I'm pretty sure that this is a common question, but I can't find an example similar to mine, so..
I have a text file called input.txt that has: 0.0001234 1.0005434 0.0005678 1.0023423 0.00063452 1.0001546 0.00074321 1.00017654 in it.
Now I want to write a program to read that into an array, and then quickly check that it worked. So far, I've got:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main ()
{
double PA[8];
int n;
ifstream myfile ("input.txt");
if (myfile.is_open())
{
while (! myfile.eof() )
{
getline (myfile,line);
cout << PA[line]<< endl;
}
myfile.close();
}
else cout << "Unable to open file";
for (n=0; n<8; n++) // to print the array, to check my work
{
cout << " {" << PA[n] << "} ";
}
system("PAUSE");
return 0;
}
My problem so far is that I keep getting the error: line was not declared. And I want to use the array later with floats to calculate new data. I think I'm doing it wrong for that.. Any help? Thanks!
declare line variable
int n, line = 0;
std::string value;
proper load data:
getline (myfile,value);
PA[line] = atof(value.c_str());
cout << PA[line]<< endl;
line++;
the variable line here
cout << PA[line]<< endl;
is not declared. and if you declare it, you also have to give it a value, otherwise it's undefined and the PA[line] is undefined, in other words: will crash.
the entire while block seems suspicious:
while (! myfile.eof() )
{
getline (myfile,line);
cout << PA[line]<< endl;
}
are you sure about the getline call?
I know a getline with this signature:
ssize_t getline(char **lineptr, size_t *n, FILE *stream);
and that does not even match the number of arguments in your version.
if your input file has more than 8 lines on input, than the while loop will have that many interations - and your array has only space for 8 elements.
You need to declare the variable you called 'line' as follows:
int i=0;
while (! myfile.eof() && i<8)
{
std::string line; // this was the missing statement
getline (myfile,line);
double value = atof(line.c_str()); // convert line form char* to double
PA[i++] = value;
cout << value << endl;
}
Note that you need to convert line as double and use increment variable 'i' (for example, as I did. Make sure not overflowing PA capacity by checking i agains the size (currently 8, which should not hard coded, btw).
Also note that you shouldn't print the result if file access failed.
Related
So I'm trying to make a program that will open up a text file based on user input (or fail trying), read the contents and create a new text file named after user input and rewrite the contents, but this time with row numbers.
I'm having trouble that I can't pin down on anything specific. My Qt Creator gives no massive errors, but the program seems to halt around line 30 without any error messages.
An online C++ compiler complained about a segmentation problem.
The code:
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
using namespace std;
int main() {
string input_file = "";
cout << "input file: ";
getline(cin, input_file);
vector<string> line_vector;
string output_file = "";
cout << "output file:";
getline(cin, output_file);
ifstream file_object(input_file);
if ( not file_object )
{
cout <<"Error! The file " << input_file <<
" cannot be opened." << endl;
} else {
int row_number = 0;
string line;
while (getline(file_object, line) )
{
cout << line << endl;
line_vector[row_number] = line;
++row_number;
}
file_object.close();
ofstream output_file;
output_file.open("");
for (int i = 0; i>=row_number;++i)
{
output_file << i << " " << line_vector[i] << "\n";
}
output_file.close();
}
}
What the code is doing on this line: line_vector[row_number] = line is not actually appending to the vector - which is what you are trying to do -, but instead accessing an element at the index i inside the vector, and trying to assign line to it.
This would be fine, however, since the vector has 0 size, it is trying to assign line to unknown memory. This leads to undefined behaviour, and often, crashes. You can do one of two things. The first is to use resize or reserve to reserve the memory for the vector, then assign the variable, like so:
while (getline(file_object, line) ) {
cout << line << endl;
line_vector.resize(row_number + 1);
line_vector[row_number] = line;
++row_number;
}
However, this is pretty pointless, and instead, you should use push_back:
while (getline(file_object, line) ) {
cout << line << endl;
line_vector.push_back(line);
}
nullifying the use of row_number in that loop.
Oh, and don't use using namespace std, it's a bad habit.
"When you define a vector like line_vector, it starts out empty. All indexing into it will be out of bounds and lead to undefined behavior. Please learn how to push back elements into a vector. "
– Some programmer dude
This is for a homework assignment, but what I am presenting is a small test program for a chunk of my assignment.
