I'm still new to advance file and structures. I'm just having trouble figuring out how to seek and read specific records from a file. and how to display the information of a specific record through it's record number?
#include <iostream>
#include <fstream>
using namespace std;
struct catalog
{
char title[50];
char author[50];
char publisher[30];
int yearpublish;
double price;
};
void showRec(catalog);
long byteNum(int);
int main()
{
catalog book;
fstream fbook("book.dat",ios::in|ios::binary);
if (!fbook)
{
cout <<"Error opening file";
return 0;
}
//Seek and read record 2 (the third record)
showRec(book); // Display record 2
//Seek and read record 0 (the first record)
showRec(book); // Display record 0
fbook.close();
return 0;
}
void showRec(catalog record)
{
cout << "Title:" << record.title << endl;
cout << "Author:" << record.author << endl;
cout << "Publisher name:" << record.publisher << endl;
cout << "Year publish:" << record.yearpublish << endl;
cout << "Price:" << record.price << endl << endl;
}
long byteNum(int recNum)
{
return sizeof(catalog) * recNum;
}
Please Help. Thank you.
The C++ istream methods you need for this are seekg and read.
// Seek and read record 2
fbook.seekg(byteNum(2));
fbook.read((char*)&book, sizeof(book));
Related
I've written a readFile function for a project I'm working on. I call it once, load in a file and read in it's contents - works fine
However, when I try to load it a second time, attempting to change the file name - it loads it in, saves it to a static string 'path' that I access in a different function - but then the function is not printing the data
The question is, how do I change the file name, and read it in successfully on the second iteration? The part that has me stumped is that it works once, but not twice
Ive attempted to use cin.ignore(); cin.clear(); cin.sync() on the second iteration of fileName function - but none of them allow a separate file to be read successfully.
Minimum Reproducible Example:
#include <iostream>
#include <fstream>
#include <string>
#include <stdlib.h>
#include <vector>
#include <sstream>
#include <iomanip>
#include <iostream>
using namespace std;
static string path;
string opt;
void readFile();
int fileName();
void menu() { // put in while loop - while True
cout << "----------------------" << endl;
cout << "R(ead) -" << "Read File" << endl;
cout << "F(ile) -" << "Set Filename" << endl;
cout << "\nPlease select from the above options" << endl;
cin >> opt;
cout << "\nInput entered: " << opt << endl;
if (opt == "R") {
readFile();
}
if (opt == "F") {
fileName();
}
}
void readFile() { // doing this twice
ifstream readFile;
readFile.open(path);
if (!readFile.is_open()) {
cout << "Could not read file" << endl;
}
string str;
int i = 0;
while (getline(readFile, str))
{
if (str[0] != '/')
{
cout << "DEBUG: Line is - " << str << endl;
}
}
readFile.clear();
readFile.close();
menu();
}
int fileName() {
cout << "File path: ";
if (path != "") {
cin.ignore();
cin.clear();
cin.sync();
}
getline(cin, path);
ifstream file(path.c_str());
if (!file) {
cout << "Error while opening the file" << endl;
return 1;
}
cout << "(File loaded)" << endl;
cout << "Path contains: " << path << endl;
file.clear();
file.close();
menu();
}
int main()
{
fileName();
}
Sample text, saved as txt file and read in using path:
Data1.txt
// standard test file
123,Frodo inc,2006, lyons,"1,021,000.16",0.0,
U2123,Sam Inc,2006, lyons,"21,600.00",13.10,123
A721,Merry Inc,2604, Kingston,"21,600.10",103.00,
U2122,Pippin Inc,2612, reid,"21,600.00",0
U1123,Huckelberry corp,2612, Turner,"21,600.00",13.10,
Data2.txt
7101003,Mike,23 boinig road,2615,48000,12000,0
7201003,Jane Philips,29 boinig cresent,2616,47000,12000,0
7301003,Philip Jane,23 bong road,2615,49000,000,0
7401004,Peta,23 bong bong road,2615,148000,19000,0
7101205,Abdulla,23 Station st,2615,80000,21000,0
The problem comes from reading in one, and trying to read in the other after the first has been executed.
