Question

Is there a particular function in c++ that can return the line number of a particular string i want to find?

ifstream fileInput;
int offset;
string line;
char* search = "a"; // test variable to search in file
// open file to search
fileInput.open(cfilename.c_str());
if(fileInput.is_open()) {
    while(!fileInput.eof()) {
        getline(fileInput, line);
        if ((offset = line.find(search, 0)) != string::npos) {
            cout << "found: " << search << endl;
        }
    }
    fileInput.close();
}
else cout << "Unable to open file.";

I want to add some codes at:

    cout << "found: " << search << endl;

That will return the line number followed by the string that was searched.

Answer 1

Just use a counter variable to keep track of the current line number. Each time you call getline you… read a line… so just increment the variable after that.

unsigned int curLine = 0;
while(getline(fileInput, line)) { // I changed this, see below
    curLine++;
    if (line.find(search, 0) != string::npos) {
        cout << "found: " << search << "line: " << curLine << endl;
    }
}

Also…

while(!fileInput.eof())

should be

while(getline(fileInput, line))

If an error occurs while reading eof will not be set, so you have an infinite loop. std::getline returns a stream (the stream you passed it) which can be implicitly converted to a bool, which tells you if you can continue to read, not only if you are at the end of the file.

If eof is set you will still exit the loop, but you will also exit if, for example, bad is set, someone deletes the file while you are reading it, etc.