struct GPattern() {
int gid;
....
}
class Example() {
public:
void run(string _filename, unsigned int _minsup);
void PrintGPattern(GPattern&, unsigned int sup);
....
};
Eample::run(string filename, unsigned int minsup) {
for(...) { // some condition
// generate one GPattern, and i want to ouput it
PrintGPattern(gp, sup);
}
}
Example::PrintGPattern(GPattern& gp, unsigned int sup) {
// I want to ouput each GPattern to a .txt file
}
run is used to generate a GPattern accordingly.
What I want to output to a file is some texts that reconstruct the original GPattern.
I can't store all GPattern in advance and output all of them. I have to output one GPattern to a file when I generate it, but I don't know how to implement it.
I have tried to declare ofstream outGPatter("pattern.txt") in class Example, but it is of no use...
Well, ofstream is the right way to go:
Example::PrintGPattern(GPattern& gp, unsigned int sup) {
ofstream outGPattern("pattern.txt")
outGPattern << gp.gid; << " " << gp.anotherGid << " " ....
outGPattern.close()
}
Have you looked at the correct place for the pattern.txt? It should either be in the folder where your .exe is, or in the folder where all your .h and .cpp files are (for VS at least).
If you want to write all patterns into the same file then you need to make sure you append (and not overwrite) the pattern.txt
ofstream outGPattern("pattern.txt",ios::app)
So you can first make an ofstream without ios::app (to clear the textfile) at the start of your program. Then you construct all other ofstreams with ios::app to append new text, instead of overwriting it.
Alternatively you can make the ofstream a member variable of Example. Then you construct it only once.
I think you can use append mode, such as:
ofstream outGPattern;
outGPattern.open("GPattern.txt", ios::app);
The way I see it, you want to append information of multiple GPattern and you simply need to set the I/O mode to ios::app in the constructor.
struct GPattern {
int gid;
friend ostream& operator <<(ostream& o, GPattern gp) {
return o << "(gid=" << gp.gid << ")";
}
...
}
Example::PrintGPattern(GPattern& gp, unsigned int sup) {
ofstream fout("pattern.txt", ios::app)
fout << gp << endl;
fout.close()
}
Related
I have a computer science assignment which requires me to have a separate function just to open the file, and then another function which will then process the data in that file and then some others to do some operations with that data. Anyways, I'm having trouble in how to be able to let other functions use that opened file. References with '&' or'*' are confusing me and I'm unsure if I have to use one or not, of course, though I'm pretty sure I'll have to pass at least something to the next function. The main intent when dealing with the file is to open it(openFile) and then have another function(getData) to sort the data into two different arrays. One for the names, and one for the amounts next to them. The file would be written as:
Johnson 6000
Brown 5000
Miller 4000
Duffy 2500
Robson 1800
My code is as follows:
'''
#include <iostream>
#include <string>
#include <iomanip>
#include <fstream>
using namespace std;
void openFile();
void getData();
void computePercentages();
void sortVotes();
void display();
void displayWinner();
int main() {
openFile();
getData();
return 0;
}
void openFile(){
string fileName;
cout << "Enter the name of the file to open: ";
cin >> fileName;
ifstream file;
file.open(fileName.c_str());
}
void getData(){
int count = 0;
while(!file.eof()){
string names[count];
int votes[count];
cin >> names[count];
cin >> votes[count];
count ++;
}
}
'''
One way is to have openFile return the file stream object, then pass it to getData.
ifstream openFile()
{
string fileName;
cout << "Enter the name of the file to open: ";
cin >> fileName;
ifstream file(fileName);
return file;
}
void getData(ifstream &file)
{
int count = 0;
while(file){
string names[count];
int votes[count];
cin >> names[count];
cin >> votes[count];
count ++;
}
}
int main()
{
ifstream file = openFile();
if (file)
{
getData(file);
}
}
Note that this answer does not fix other issues in your code. For example, in getData you're using variable-length arrays which are non-standard and won't work on all compilers, and those arrays are constructed and destroyed each time through the while loop.