Enter Filename
Hit Readfile
Return to menu, hit Set Filename
Change to Data2.txt
Readfile again. Not working
My tutor told me "That's not how functions work in c++" but didn't elaborate further, and is unavailable for contact.
In general, do not use global variables. The path variable should be passed as a parameter, not kept as a global variable altered between function calls, as this leads to many side effects and is the source of countless bugs. See the following refactoring:
void menu() { // put in while loop - while True
while(true)
{
//Keep this as a local variable!
std::string opt;
std::string filename;
cout << "----------------------\n";
cout << "R(ead) -" << "Read File\n";
cout << "F(ile) -" << "Set Filename\n";
cout << "\nPlease select from the above options\n";
cin >> opt;
cout << "\nInput entered: " << opt << '\n';
if (opt == "R") {
readFile(filename);
}
if (opt == "F") {
filename = getFileName();
}
}
}
void readFile(const std::string & filename) {
ifstream readFile;
readFile.open(filename);
if (!readFile.is_open()) {
cout << "Could not read file " << filename << '\n';
}
string str;
int i = 0;
while (getline(readFile, str))
{
if (str[0] != '/')
{
cout << "DEBUG: Line is - " << str << '\n';
}
}
readFile.close();
//just return to get back to menu
return;
}
std::string getFileName() {
cout << "File path: ";
std::string path;
getline(cin, path);
ifstream file(path.c_str());
if (!file) {
cout << "Error while opening the file" << '\n';
//Instead of returning an error code use an exception preferably
}
cout << "(File loaded)" << '\n';
cout << "Path contains: " << path << '\n';
file.close();
return path;
}
Other notes:
Ideally, do input in output in just one function, not all three as it gets confusing exactly what each function is responsible for.
If you want something to hold a file and print the contents, you can use an class.
The file is checked if it is openable twice, not really any reason to do this just delegate that responsibility to one function.
One of the best things about C++ is RAII and deterministic lifecycles for objects and primitives - use it!! Do not give everything a long life with global variables - use smart parameters and return values instead.
I'd like to get some assistance regarding the following line of code.
for the function constructArray,I'm unable to run it as shows the !aFile as true message but I have no idea what's the error.
Really appreciative of your assistance
Also how do I create inFile fileName with .txt,i've tried indentation with +".txt" but due to the file type in the argument i'm unable to do so.
Compiler run image: https://imgur.com/a/wkGwL
using namespace std;
enum NumType {Odd, Even};
struct Number
{
int no;
NumType type;
int oddDigits;
int evenDigits;
int sumDigits;
int noDigits;
};
// Create inFile data file with certain number of integers which are randomly generated
void constructInfile (fstream& aFile, char fileName[]);
// Read data from infile txt file and transfer to array of numbers
int constructArray (fstream& aFile,const char fileName[], Number ran[]);
/*
void processArray (Number [ ], int);
// Transfer information from array and store into output file called outfile txt with specific information format
void arrayToOutfile (fstream&, char [ ], Number [ ], int);
*/
const int MAX = 50;
int main()
{
srand(time(NULL));
fstream aFile;
char fileName [MAX];
cout << "Enter designated file name to be created" << endl;
cin >> fileName;
constructInfile (aFile,fileName);
Number ran[MAX];
int recNo = constructArray(aFile,fileName,ran);
cout << recNo << " of records transferred" << endl;
}
// Create inFile data file with certain number of integers which are randomly generated
void constructInfile (fstream& aFile,char fileName[]){
aFile.open(fileName, ios::out);
if(aFile.fail()){
cout << "File open unsuccessful" << endl;
aFile.close();
exit(1);
}
cout << "Begin creation of " << fileName << " file" << endl << endl;
int size = rand()%51+50;
for(int a = 0;a < size;a++){
aFile << rand()%1000+1 << endl;
}
cout << fileName << " file successfully created" << endl;
}
// Read data from infile txt file and transfer to array of numbers
int constructArray (fstream& aFile,const char fileName[], Number ran[]){
aFile.open (fileName, ios::in);
if (!aFile)
{
cout << fileName << " failed to open" << endl;
aFile.close ();
return 0;
}
cout << "Begin from " << fileName << " to array" << endl;
int i = 0;
char tabKey;
while (aFile >> ran[i].no)
{
aFile.get (tabKey); // read and discard
i++;
}
aFile.close ();
cout << fileName << " to array done" << endl;
return i;
}
You are opened a file in ios::out mode and never closed it!
