So currently I am working on an assignment, and for a section of it I need to be able to read a .txt file into a linked list of type char. I was already confused trying to do this, so I set out on a different path and decided to try to copy the text from the file into a char array, and then one by one copy the values in the array into the linked list.
So far I have my program compiling and running up to a certain point, before I receive the error Segmentation fault (core dumped).
The code for reading the file is as follow:
void readFile(list<char> &originList, string fileName){
ifstream fileInput;
fileInput.open(fileName.c_str());
int arraySize = fileInput.gcount();
char tempHold[arraySize];
if (!fileInput) {
cout << "Can't open file: " << fileName << "\n";
} else {
string contents((istreambuf_iterator<char>(fileInput)), istreambuf_iterator<char>());
strcpy (tempHold, contents.c_str());
for (int x = 0; x < fileInput.gcount(); x++) {
originList.push_back(tempHold[x]);
}
}
fileInput.close();
}
Also to add some context, using cout I determined that the code stops running, instead presenting the error, at the following point:
strcpy (tempHold, contents.data());
Also, I am not 100% on how exactly they work, only a loose idea to be honest. I mostly sourced the idea from this Stack Overflow question,
How to copy a .txt file to a char array in c++, but got confused somewhere a long the way.
Thanks in advance for any help you can provide.
istream::gcount returns the number of characters extracted by the last unformatted input operation performed on the object. Since you did not read anything from the file, you should get something wrong.Call, for example, istream.getline(); before calling gcount()
Like my comment to your question says, read each character from the file and add it to the list.
void readFile(list<char> &originList, string fileName) {
ifstream fileInput(fileName.c_str());
if (!fileInput) {
cout << "Can't open file: " << fileName << "\n";
}
char c;
while (fileInput.get(c)) {
originList.push_back(c);
}
}
Note: while (fileInput.get(c)) This reads the character and returns the stream. When a stream is used as a bool value it checks to see if the stream is valid. Valid means eof() and bad() are both false. - From a comment to the answer to the question linked below.
My answer was adapted from this other Stack Overflow question: Reading from text file until EOF repeats last line
Related
I began learning strings yesterday and wanted to manipulate it around by filling it with a text from a text file. However, upon filling it the cstring array only prints out the last word of the text file. I am a complete beginner, so I hope you can keep this beginner friendly. The lines I want to print from the file are:
"Hello World from UAE" - First line
"I like to program" - Second line
Now I did look around and eventually found a way and that is to use std::skipary or something like that but that did not print it the way I had envisioned, it prints letter by letter and skips each line in doing so.
here is my code:
#include <fstream>
#include <iostream>
#include <cstring>
#include <cctype>
using namespace std;
int main() {
ifstream myfile;
myfile.open("output.txt");
int vowels = 0, spaces = 0, upper = 0, lower = 0;
//check for error
if (myfile.fail()) {
cout << "Error opening file: ";
exit(1);
}
char statement[100];
while (!myfile.eof()) {
myfile >> statement;
}
for (int i = 0; i < 30; ++i) {
cout << statement << " ";
}
I'm not exactly sure what you try to do with output.txt's contents, but a clean way to read through a file's contents using C++ Strings goes like this:
if (std::ifstream in("output.txt"); in.good()) {
for (std::string line; std::getline(in, line); ) {
// do something with line
std::cout << line << '\n';
}
}
You wouldn't want to use char[] for that, in fact raw char arrays are hardly ever useful in modern C++.
Also - As you can see, it's much more concise to check if the stream is good than checking for std::ifstream::fail() and std::ifstream::eof(). Be optimistic! :)
Whenever you encounter output issues - either wrong or no output, the best practise is to add print (cout) statements wherever data change is occurring.
So I first modified your code as follows:
while (!myfile.eof()) {
myfile >> statement;
std::cout<<statement;
}
This way, the output I got was - all lines are printed but the last line gets printed twice.
So,
We understood that data is being read correctly and stored in statement.
This raises 2 questions. One is your question, other is why last line is printed twice.
To answer your question exactly, in every loop iteration, you're reading the text completely into statement. You're overwriting existing value. So whatever value you read last is only stored.
Once you fix that, you might come across the second question. It's very common and I myself came across that issue long back. So I'm gonna answer that as well.
