String turns up empty after find_last_of() and substr()? - c++

Question

Self-teaching myself C++, and I know I'm missing something critical, but I can't for the life of me figure out what it is.
Forgive the huge block of code, I was tempted to trim it down to the critical elements, but I figured if I left it intact, you folks might have other educational criticisms about my coding style...
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
// main routine
int main(int argc, char *argv[]) {
// will store filetype here for later
string filetype = "";
string filename;
// if no arguments, die.
if (argc < 2) {
cout << "ERROR: Nothing to do." << endl;
return 1;
}
// if more than one argument, die.
else if (argc > 2) {
// TODO: support for multiple files being checked would go here.
cout << "ERROR: Too many arguments." << endl;
return 1;
}
// otherwise, check filetype
else {
string filename = argv[1];
cout << "Filename: " << filename << endl;
//searching from the end, find the extension of the filename
int dot = filename.find_last_of('.');
if (dot == -1){
// TODO: Add support for filenames with no extension
cout << "ERROR: Filename with no extension." << endl;
return 1;
}
string extension = filename.substr(dot);
if (extension == ".htm" || extension == ".html"){
filetype = "html";
}
else if (extension == ".c"){
filetype = "c";
}
else if (extension == ".c++" || extension == ".cpp") {
filetype = "cpp";
}
else {
cout << "ERROR: unsupported file extension" << endl;
// TODO: try to guess filetype from file headers here
}
}
cout << "Determined filetype: " << filetype << endl;
cout << "Filename: " << filename << endl;
return 0;
}
// All done :]
The issue I'm having is mysterious. I put the argument passed into a string like so:
string filename = argv[1];
and then search it for an extension, starting from the end and working my way to the beginning:
int dot = filename.find_last_of('.');
string extension = filename.substr(dot);
This all works as expected, but afterwards, when I try to output filename, it is mysteriously empty? I tried debugging with cout. When I print out the string BEFORE I search it, it prints properly. After, nothing prints. Like so:
$ g++ test.cpp -o test.out; ./test.out foo.html
Filename: foo.html
Determined filetype: html
Filename:
I remembered something about iterators in the past, and tried using filename.begin() to reset it, but this did nothing. Can someone shed light onto this puzzling issue?

Answer 1

You are declaring a second variable called filename here, after the else:
string filename = argv[1];
This goes out of scope by the time you get here:
cout << "Filename: " << filename << endl;
You are now printing the contents of the 1st variable you declared called filename, just under main.