{
while ( getline (gridData, allData) )
{
size_t gridNum = allData.find("Grid Receiver 34");
string receiverX = allData.substr (gridNum, 40);
cout << receiverX << endl;
}
gridData.close();
}
else cout << "Unable to open file..." << endl;
return 0;
}
error in the console...
libc++abi.dylib: terminating with uncaught exception of type std::out_of_range: basic_string
(lldb)
I am trying to read from a text file into a string variable. I only want to read in 40 characters after the words "Grid receiver 34", then print the contents of the new string.

Answer 1

while ( getline (gridData, allData) )
{
size_t gridNum = allData.find("Grid Receiver 34");
string receiverX = allData.substr (gridNum, 40);
cout << receiverX << endl;
}
Here you read the file line by line, searching for "Grid Receiver 34", however, if that string isn't found then std::string::find will return std::string::npos. Using that as argument for substr gets you in trouble. You should check if it's found before using it:
while ( getline (gridData, allData) )
{
size_t gridNum = allData.find("Grid Receiver 34");
if(gridNum != std::string::npos)
{
string receiverX = allData.substr (gridNum, 40);
cout << receiverX << endl;
}
}
Also, stop using using namespace std;.

Answer 2

You're probably getting an exception on the lines where the search string is not found.
You want to only try to extract the substring on lines where the string is found.
Modify your code as follows:
int main()
{
string allData, gridNum;
ifstream gridData;
gridData.open ("/Users/Neo/Documents/UNi/Year_3/Grid Data Analysis Program/gridData.txt");
if (gridData.is_open())
{
while ( getline (gridData, allData) )
{
size_t gridNum = allData.find("Grid Receiver 34");
if (gridNum != std::string::npos) // add this condition :-)
{
string receiverX = allData.substr (gridNum, 40);
cout << receiverX << endl;
}
}
gridData.close();
}
else cout << "Unable to open file..." << endl;
return 0;
}