Let's say your file has 3 lines:
line1
line2
line3
Initially your file control (pointer) is at the beginning, exactly where line 1 starts. After iterations when it comes to line3, we know it's last line as we input the data. But the loop control doesn't know that. For all it knows, there could be a million more lines. Only after it enters the loop condition THE NEXT TIME will it come to know that the file has ended. So the final value will be printed twice.
I know how to pass in strings from a text file. In a previous project I read in strings and then tested them on either being "t" or "f", which the result of would set a variable to true or false.
Now I am wondering if it is efficiently possible to read numbers from a text file and pass them into an int? All I can think of is checking for the string "1" and returning 1 in a function, but that would have to be done for every possible integer I could expect in my program, which is not an effective solution.
For context, I am trying to make a save system for a game, and ints/floats that are read in would be variables such as player health, how much of an item they have, etc.
If you already know how to read a string str from a text file, reading numbers is not that difficult: jsut read the string as you did and use stoi() to convert the string into an int, or stof() into float.
int i; double d;
i=stroi(str); d=strod(str2);
Another technique is to use file streams to read or write from a file exactly as you would do from cin and cout:
ifstream file("mytext.txt");
file>>i>>d;
The previous method doesn't care so much about lines. So still another technique is to read a string, convert it into a string stream and use the stringstream as you would with cin:
if (getline(file, str)){ // read a full line
stringstream sst(str);
sst>>i>>d;
}
Using std::fstream. You can open a file, and stream input or output based on how you opened the file.
Example:
#include <iostream>
#include <fstream>
int main(int argc, char** argv)
{
// Pretend we are passed the file location as a command-line argument to our program:
std::fstream file { argv[1], std::ios::in };
if (file.is_open())
{
int value;
file >> value;
std::cout << value << std::endl;
}
else
{
std::cout << "Could not open file " << argv[1] << std::endl;
}
}
Provided that the information is correctly formatted in the file, this should work.
I didn't run it, so there might be syntax errors, but the basics are there. Check out cppreference for some help, they will have further examples.
I'm fairly new to C++ so please forgive me if my terminology or methodology isn't correct.
I'm trying to write a simple program that:
Opens two input files ("infileicd" and "infilesel").
Opens a single output file "list.txt".
Compares "infilesel" to "infileicd" line by line.
If a line from "infilesel" is found in "infileicd", it writes that line from "infileicd" to "list.txt", effectively making a separate log file.
I am using the getline() function to do this but have run into trouble when trying to compare each file line. I think it might be easier if I could use only the substring of interest to use as a comparison.
The problem is that there are multiple words within the entire getline string and I am only really interested in the second one. Here are two examples:
"1529 nic1_mau_op_mode_3 "8664afm007-01" "1" OUTPUT 1 0 LOGICAL 4 4136"
"1523 pilot_mfd_only_sel "8664afm003-02" "1" OUTPUT 1 0 LOGICAL 4 4112"
"nic1_mau_op_mode_3" and "pilot_mfd_only_sel" are the only substrings of interest.
It would make it a lot easier if I could only use that second substring to compare but I don't know how to extract it specifically from the getline() function. I haven't found anything suggesting it is impossible to do this, but if it is impossible, what would be an alternative method for extracting that substring?
This is a personal project so I'm under no time contstraints.
Any assistance is greatly apprecated in advance. Here is my code (so far):
int main()
{
//Open the file to write the selected variables to.
ofstream writer("list.txt");
//Open the selected variabels file to be read.
ifstream infilesel;
infilesel.open("varsel.txt");
//Open the icd file to be read.
ifstream infileicd;
infileicd.open("aic_fdk_host.txt");
//Check icd file for errors.
if (infileicd.fail()){
cerr << "Error opening icd.\n" << endl;
return 1;
}
else {
cout << "The icd file has been opened.\n";
}
//Check selected variables file for errors.
if (infilesel.fail()){
cerr << "Error opening selection file.\n" << endl;
return 1;
}
else {
cout << "The selection file has been opened.\n";
}
//Read each infile and copy contents of icd file to the list file.
string namesel;
string nameicd;
while(!infileicd.eof()){
getline(infileicd, nameicd);
getline(infilesel, namesel);
if (nameicd != namesel){ //This is where I would like to extract and compare the two specific strings
infileicd; //Skip to next line if not the same
} else {
writer << nameicd << namesel << endl;
}
}
writer.close();
infilesel.close();
infileicd.close();
return 0;
}
So, based on what we discussed in the comments, you just need to toss the stuff you don't want. So try this:
string namesel;
string nameicd;
string junk;
while(!infileicd.eof()){
// Get the first section, which we'll ignore
getline(infileicd, junk, ' ');
getline(infilesel, junk, ' ');
// Get the real data
getline(infileicd, nameicd, ' ');
getline(infilesel, namesel, ' ');
// Get the rest of the line, which we'll ignore
getline(infileicd, junk);
getline(infilesel, junk);
Basically, getline takes a delimiter, which by default is a newline. By setting it as a space the first time, you get rid of the first junk section, using the same method, you get the part you want, and then the final portion goes to the end of the line, also ignoring it.