Again you are trying to open the same file in ios::in mode. How a file could be opened twice in 2 different mode without properly closing the fstream?
You need to close the filestream inside the function constructInfile()!!!
You have opened the file but forget to close it. At the end of both of the functions, close the aFile. That should solve the problem.
I am new in C++ in general, and thus, also in file handling in C++.
I need to read a .raw file which has 16-bit integers, and have dimension 512 x 512.
For that I am using following code:
ifstream myData("myData.raw");
short value;
int i = 0;
char buf[sizeof(short)];
while (myData.read(buf,sizeof(buf)))
{
memcpy(&value, buf, sizeof(value));
cout << value << " ";
i++;
}
cout << endl << "Total count: " << i << endl;
The value i am getting for i is not 512 x 512. So I guess something is not right.
Can someone please help me in this regard?
The default open mode is "text" and some characters will be possibly dropped or treated as end of file. ios::binary stops these alterations.
#include <iostream>
#include <fstream>
using namespace std;
int main() {
ifstream myData("myData.raw", ios::binary);
short value;
int i = 0;
char buf[sizeof(short)];
while (myData.read(buf, sizeof(buf)))
{
memcpy(&value, buf, sizeof(value));
cout << value << " ";
i++;
}
cout << endl << "Total count: " << i << endl;
}
the program should read from 2 files (author.dat and citation.dat) and save them into a map and set;
first it reads the citationlist without problem, then it seems to properly read the authors and after it went through the whole list (author.dat) a floating point exception arises .. can't quite figure out why
seems to happen in author.cpp inside the constructor for authorlist
author.cpp:
#include <fstream>
#include <iostream>
#include "authors.h"
using namespace std;
AuthorList::AuthorList(char *fileName) {
ifstream s (fileName);
int idTemp;
int nrTemp;
string nameTemp;
try {
while (true){
s >> idTemp >> nrTemp >> nameTemp;
cout << idTemp << " " << nrTemp << " " << nameTemp << " test_string";
authors.insert(std::make_pair(idTemp,Author(idTemp,nrTemp,nameTemp)));
if (!s){
cout << "IF-CLAUSE";
throw EOFException();
}
cout << "WHILE-LOOP_END" << endl;
}
} catch (EOFException){}
}
author.h:
#ifndef CPP_AUTHORS_H
#define CPP_AUTHORS_H
#include <iostream>
#include <map>
#include <string>
#include "citations.h"
class Author {
public:
Author (int id, int nr, std::string name) :
articleID(id),
authorNR(nr),
authorName(name){}
int getArticleID() const {
return articleID;
}
std::string getAuthorName() const {
return authorName;
}
private:
int articleID;
int authorNR;
std::string authorName;
};
class AuthorList {
public:
AuthorList(char *fileName);
std::pair<std::multimap<int,Author>::const_iterator, std::multimap<int,Author>::const_iterator> findAuthors(int articleID) {
return authors.equal_range(articleID);
}
private:
std::multimap<int,Author> authors;
};
#endif //CPP_AUTHORS_H
programm.cpp:
#include <iostream>
#include <cstdlib>
#include "citations.h"
#include "authors.h"
#include "authorCitation.h"
using namespace std;
int main(int argc, char *argv[]){
CitationList *cl;
AuthorList *al;
//check if argv array has its supposed length
if (argc != 4){
cerr << "usage: programm article.dat citation.dat author.dat";
return EXIT_FAILURE;
}
//inserting citation.dat and author.dat in corresponding lists (article.dat not used)
cl = new CitationList(argv[2]);
al = new AuthorList(argv[3]);
try {
AuthorCitationList *acl;
acl->createAuthorCitationList(al,cl);
acl->printAuthorCitationList2File("authorcitation.dat");
} catch (EOFException){
cerr << "something went wrong while writing to file";
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
All files:
https://drive.google.com/file/d/0B734gx5Q_mVAV0xWRG1KX0JuYW8/view?usp=sharing
I am willing to bet that the problem is caused by the following lines of code:
AuthorCitationList *acl;
acl->createAuthorCitationList(al,cl);
You are calling a member function using an uninitialized pointer. I suggest changing the first line to:
AuthorCitationList *acl = new AuthorCitationList;
Add any necessary arguments to the constructor.