There are many ways to do it..
Here is a simple way.. using global variables.
I made ifstream file; as global..
This is not good way.. but simple..
#include <iostream>
#include <string>
#include <iomanip>
#include <fstream>
using namespace std;
void openFile();
void getData();
void computePercentages();
void sortVotes();
void display();
void displayWinner();
ifstream file;
int main() {
openFile();
getData();
return 0;
}
void openFile(){
string fileName;
cout << "Enter the name of the file to open: ";
cin >> fileName;
file.open(fileName.c_str());
}
void getData(){
int count = 0;
while(!file.eof()){
string names[count];
int votes[count];
cin >> names[count];
cin >> votes[count];
count ++;
}
}
Your getData() function has some problems:
void getData(){
int count = 0;
while(!file.eof()){ // this is almost never the correct check
string names[count]; // you declare new VLA:s (non-standard) every iteration
int votes[count]; // -"-
cin >> names[count]; // and you put a value in it out of bounds.
cin >> votes[count]; // -"-
count ++;
} // both arrays are destroyed here
}
file.eof() does not return true until you've tried to read beyond the end of the file. If you've read the last value, it will not be set. Only when you try next time will it be set.
The arrays you declare inside the while loop will be destroyed at the end of the loop. After the loop is finished, you have no arrays.
When you declare an array of count elements, you can access those elements using 0 to count-1 inclusive. You access element count which is out of bounds so your program has undefined behaviour.
VLA:s (variable length arrays) does not exist in standard C++ (but does as an extension in some compilers). If you know exactly how many elements you need to store, you can use std::array instead, but in this case, use a std::vector.
It uses a global file variable (that doesn't even exist). Try to stay away from global variables if you can.
The records in your data file should be kept together instead of putting each column in a separate array. A simple placeholder for each record in your file could look like this:
struct record {
std::string name{};
int vote{};
};
With that, you only need one array (or std::vector).
std::vector<record> records;
It'd also be good if one could extract one complete record from a stream using the same >> operator as you used for int and std::string. Like this:
record temp; // declare a variable using your own type, "record"
while(file >> temp) { // read records until no more can be read
records.push_back(temp) // add one record to records
}
A function to read one record from an istream, like an ifstream:
std::istream& operator>>(std::istream& is, record& r) {
// You may want to use getline here instead in case the names contain spaces.
return is >> r.name >> r.vote; // extract name and vote from is and return is
}
The function takes both parameters (is and r) by reference. That means that whatever is done to the parameters inside the function affects the variables that were used to call the function. file >> temp results in a call to the above function where is is a reference to file and r is a reference to temp.
For openFile() I'd suggest:
std::ifstream openFile(const std::string& fileName) { // return the ifstream by value
return std::ifstream{fileName};
}
Getting the filename from the user doesn't have anything to do with opening the file, so get the filename before calling the function. The above function lets you call openFile() and get an ifstream in return:
std::ifstream file = openFile(fileName);
You can now call getData() using file, but it needs to be able to receive it. Standard stream objects can't be copied (passed by value), but we don't need to. Just make getData() receive a reference to the stream. I'd make it an istream instead of an ifstream to be able to read from any istream decendant:
std::vector<record> getData(std::istream& is) {
// create a vector, read data from "is" and put it in vector and return vector when done
}
When all is pieced together, you could have a main() looking something like this:
int main() {
std::vector<record> records;
std::cout << "Enter the name of the file to open: ";
// use getline since a filename may contain spaces
if(std::string fileName; std::getline(std::cin, fileName)) {
// if "file" is in a good state after openFile(), call getData()
if(std::ifstream file = openFile(fileName)) {
records = getData(file);
} // "file" is automatically closed when it goes out of scope
}
// print what you collected
for(const record& r : records) {
std::cout << r.name << "\t" << r.vote << "\n";
}
}
The above uses If Statements with Initializer which is a C++17 feature to help create a narrow scope for variables.