EDIT: Problem solved! Turns out Windows 7 wont let me read/ write to files without explicitly running as administrator. So if i run as admin it works fine, if i dont i get the weird results i explain below.
I've been trying to get a part of a larger program of mine to read a file.
Despite trying multiple methods(istream::getline, std::getline, using the >> operator etc) All of them return with either /0, blank or a random number/what ever i initialised the var with.
My first thought was that the file didn't exist or couldn't be opened, however the state flags .good, .bad and .eof all indicate no problems and the file im trying to read is certainly in the same directory as the debug .exe and contains data.
I'd most like to use istream::getline to read lines into a char array, however reading lines into a string array is possible too.
My current code looks like this:
void startup::load_settings(char filename[]) //master function for opening a file.
{
int i = 0; //count variable
int num = 0; //var containing all the lines we read.
char line[5];
ifstream settings_file (settings.inf);
if (settings_file.is_open());
{
while (settings_file.good())
{
settings_file.getline(line, 5);
cout << line;
}
}
return;
}
As said above, it compiles but just puts /0 into every element of the char array much like all the other methods i've tried.
Thanks for any help.
Firstly your code is not complete, what is settings.inf ?
Secondly most probably your reading everything fine, but the way you are printing is cumbersome
cout << line; where char line[5]; be sure that the last element of the array is \0.
You can do something like this.
line[4] = '\0' or you can manually print the values of each element in array in a loop.
Also you can try printing the character codes in hex for example. Because the values (character codes) in array might be not from the visible character range of ASCII symbols. You can do it like this for example :
cout << hex << (int)line[i]
I have been having a very hard time writing to a binary file and reading back. I am basically writing records of this format
1234|ABCD|efgh|IJKL|ABC
Before writing this record, I would write the length of this entire record ( using string.size()) and then I write the record to the binary file using ofstream as follows:
int size;
ofstream studentfile;
studentfile.open( filename.c_str(),ios::out|ios::binary );
studentfile.write((char*)&size,sizeof(int));
studentfile.write(data.c_str(),(data.size()*(sizeof(char))));
cout << "Added " << data << " to " << filename << endl;
studentfile.close();
And I read this data at some other place
ifstream ifile11;
int x;
std::string y;
ifile11.open("student.db", ios::in |ios::binary);
ifile11.read((char*)&x,sizeof(int));
ifile11.read((char*)&y,x);
cout << "X " << x << " Y " << y << endl;
first I read the length of the record into the variable x, and then read the record into string y. The problem is, the output shows x as being '0' and 'y' is empty.
I am not able figure this out. Someone who can look into this problem and provide some insight will be thanked very much.
Thank you
You can't read a string that way, as a std::string is really only a pointer and a size member. (Try doing std::string s; sizeof(s), the size will be constant no matter what you set the string to.)
Instead read it into a temporary buffer, and then convert that buffer into a string:
int length;
ifile11.read(reinterpret_cast<char*>(&length), sizeof(length));
char* temp_buffer = new char[length];
ifile11.read(temp_buffer, length);
std::string str(temp_buffer, length);
delete [] temp_buffer;
I know I am answering my own question, but I strictly feel this information is going to help everyone. For most part, Joachim's answer is correct and works. However, there are two main issues behind my problem :
1. The Dev-C++ compiler was having a hard time reading binary files.
2. Not passing strings properly while writing to the binary file, and also reading from the file. For the reading part, Joachim's answer fixed it all.
The Dev-C++ IDE didn't help me. It wrongly read data from the binary file, and it did it without me even making use of a temp_buffer. Visual C++ 2010 Express has correctly identified this error, and threw run-time exceptions and kept me from being misled.
As soon as I took all my code into a new VC++ project, it appropriately provided me with error messages, so that I could fix it all.
So, please do not use Dev-C++ unless you want to run into real troubles like thiis. Also, when trying to read strings, Joachim's answer would be the ideal way.