So, quick question:
I am tasked with making a program that reads in a file, does some fancy things, and writes out to an html file with the re-purposed text, all simple stuff.
Furthermore the program must be able to accept up to four command line arguments (but a minumum of two). The executable of course, the file it is reading in, the file name it will be reading out to, and finally a "-r" argument for more information on the file (the amount of paragraphs, etc).
So the question I have is as follows:
the "-r" argument can be anywhere in the arguments (as long as it comes after argv[0] of course), or it can be completely non-existent (as can the output file name).
This is still simple to do, a little tedious writing a bunch of if's or even a switch or two, but I can't help but think that there may be an easier way to accomplish this, rather than having a plethora of if statements.
Any help would be greatly appreciated. (I also suppose you don't need any code considering I don't exactly have a problem.)
Here is roughly how one might do it without a library:
GCC 4.8.2: g++ -Wall -Wextra -std=c++0x main.cpp
#include <iostream>
#include <string>
int main(int argc, char* argv[]) {
char* input = nullptr;
char* output = nullptr;
char* r_arg = nullptr;
--argc; ++argv; // Skip the program name.
const char* def_in = "default in";
const char* def_out = "default out";
const char* def_r = "default r";
while (0 < argc) {
if (std::string(argv[0]) == "-r") {
// This code requires a space after the "-r", which is unusual.
++argv;
--argc;
r_arg = argv[0]; }
else if (input == nullptr) {
input = argv[0]; }
else if (output == nullptr) {
output = argv[0]; }
else {
std::cerr << "error: unexpected arg '" << argv[0] << "'\n"; }
++argv;
--argc; }
if (input == nullptr) {
input = const_cast<char*>(def_in); }
if (output == nullptr) {
output = const_cast<char*>(def_out); }
if (r_arg == nullptr) {
r_arg = const_cast<char*>(def_r); }
std::cout << "input: " << input << "\n"
<< "output: " << output << "\n"
<< "r arg: " << r_arg << "\n";
return 0; }
Related
I'm trying to dump the contents of a file to cout.
#include <iostream>
#include <fstream>
int main(int argc, char** argv)
{
if (argc > 1) {
std::ifstream fin(argv[1]);
if (fin) {
std::cout << "---file contents---\n";
std::cout << fin.rdbuf();
std::cout << "---end contents---\n";
} else {
std::cout << "The file does not exist\n";
}
}
else {
std::cout << "Usage: " << argv[0] << " FILE\n";
}
if (std::cout.good()) {
return 0;
}
else if (std::cout.fail()) {
return 1;
}
else {
return 2;
}
}
This code does not work as intended when the input file is empty. It prints the initial "---file contents---", but never prints the trailing "---end contents---". After debugging, I found the application is not crashing, but instead is putting std::cout in an error state (the return code is 1).
How can I print the contents of an empty file without putting cout in an error state?
This operator<< reference (overload number 10 in the list) explains it all:
If no characters were inserted, executes setstate(failbit).
Since the input file is empty, there's no characters to insert into the output stream. And the failbit is set.
You need to add a specific check for failbit after
std::cout << fin.rdbuf();
to see if the input file was empty or not.
This question already has answers here:
How to compare strings
(4 answers)
Closed 1 year ago.
I am trying to make a CLI app in C++. This is my first time coding in C++.
I have this c++ code:
#include <iostream>
// using namespace std;
static void help(std::string argv)
{
std::cerr << "Usage:" << argv << " [options]\n"
<< "Options:\n"
<< "-h (--help): Displays this help message.\n"
<< "-o (--output=[output file]): Specifies the output file.\n"
<< "-p (--ports=[ports]) Sets the ports to scan.\n"
<< std::endl;
}
int main(int argc, char** argv)
{
if (argc > 1)
{
std::cout << argv[1] << "\n";
if (argv[1] == "-h" || argv[1] == "--help")
{
help(argv[0]);
return 0;
}
}
else
{
std::cout << "No arguments were given" << "\n";
};
};
// g++ -o cli main.cpp
It works! When I compile it, it successfully outputs No arguments were given, but when I run cli -h, I can see argv[1] is -h, but nothing is outputted.
What did I do wrong?
In your string comparison, argv[1] is a C string: a null-terminated char array. You cannot compare these with == and get the result you expect. If, however, you assign it to a std::string you can compare it with "-h" and "--help" the way you want.
std::string arg1 = argv[1];
if (arg1 == "-h" || arg1 == "--help") {
help(argv[0]);
return 0;
}
Alternatively you could use std::strcmp to compare C strings without creating a new std::string. In order to do this, you'll need #include <cstring>.
if (std::strcmp(argv[1], "-h") == 0 || std::strcmp(argv[1], "--help") == 0) {
help(argv[0]);
return 0;
}
I'm new to C++, and I'm trying to write a project that interacts through command line. Right now, whenever I run my main (which is the executable), I always receive a segmentation fault error when the main program finished.