While you are at it, change the loop for reading the data also. You have:
while (true){
s >> idTemp >> nrTemp >> nameTemp;
cout << idTemp << " " << nrTemp << " " << nameTemp << " test_string";
authors.insert(std::make_pair(idTemp,Author(idTemp,nrTemp,nameTemp)));
if (!s){
cout << "IF-CLAUSE";
throw EOFException();
}
cout << "WHILE-LOOP_END" << endl;
}
When you do that, you end up adding data once after the end of line has been reached. Also, you seem to have the last line in the wrong place. It seems to me that it should be outside the while loop.
You can use:
while (true){
s >> idTemp >> nrTemp >> nameTemp;
// Break out of the loop when reading the
// data is not successful.
if (!s){
cout << "IF-CLAUSE";
throw EOFException();
}
cout << idTemp << " " << nrTemp << " " << nameTemp << " test_string";
authors.insert(std::make_pair(idTemp,Author(idTemp,nrTemp,nameTemp)));
}
cout << "WHILE-LOOP_END" << endl;
You can simplify it further by using:
while (s >> idTemp >> nrTemp >> nameTemp){
cout << idTemp << " " << nrTemp << " " << nameTemp << " test_string";
authors.insert(std::make_pair(idTemp,Author(idTemp,nrTemp,nameTemp)));
}
cout << "WHILE-LOOP_END" << endl;
I am trying to load a text file and import the contents into a vector of structs.
Here are my definitions
typedef struct
{
string pcName, pcUsername, pcPassword, pcMessage, pcAdvertisement; //I know that
//this is incorrect convention. It was originally a char*
}
ENTRY;
vector<ENTRY> entries;
fstream data;
Here is my display data function
void DisplayData()
{
std::cout << (int)(entries.size() / 5) <<" entries" << endl;
for(int i = 1; i <=(int)entries.size()/5; i++)
{
cout << endl << "Entry " << i << ":" << endl
<< "Name: " << entries[i].pcName << endl
<< "Username: " << entries[i].pcUsername << endl
<< "Password: " << entries[i].pcPassword << endl
<< "Message: " << entries[i].pcMessage << endl
<< "Advertisement: " << entries[i].pcAdvertisement << endl;
}
}
and here is my Load Data function
bool LoadData(const char* filepath)
{
std::string lineData ;
int linenumber = 1 ;
data.open(filepath, ios::in);
ENTRY entry_temp;
if(!data.is_open())
{
cerr << "Error loading file" << endl;
return false;
}
while(getline(data, lineData))
{
if(linenumber==1) {entry_temp.pcName = lineData;}
else if(linenumber==2) {entry_temp.pcUsername = lineData;}
else if(linenumber==3) {entry_temp.pcPassword = lineData;}
else if(linenumber==4) {entry_temp.pcMessage = lineData;}
else if(linenumber==5) {entry_temp.pcAdvertisement = lineData;}
entries.push_back(entry_temp);
if(linenumber == 5)
{
linenumber = 0;
}
linenumber++;
}
data.close();
puts("Database Loaded");
return true;
}
Here is the text file I am loading:
Name1
Username1
Password1
Message1
Ad1
And here is the result of the display data function after calling load data:
1 entries
Entry 1:
Name: Name1
Username Username1
Password:
Message:
Advertisement:
As you can see, the first two load but the last three don't. When I did this with an array instead of a vector, it worked fine so I don't know what I'm doing wrong. Thanks.