In the below class error is in the init function where i load the class object I stored in the file to the vector Items.
class Item
{
std::string item_code;
std::string item_name;
std::string unit_name;
unsigned int price_per_unit;
double discount_rate;
static std::vector<Item> Items;
friend std::ostream& operator<< (std::ostream&, Item&);
public:
static void PrintAll();
static void Init();
~Item();
};
Default constructor is the one which reads data from user and writes into file. Below is the code of default constructor.
Item::Item(int a)
{
std::cout << "Item name : ";
std::getline(std::cin, item_name);
std::cout << "Unit (Kg/g/Qty) : ";
std::getline(std::cin, unit_name);
std::cout << "Price per unit : ";
std::cin >> price_per_unit;
std::cout << "Discount Rate : ";
std::cin >> discount_rate;
std::cin.ignore();
std::cout << "Product code (has to be unique) : ";
std::getline(std::cin, item_code);
std::ofstream outfile;
outfile.open("Files\\Items.txt", std::ios::out | std::ios::app);
outfile.write((char*)&(*this), sizeof(Item));
outfile.close();
}
Below is the Init() function for which read access violation is thrown at.
void Item::Init()
{
std::ifstream infile("Files\\Items.txt", std::ios::in);
if (!infile.is_open())
{
std::cout << "Cannot Open File \n";
infile.close();
return;
}
else
{
Item temp;
while (!infile.eof())
{
infile.read((char*)&temp, sizeof(temp));
Item::Items.push_back(temp);
}
}
infile.close();
}
Even though i am checking for eof, read access violation is thrown. Please give me some advice on this issue.
infile.read((char*)&temp, sizeof(temp));
This fills the temp object with junk from the file. It's supposed to contain valid std::string objects and whatever is in the file, it can't possibly be a valid std::string object. If you don't see why, consider that creating a valid std::string object requires allocating memory to hold the string data -- that's what the std::string constructor does. Reading data from a file can't possibly do this.
A file is a stream of bytes. To write data to a file, you need to define some way to represent that data as a stream of bytes. You need to encode its length if it is variable length. To read it back in, you need to handle the variable length case as well. You need to convert the file data to an appropriate internal representation, such as std::string. This is called "serialization".
std::string size is variable, you can try the following definition
char item_code[20];
char item_name[20];
char unit_name[20];
If I have a simple function that prints to standard output maybe something like an integer or lets say a string like "Flipy flops", then is there a way to call the same function but instead of printing to standard output it prints the string "Flipy flops" to a file stream? (provided of course I've opened the file and all that stuff).
Yes just give it an ostream& parameter
void my_function(...)
{
cout << ...
}
becomes
void my_function(ostream& out, ...)
{
out << ...
}
Using fstream works just like cin and cout.
void SomeMethod( )
{
ofstream myFile( "myFile.txt" );
myFile << FlipyFlops( );
myFile.close( );
}
char* FlipyFlops( )
{
return "flipy flops"; // or "flippy floppies", whichever you prefer
}
ofstream is for output to a file and ifstream is for reading from a file.
