C++ ifstream type error - c++

I want to read the content of a file with c++. Im using ifstream, but here is an error at compiling:
Code:
#include <Python.h>
#include <iostream>
#include <fstream>
using namespace std;
ifstream script;
script.open("main.py");
const char *programm;
if (script.is_open()) {
while (!script.eof()) {
script >> programm;
}
}
script.close();
And the Error:
main.cpp:8:1: error: 'script' does not name a type
script.open("main.py");
^
main.cpp:10:1: error: expected unqualified-id before 'if'
if (script.is_open()) {
^
I hope you can help me, thanks!

#include <Python.h>
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main(){
ifstream script;
script.open("main.py");
// const char *programm; // You don't need a C string unless you have a reason.
string programm;
if (script.is_open()) {
while (!script.eof()) {
string line;
script >> line;
line += '\n';
programm += line;
}
}
script.close();
// Now do your task with programm;
return 0;
}

There are a few issues. The main one (causing the error) is that in C++ you can't just have code living on its own. It all goes into functions. In particular, you have to have the main function.
Also, your read loop is not going to work correctly. You should read into a std::string, that will keep track of the memory for you, and the current way you would miss the last string. I would suggest reading a line at a time. Something like this:
#include <Python.h>
#include <iostream>
#include <fstream>
#include <string>
int main()
{
std::ifstream script ("main.py");
std::string line;
if (!script) // Check the state of the file
{
std::cerr << "Couldn't open file\n";
return 1;
}
while (std::getline(script, line)) // Read a line at a time.
// This also checks the state of script after the read.
{
// Do something with line
}
return 0; // File gets closed automatically at the end
}

Related

Getting Information from input file C++

I'm pretty new to coding so I'm not entirely sure if I'm doing file extraction correct. I'm getting lldb as my output for this code. Instead of prompting the user with the words in the hangman.dat file.
#include <iostream>
#include <fstream>
#include <cstdlib>
using namespace std;
int main()
{
ifstream sourceFile;
sourceFile.open("hangman.dat");
if (sourceFile.fail())
{
cout<<"File didn't open" ;
}
else
{
string words;
sourceFile >> words;
while(sourceFile>>words)
{
cout<<words<<endl;
}
}
}
The file hangman.dat contains the following information:
Fall
leaves
Thanksgiving
pumpkins
turkey
Halloween

Reading a file to a string in C++

As somebody who is new to C++ and coming from a python background, I am trying to translate the code below to C++
f = open('transit_test.py')
s = f.read()
What is the shortest C++ idiom to do something like this?
The C++ STL way to do this is this:
#include <string>
#include <iterator>
#include <fstream>
using namespace std;
wifstream f(L"transit_test.py");
wstring s(istreambuf_iterator<wchar_t>(f), (istreambuf_iterator<wchar_t>()) );
I'm pretty sure I've posted this before, but it's sufficiently short it's probably not worth finding the previous answer:
std::ifstream in("transit_test.py");
std::stringstream buffer;
buffer << in.rdbuf();
Now buffer.str() is an std::string holding the contents of transit_test.py.
You can do file read in C++ as like,
#include <iostream>
#include <fstream>
#include <string>
int main ()
{
string line;
ifstream in("transit_test.py"); //open file handler
if(in.is_open()) //check if file open
{
while (!in.eof() ) //until the end of file
{
getline(in,line); //read each line
// do something with the line
}
in.close(); //close file handler
}
else
{
cout << "Can not open file" << endl;
}
return 0;
}

Most Compact Way to Count Number of Lines in a File in C++

What's the most compact way to compute the number of lines of a file?
I need this information to create/initialize a matrix data structure.
Later I have to go through the file again and store the information inside a matrix.
Update: Based on Dave Gamble's. But why this doesn't compile?
Note that the file could be very large. So I try to avoid using container
to save memory.
#include <iostream>
#include <vector>
#include <fstream>
#include <sstream>
using namespace std;
int main ( int arg_count, char *arg_vec[] ) {
if (arg_count !=2 ) {
cerr << "expected one argument" << endl;
return EXIT_FAILURE;
}
string line;
ifstream myfile (arg_vec[1]);
FILE *f=fopen(myfile,"rb");
int c=0,b;
while ((b=fgetc(f))!=EOF) c+=(b==10)?1:0;
fseek(f,0,SEEK_SET);
return 0;
}
I think this might do it...
std::ifstream file(f);
int n = std::count(std::istreambuf_iterator<char>(file), std::istreambuf_iterator<char>(), '\n') + 1;
If the reason you need to "go back again" is because you cannot continue without the size, try re-ordering your setup.
That is, read through the file, storing each line in a std::vector<string> or something. Then you have the size, along with the lines in the file:
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
int main(void)
{
std::fstream file("main.cpp");
std::vector<std::string> fileData;
// read in each line
std::string dummy;
while (getline(file, dummy))
{
fileData.push_back(dummy);
}
// and size is available, along with the file
// being in memory (faster than hard drive)
size_t fileLines = fileData.size();
std::cout << "Number of lines: " << fileLines << std::endl;
}
Here is a solution without the container:
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
int main(void)
{
std::fstream file("main.cpp");
size_t fileLines = 0;
// read in each line
std::string dummy;
while (getline(file, dummy))
{
++fileLines;
}
std::cout << "Number of lines: " << fileLines << std::endl;
}
Though I doubt that's the most efficient way. The benefit of this method was the ability to store the lines in memory as you went.
FILE *f=fopen(filename,"rb");
int c=0,b;while ((b=fgetc(f))!=EOF) c+=(b==10)?1:0;fseek(f,0,SEEK_SET);
Answer in c.
That kind of compact?
#include <stdlib.h>
int main(void) { system("wc -l plainfile.txt"); }
Count the number of instances of '\n'. This works for *nix (\n) and DOS/Windows (\r\n) line endings, but not for old-skool Mac (System 9 or maybe before that), which used just \r. I've never seen a case come up with just \r as line endings, so I wouldn't worry about it unless you know it's going to be an issue.
Edit: If your input is not ASCII, then you could run into encoding problems as well. What's your input look like?