Edit comment:
I'm told by tutor to use as little as C++ features such as vectors or strings ... I'm also very new to C++, so i'm trying to utilize as many basic C functions as I can.
I'm
My main function looks like this:
int main(int argc, char** argv) {
cout << "starting mvote..." << endl;
int run_flag = 1;
char* actionBuffer = (char*)malloc(100 * sizeof(char));
char* action = (char*)malloc(16 * sizeof(char));
char* readPtr;
char exit[4] = { 'e','x','i','t' };
//parse command line argumentand get the filename
char* filename = argv[2];
cout << filename;
FILE* fp;
char line[64];
//from here, I'm opening the file and read it by lines
fp = fopen(filename, "r");
if (fp == NULL) {
cout << "file not exists";
return -1;
}
while (fgets(line, 64, fp) != NULL) {
cout << line << "\n";
}
fclose(fp);
while (run_flag == 1) {
cout << "what do you want?\n " << endl;
cin.getline(actionBuffer, 1024);
if (strcmp(actionBuffer, exit) == 0) {
cout << "bye!";
run_flag = 0;
break;
}
//if not exit, Look for the space in the input
readPtr = strchr(actionBuffer, ' ');
int size = readPtr - actionBuffer;
//extract the operation
strncpy(action, actionBuffer, size);
for (int i = 0; i < size; i++) {
cout << "operation:" << action[i];
}
// depend on the operation specified before the first empty space
run_flag = 0;
}
free(actionBuffer);
free(action);
return 0;
}
Description:
I first try to open up a csv file which lies in the same folder as main, and I read the file line by line. Then, I just implement a simple command where you can type exit and quit the program.
I allocate two memory, actionBuffer and action, which are used to hold command
Problem: a segmentation fault [core dumped] always exists when I type exit and hit enter, and then the process finished.
Research: So I learned that segmentation fault is due to accessing a memory that does not belongs to me. But where in my program am I trying to access such a memory?
Any advice is appreciated! Thank you.
Just to give you an idea, this would be an example of C++ code
#include<iostream>
#include<fstream>
#include<string_view>
#include<string>
#include<sstream>
#include<exception>
int main(int argc, char** argv) {
std::cout << "starting mvote...\n";
//parse command line argumentand get the filename
std::string filename = argv[2]; // NO CHECKS!
std::cout << filename <<'\n';
//from here, I'm opening the file and read it by lines
{
std::ifstream ifs(filename);
if (!ifs) {
throw std::invalid_argument("file not exists");
}
std::string line;
while (std::getline(ifs, line)) {
std::cout << line << '\n';
}
}
bool run_flag = true;
while (run_flag) {
std::cout << "what do you want?\n";
std::string userInput;
std::getline(std::cin, userInput);
if (userInput == "exit") {
std::cout << "bye!\n";
return 0;
}
std::stringstream userInputSs(userInput);
std::string operation;
while(userInputSs >> operation){
std::cout << "operation: " << operation << '\n';
}
}
}
I am new at C/C++,
So basically I want to call an .exe file that displays 2 numbers and be able to grab those two numbers to use them in my code.
To call the .exe file I've used the system command, but I am still not able to grab those two numbers that are displayed by the .exe file
char *files = "MyPath\file.exe";
system (files);
I think this is better aproach:
Here you just create new process, and you read data that process gives you. I tested this on OS X 10.11 with .sh file and works like a charm. I think that this would probably work on Windows also.
FILE *fp = popen("path to exe","r");
if (fp == NULL)
{
std::cout << "Popen is null" << std::endl;
}else
{
char buff[100];
while ( fgets( buff, sizeof(buff), fp ) != NULL )
{
std::cout << buff;
}
}
You need to escapr back slashes in C++ string literals so:
// note the double "\\"
char* files = "MyPath\\file.exe";
Or just use forward slashes:
char* files = "MyPath/file.exe";
Its not very efficient but one thing you can to with std::system is redirect the output to a file and then read the file:
#include <cstdlib>
#include <fstream>
#include <iostream>
int main()
{
// redirect > the output to a file called output.txt
if(std::system("MyPath\\file.exe > output.txt") != 0)
{
std::cerr << "ERROR: calling system\n";
return 1; // error code
}
// open a file to the output data
std::ifstream ifs("output.txt");
if(!ifs.is_open())
{
std::cerr << "ERROR: opening output file\n";
return 1; // error code
}
int num1, num2;
if(!(ifs >> num1 >> num2))
{
std::cerr << "ERROR: reading numbers\n";
return 1; // error code
}
// do something with the numbers here
std::cout << "num1: " << num1 << '\n';
std::cout << "num2: " << num2 << '\n';
}
NOTE: (thnx #VermillionAzure)
Note that system doesn't always work everywhere because unicorn
environments. Also, shells can differ from each other, like cmd.exe
and bash. – VermillionAzure
When using std::system the results are platform dependant and not all shells will have redirection or use the same syntax or even exist!
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?
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.