Below is the basic structure code for my super market billing and stock editing program
class:
class Admin
{
public:
admin();
private:
int Pid;
char name[20];
double quant;
double price;
double disc;
double net_price;
friend istream &read(istream&, Admin&);
friend ostream &show(ostream&, const Admin);
};
istream &read(istream&, Admin&);
ostream &show(ostream&, const Admin);
definition of friend functions:
istream &read(istream &is, Admin &commodity)
{
double dis;
fflush(stdin);
is>>commodity.Pid;
is.getline(commodity.name,30);
is>>commodity.quant
>>commodity.price
>>commodity.disc;
dis=grs_pr*(commodity.disc/100);
commodity.net_price=grs_pr-dis;
return is;
}
ostream &show(ostream &os, const Admin thing)
{
os << thing.name <<" " << thing.quant <<" "
<< thing.price <<" " << thing.disc <<" "
<< thing.net_price << endl;
return os;
}
Main function:
int main()
{
admin item;
while(read(cin,item)
{
ofstream file;
file.open("Stock.dat",ios::binary | ios::app);
file.write(reinterpret_cast<const char*>(&item),sizeof(Admin));
}
ifstream readFile("Stock.dat",ios::in|ios::binary);
while(!readFile.eof())
{
readFile.read(reinterpret_cast<char*> (&item),sizeof(Admin));
Admin readedItem;
read(readFile,readedItem); /*i have used read and show function to watch the values that are read by readFile but every time show function output some five exponential values like 5.23689e-301*/
show(cout,readedItem);
}
return 0;
}
Please tell me what is wrong in above code and what should i do to correct it,
my motive is to read from the file created and to alter or edit the data of the item that is to be selected by mentioning the product ID by the user, please help me how to attain this functionality.
you shouldn't implement a naiive serialization yourself. Use a library.
See i.e. answers 1, 2+comments, 3, 4
Also, don't put logic of data manipulation (i.e. commodity.net_price=grs_pr-dis;) in your data serialization
Assuming I'm understanding right then your read and show loop should be
for (;;)
{
readFile.read(reinterpret_cast<char*> (&item),sizeof(Admin));
if (readFile.eof())
break;
show(cout,item);
}
For some reason you were reading twice, first into a variable called item and then into another variable called readItem. You only need to read once. Also while (!readFile.eof()) is wrong because you must test for end of file after you read not before.
I am quite new to C++ and am trying to work out how to write a record in the format of this structure below to a text file:
struct user {
int id;
char username [20];
char password [20];
char name [20];
char email [30];
int telephone;
char address [70];
int level;
};
So far, I'm able to write to it fine but without an incremented id number as I don't know how to work out the number of records so the file looks something like this after I've written the data to the file.
1 Nick pass Nick email tele address 1
1 user pass name email tele address 1
1 test test test test test test 1
1 user pass Nick email tele addy 1
1 nbao pass Nick email tele 207 1
Using the following code:
ofstream outFile;
outFile.open("users.dat", ios::app);
// User input of data here
outFile << "\n" << 1 << " " << username << " " << password << " " << name << " "
<< email << " " << telephone << " " << address << " " << 1;
cout << "\nUser added successfully\n\n";
outFile.close();
So, how can I increment the value for each record on insertion and how then target a specific record in the file?
EDIT: I've got as far as being able to display each line:
if (inFile.is_open())
{
while(!inFile.eof())
{
cout<<endl;
getline(inFile,line);
cout<<line<<endl;
}
inFile.close();
}
What you have so far is not bad, except that it cannot handle cases where there is space in your strings (for example in address!)
What you are trying to do is write a very basic data base. You require three operations that need to be implemented separately (although intertwining them may give you better performance in certain cases, but I'm sure that's not your concern here).
Insert: You already have this implemented. Only thing you might want to change is the " " to "\n". This way, every field of the struct is in a new line and your problem with spaces are resolved. Later when reading, you need to read line by line
Search: To search, you need to open the file, read struct by struct (which itself consists of reading many lines corresponding to your struct fields) and identifying the entities of your interest. What to do with them is another issue, but simplest case would be to return the list of matching entities in an array (or vector).
Delete: This is similar to search, except you have to rewrite the file. What you do is basically, again read struct by struct, see which ones match your criteria of deletion. You ignore those that match, and write (like the insert part) the rest to another file. Afterwards, you can replace the original file with the new file.
Here is a pseudo-code:
Write-entity(user &u, ofstream &fout)
fout << u.id << endl
<< u.username << endl
<< u.password << endl
<< ...