Starting out, I am to have a list of songs in file "songs.txt". My current file looks like this.
Maneater;4;32
Whip It;2;41
Wake Me Up Before You Go-Go;3;45
The file simply contains a song title, and the duration in minutes and seconds, with the title, minutes, and seconds separated by semicolons. The full file is supposed to contain the Artists and Album as well, all separated by semicolons. Anyways, the code.
#include<iostream>
#include<cstring>
#include<fstream>
#include<cstdlib>
using namespace std;
const int CAP = 100;
const int MAXCHAR = 101;
struct songInfo
{
char title[CAP];
char durMin[CAP];
char durSec[CAP];
};
void getData(songInfo Song[], int listSize, int charSize);
int main()
{
string fileName;
songInfo Song[CAP];
ifstream inFile;
cout << "What is the file location?: ";
cin >> fileName;
inFile.open(fileName.c_str());
if (inFile.fail())
{
cout << "Cannot open file " << fileName << endl;
exit(1);
}
getData(Song, CAP, MAXCHAR);
for (int i=0;i<CAP;i++)
{
cout << Song[i].title << " - "
<< Song[i].durMin << ":"
<< Song[i].durSec << endl;
}
cout << "Press any button to continue..." << endl;
cin.get(); cin.get();
return 0;
}
void getData(songInfo Song[], int listSize, int charSize)
{
for (int i = 0; i < listSize; i++)
{
cin.get(Song[i].title, charSize, ';');
cin.get(Song[i].durMin, charSize, ';');
cin.get(Song[i].durSec, charSize, '\n');
i++;
cin.ignore();
}
}
The program compiles correctly without incident, but the output is not what I want it to be. What should happen:
Test.cpp opens songs.txt
Read the first char array into Song[i].title, delimited by ';'
Read the second char into Song[i].durMin, delimited by ';'
Read the third char into Song[i].durSec, delimited by newline
After compiling the code and running it, I get this as my output:
~/project2Test> ./test
What is the file location?: songs.txt
The program then hangs here and I have to ctrl+C out
First, what am I doing wrong?
Second, how do I go about fixing what I screwed up?
Also, as a note for class rules, I am not allowed to use any strings except for the filename. Other than that, all words must be chars.
A debugger is definitely a good thing to use for a problem like this.
Your hanging problem is occurring because in your get_data function you are using cin.get instructing your program to get input from the standard input file. You intended to use the file you defined, "inFile" not the standard input cin.
As an aside it is not clear to me why you are incrementing i twice per iteration of the for loop.
Use inFile.get() instead of cin. You need to pass inFile to the function first.
Put a print statement in the for loop to see what is happening.. A future issue that might crop up is that if you are on a Windows machine and have \r\n line endings. Unix uses \n, Windows uses \r\n
I'm just learning about text file input/output. I have outputted a file which contains a header and 10 rows of data underneath it.
I now want to read this back to the main function. This works for me if I leave out the header in the text file, but if I leave the header in, I get an infinite loop.
How can I skip the 1st line (the header line) in reading this data back, or if possible, read back the header as well as the data?
Here is what I have so far:
void fileRead(int x2[], double y2[], int& n, char filename)
{
ifstream fin ("pendulum.txt"); // fin is an input file stream
if(!fin) //same as fin.fail()
{
cerr << "Failure to open pendulum.txt for input" << endl;
exit(1);
}
int j = 0, dummy = 0; //index of the first value j and dummy value
while(!fin.eof()) //loop while not end of file
{
fin >> dummy >> x2[j] >> y2[j];
cout << setw(5) << fixed << j
<< setw(12) << scientific << x2[j] << " "
<< setw(12) << y2[j] << endl; //print a copy on screen
j += 1;
}
fin.close(); //close the input file
}
You can first read the header of the file then the real contents you want as follows:
string line;
getline(fin, line);//just skip the line contents if you do not want header
while (fin >> dummy >> x2[j] >> y2[j] )
{ //^^if you do not always have a dummy at the beginning of line
//you can remove dummy when you read the rest of the file
//do something
}
Your best bet would be to use
fin.ignore(10000,'\n');
http://www.cplusplus.com/reference/istream/istream/ignore/
This will ignore the first 10000 character in the file, or ignore the characters until a newline is reached. The 10000 is fairly arbitrary and should be a number that will always be longer than the maximum line length.
man, this gentleman over there helped me quite a lot. You see, everyone says to use getline(); to skip one line, but the problem is that sometimes you dont want to store anything in a buffer, so ignore() makes much more sense to me. Well so I would like to back up our fella's answer by adding that, you could use " numeric_limits::max()" which will make it have no limit, it will ignore until it finds the delimiter...