I suggest that you read each line directly into the data field where it goes:
getline(data, entry_temp.pcName);
getline(data, entry_temp.pcUsername);
getline(data, entry_temp.pcPassword);
getline(data, entry_temp.pcMessage);
getline(data, entry_temp.pcAdvertisement);
entries.push_back(entry_temp);
This makes your intent much clearer than your current while loop. It also creates a single entry for all 4 input lines rather than one for each input line (with the other three blank). Now you can read several "entries" by using a while loop that checks if you have reached the end of the file.
Doing this will also make printing out the data much easier since the vector will have exactly the number of entries rather than five times as many as you expect (which also eats up a lot more memory than you need to).
Your DisplayData function is a little weird, and so is your LoadData.
Your LoadData pushes back a new copy of the current ENTRIES entry with every line. Your DisplayData starts at 1 (which is not the beginning of any vector or array), and iterates only up to the 1/5th entry of the entire vector.
This needs a heavy rework.
First, the size() member of any standard container returns the number of elements that it contains, and will not take the number of fields in a contained struct into account.
For future reference, you'll want to post your question in a complete, standalone example that we can immediately compile to help. (see http://sscce.org/)
Try this modified data, which runs correctly, and see if you can tell what is being done differently:
#include <iostream>
#include <fstream>
#include <vector>
#include <string>
using namespace std;
typedef struct
{
string pcName, pcUsername, pcPassword, pcMessage, pcAdvertisement;
}
ENTRY;
vector<ENTRY> entries;
fstream data;
bool LoadData(const char* filepath)
{
std::string lineData ;
int linenumber = 1 ;
data.open(filepath, ios::in);
ENTRY entry_temp;
if(!data.is_open())
{
cerr << "Error loading file" << endl;
return false;
}
while(getline(data, lineData))
{
if(linenumber==1) {entry_temp.pcName = lineData;}
else if(linenumber==2) {entry_temp.pcUsername = lineData;}
else if(linenumber==3) {entry_temp.pcPassword = lineData;}
else if(linenumber==4) {entry_temp.pcMessage = lineData;}
else if(linenumber==5) {entry_temp.pcAdvertisement = lineData;}
if(linenumber == 5)
{
entries.push_back(entry_temp);
linenumber = 0;
}
linenumber++;
}
data.close();
puts("Database Loaded");
return true;
}
void DisplayData()
{
std::cout << entries.size() <<" entries" << endl;
for(int i = 0; i < entries.size(); i++)
{
cout << endl << "Entry " << i << ":" << endl
<< "Name: " << entries[i].pcName << endl
<< "Username: " << entries[i].pcUsername << endl
<< "Password: " << entries[i].pcPassword << endl
<< "Message: " << entries[i].pcMessage << endl
<< "Advertisement: " << entries[i].pcAdvertisement << endl;
}
}
int main()
{
LoadData("/tmp/testdata");
DisplayData();
return (0);
}
While I think #code-guru has the right idea, I'd take the same idea just a little further, and make your code work a little more closely with the standard library. I'd do that by reading a data item with a stream extractor, and displaying it with stream inserter. So, the extractor would look something like this:
std::istream &operator>>(std::istream &is, ENTRY &e) {
getline(is, e.pcName);
getline(is, e.pcUsername);
getline(is, e.pcPassword);
getline(is, e.pcMessage);
getline(is, e.pcAdvertisement);
return is;
}
..and the inserter would look something like this:
std::ostream &operator<<(std::ostream &os, ENTRY const &e) {
os << e.pcName << "\n";
os << e.pcUsername << "\n";
os << e.pcPassword << "\n";
os << e.pcMessage << "\n";
os << e.pcAdvertisement << "\n";
return os;
}
With those in place, loading and displaying the data becomes fairly straightforward.
Load the data:
std::ifstream in("yourfile.txt");
std::vector<ENTRY> data((std::istream_iterator<ENTRY>(in)),
std::istream_iterator<ENTRY>());
Display the data:
for (auto const & e: data)
std::cout << e << "\n";
For the moment, I haven't tried to duplicate the format you were using to display the data -- presumably the modifications for that should be fairly obvious.