Read-entity(user &u, ifstream &fin)
char ignore_new_line
fin >> u.id >> ignore_new_line
fin.getline(u.username, 20);
fin.getline(u.password, 20);
...
if end of file
return fail
Insert(user &u)
ofstream fout("db.dat");
Write-entity(u, fout);
fout.close();
Search(char *username) /* for example */
ifstream fin("db.dat");
user u;
vector<user> results;
while (Read-entity(u))
if (strcmp(username, u.username) == 0)
results.push_back(u);
fin.close();
return results;
Delete(int level) /* for example */
ifstream fin("db.dat");
ofstream fout("db_temp.dat");
user u;
while (Read-entity(u))
if (level != u.level)
Write-entity(u, fout);
fin.close();
fout.close();
copy "db_temp.dat" to "db.dat"
Side note: It's a good idea to place the \n after data has been written (so that your text file would end in a new line)
Using typical methods at least you will need to use fix size records if you want to have random access when reading the file so say you have 5 characters for name it will be stored as
bob\0\0
or whatever else you use to pad, this way you can index with record number * record size.
To increment the index you in the way you are doing you will need to the read the file to find the high existing index and increment it. Or you can load the file into memory and append the new record and write the file back
std::vector<user> users=read_dat("file.dat");
user user_=get_from_input();
users.push_back(user_);
then write the file back
std::ofstream file("file.dat");
for(size_t i=0; i!=users.size(); ++i) {
file << users.at(i);
//you will need to implement the stream extractor to do this easily
}
I suggest to wrap the file handler into a Class, and then overload the operator >> and << for your struct, with this was you will control the in and out.
For instance
struct User{
...
};
typedef std::vector<User> UserConT;
struct MyDataFile
{
ofstream outFile;
UserConT User_container;
MyDataFile(std::string const&); //
MyDataFile& operator<< (User const& user); // Implement and/or process the record before to write
MyDataFile& operator>> (UserConT & user); // Implement the extraction/parse and insert into container
MyDataFile& operator<< (UserConT const & user); //Implement extraction/parse and insert into ofstream
};
MyDataFile& MyDataFile::operator<< (User const& user)
{
static unsigned myIdRecord=User_container.size();
myIdRecord++;
outFile << user.id+myIdRecord << ....;
return *this;
}
int main()
{
MydataFile file("data.dat");
UserConT myUser;
User a;
//... you could manage a single record
a.name="pepe";
...
file<<a;
..//
}
A .Dat file is normally a simple text file itself that can be opened with notepad . So , you can simply read the Last Line of the file , read it , extract the first character , convert it into integer . THen increment the value and be done .
Some sample code here :
#include <iostream.h>
#include <fstream.h>
using namespace std;
int main(int argc, char *argv[])
{
ifstream in("test.txt");
if(!in) {
cout << "Cannot open input file.\n";
return 1;
}
char str[255];
while(in) {
in.getline(str, 255); // delim defaults to '\n'
//if(in) cout << str << endl;
}
// Now str contains the last line ,
if ((str[0] >=48) || ( str[0] <=57))
{
int i = atoi(str[0]);
i++;
}
//i contains the latest value , do your operation now
in.close();
return 0;
}
Assuming your file format doesn't not need to be human readable.
You can write the struct out to file such as.
outFile.open("users.dat", ios::app | ios::binary);
user someValue = {};
outFile.write( (char*)&someValue, sizeof(user) );
int nIndex = 0;
user fetchValue = {};
ifstream inputFile.open("user.data", ios::binary);
inputFile.seekg (0, ios::end);
int itemCount = inputFile.tellg() / sizeof(user);
inputFile.seekg (0, ios::beg);
if( nIndex > -1 && nIndex < itemCount){
inputFile.seekg ( sizeof(user) * nIndex , ios::beg);
inputFile.read( (char*)&fetchValue, sizeof(user) );
}
The code that writes to the file is a member function of the user struct?
Otherwise I see no connection with between the output and the struct.
Possible things to do:
write the id member instead of 1
use a counter for id and increment it at each write
don't write the id and when reading use the line number as id