`
#include <iostream>
#include <fstream>
#include <limits>
using std::streamsize;
int main() {
ifstream fin ("pendulum.txt");
fin.ignore(numeric_limits<streamsize>::max(),'\n');
}
`
http://www.cplusplus.com/reference/limits/numeric_limits/
I have created a file hangman_word_collection.txt and stored all the content of file into the string line.
Now I want to use the line string in my program but line[0] is not having any value into it or I don't know if it have something in it.
I am new to this please help.
Here is the code:
#include <iostream>
#include <fstream>
using namespace std;
int main() {
string line;
ifstream myfile ("hangman_word_collection.txt");
if (myfile.is_open()) {
while (myfile.good()) {
getline (myfile,line);
cout << line << endl;
}
}
for(int i=0; i <= 79; i++) {
cout << "\n" << i;
cout << ":" << line[i];
}
return 0;
}
And the output:
actingraringbackupcampusdacoiteasilyfabricgardenhackediceboxprimeralwaysupload.
0:
1:c
2:t
3:i
4:n
5:g
6:r
7:a
8:r
9:i
10:n
11:g
12:b
13:a
14:c
15:k
Press <RETURN> to close this window...
When getline fails on writing to your target line you are assuming it will not modify what is in that string but it is blanking the string, which internally is replacing character 0 with a null character.
The rest is undefined behaviour as you are reading characters off the end of the logical string.
To fix this issue change your code to;
string line;
ifstream myfile ("hangman_word_collection.txt");
if (myfile.is_open())
{
while (myfile.good())
{
std::string temp;
if( getline( myfile, temp ) )
{
temp.swap( line );
cout <<line<<endl;
}
}
}
Note that it is bad practice to hard-code in magic numbers like 79. If you had put line.size() instead you would have seen what size the string actually is, and there would be no undefined behaviour. You can store this in a variable outside the loop if you are worried about performance, although chances are it makes little difference.
I wrote the code below that successfully gets a random line from a file; however, I need to be able to modify one of the lines, so I need to be able to get the line character by character.
How can I change my code to do this?
Use std::istream::get instead of std::getline. Just read your string character by character until you reach \n, EOF or other errors. I also recommend you read the full std::istream reference.
Good luck with your homework!
UPDATE:
OK, I don't think an example will hurt. Here is how I'd do it if I were you:
#include <string>
#include <iostream>
#include <fstream>
#include <cstdlib>
using namespace std;
static std::string
answer (const string & question)
{
std::string answer;
const string filename = "answerfile.txt";
ifstream file (filename.c_str ());
if (!file)
{
cerr << "Can't open '" << filename << "' file.\n";
exit (1);
}
for (int i = 0, r = rand () % 5; i <= r; ++i)
{
answer.clear ();
char c;
while (file.get (c).good () && c != '\n')
{
if (c == 'i') c = 'I'; // Replace character? :)
answer.append (1, c);
}
}
return answer;
}
int
main ()
{
srand (time (NULL));
string question;
cout << "Please enter a question: " << flush;
cin >> question;
cout << answer (question) << endl;
}
... the only thing is that I have no idea why do you need to read string char by char in order to modify it. You can modify std::string object, which is even easier. Let's say you want to replace "I think" with "what if"? You might be better off reading more about
std::string and using find, erase, replace etc.
UPDATE 2:
What happens with your latest code is simply this - you open a file, then you get its content character by character until you reach newline (\n). So in either case you will end up reading the first line and then your do-while loop will terminate. If you look into my example, I did while loop that reads line until \n inside a for loop. So that is basically what you should do - repeat your do-while loop for as many times as many lines you want/can get from that file. For example, something like this will read you two lines:
for (int i = 1; i <= 2; ++i)
{
do
{
answerfile.get (answer);
cout << answer << " (from line " << i << ")\n";
}
while (answer != '\n');
}