How create file in C++ in a specific place in the PC

Hey all, I have a problem, I don't know how to create a file in C++ in a specific place in the PC. For example a file (.txt) in C:\file.txt. Can anybody help me? Thank you :)
#include <iostream>
#include <fstream>
using namespace std;
int main() {
ofstream ofs("c:\\file.txt");
if (ofs) {
ofs << "hello, world!\n";
}
return 0;
}
It's probably fooling you because it's easier than you think. You just open a file for create and give it that path name. Voila.
See, eg,
// fstream::open
#include <fstream>
using namespace std;
int main () {
fstream filestr;
// You need a doubled backslash in a C string
filestr.open ("C:\\file.txt", fstream::out);
// >> i/o operations here <<
filestr.close();
return 0;
}
#include <stdio.h>
....
FILE *file;
file = fopen("c:/file.txt", "w");

C++ Reading Files

What is the minimum code required to read a file and assign its contents to a string in c++?
I did read a lot of tutorials that worked but they were all different in a way so i am trying to see why, so if you could please include some explanatory comments that would be great.
Related: What is the best way to read an entire file into a std::string in C++?
#include <fstream>
#include <string>
int main()
{
std::ifstream file("myfile.txt"); // open the file
std::string line, whole_file;
// Read one line at a time from 'file' and store the result
// in the string called 'line'.
while (std::getline(file, line))
{
// Append each line together so the entire file will
// be in one string.
whole_file += line;
whole_file += '\n';
}
return 0;
// 'file' is closed automatically when the object goes out of scope.
}
A couple of things to note here. getline() returns a reference to the stream object, which fails the while-test if anything bad happens or if you reach the end of the file. Also, the trailing newline is not included in the string, so you have to append it manually.
The shortest code: (not effecient)
#include <iostream>
#include <string>
#include <algorithm>
#include <iterator>
#include <fstream>
int main()
{
std::ifstream f("plop");
std::string buffer;
std::copy(std::istreambuf_iterator<char>(f),
std::istreambuf_iterator<char>(),
std::back_inserter(buffer));
}
How I would probably do it:
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <iterator>
#include <fstream>
int main()
{
// Find the size of the file
std::ifstream file("Plop");
file.seekg(0,std::ios_base::end);
std::streampos size = file.tellg();
// Read the file in one go.
file.seekg(0);
std::vector<char> buffer(size); // pre-szie the vector.
file.read(&buffer[0],size);
// or
// Until the next version of the standard I don't think string gurantees contigious storage.
// But all the current versions I know do use continious storage so it should workd.
file.seekg(0);
std::string buffer1(size);
file.read(&buffer1[0],size);
}
I'm not seeing as much:
#include <fstream>
#include <sstream>
#include <string>
using namespace std;
int main() {
ifstream ifs("filename");
stringstream ss;
ss << ifs.rdbuf();
string s = ss.str();
}
... as I'd expect. You'd want some error-checking too.
Konrad Rudolph gave this as the answer to the "related question" linked above. I suppose this isn't a duplicate, since this asks for the shortest code, but the answer is the same either way. So I repost it here as wiki.
I am reading a word from each line.
#include<fstream>
#include<string>
using namespace std;
int main(int argc, char **argv)
{
fstream inFile;
string str;
while(!inFile.eof())
{
inFile.open("file.txt");
infile>>str;
}
inFile.close();
return 0;
}
This is longer than the short solutions, but is possibly slightly more efficient as it does a bit less copying - I haven't done any timing comparisons though:
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
using namespace std;;
unsigned int FileRead( istream & is, vector <char> & buff ) {
is.read( &buff[0], buff.size() );
return is.gcount();
}
int main() {
ifstream ifs( "afile.dat", ios::binary );
const unsigned int BUFSIZE = 64 * 1024;
std::vector <char> buffer( BUFSIZE );
unsigned int n;
string s;
while( n = FileRead( ifs, buffer ) ) {
s.append( &buffer[0], n );
}
cout << s;
}
If you know that your file contains text, then you can use STLSoft's platformstl::memory_mapped_file:
platformstl::memory_mapped_file file("your-file-name");
std::string contents(static_cast<char const*>(file.memory()), file.size());
or
platformstl::memory_mapped_file file("your-file-name");
std::wstring contents(static_cast<wchar_t const*>(file.memory()),
file.size() / sizeof(wchar_t));
On WIndows, that will leave your string containing \r\n sequences, so you could instead use the winstl::load_text_file() function:
std::string contents;
winstl::load_text_file("your-file-name", contents);
If you want it loaded into a collection of lines, then use platformstl::read_lines():
platformstl::basic_file_lines<char> lines("your-file-name");
size_t n = lines.size();
std::string line3 = lines[3];