What I want to do:
Store records in a file. These records have two things.
time_t rt; //which stores the time the record was entered by the user
and along with this I want to store one string. But I don't know the length of the string.
It will be decided on run time and will depend on how many characters the user enters.
What needs to be done(According to me):
I have no clue. I know about dynamic memory allocation but did not know how to apply this to such a problem.
What I have tried:
I have tried to take one charachter at a time from the user and store it in a text file(Temporarily).
ofstream fileObject;
fileObject.open("temp.txt");
for(int j=0;;j++)
{
ch = _getche();
if( ch == 13) break; //user has pressed the return key
fileObject<<ch;
}
Then I found out the size of the file using the following code:
fileObject.seekp(0,ios::end);
long pos = fileObject.tellg(); //this is the size of the file
Then I declared a dynamic array of the size of the file.
char * entry;
entry = new char[pos]
Closed the file in the "out" mode and opened it again in the "in" mode.
fileObject.close();
ifstream fout;
fout.open("temp.txt"); //this is the name of the text file that i had given
Then character wise I copied the content of the text file into the character array:
for(int i=0;i<pos;i++)
fout>>info[i];
info[i] = '\0';
fout.close();
But now i dont know what to do further.
What I need you to help me with:
Help me to write this record as a class object into a binary ".dat" file.
My specs:
Windows XP SP 3
IDE: Visual C++ 2010 Express
What are the restrictions on the string? And how do you
recognize that the user has entered all of the data he wants in
the string?
If the string has to be a single line, and we can assume
"reasonable" length (i.e. it will easily fit into memory), then
you can use std::getline to get the string into an
std::string (for input), and then define the output format,
say "%Y-%m-%d %H:%M:%S: user string\n" for
the file. If the user string can be several lines, you'll have
to define a protocol to input them (so you can know when
a single record is finished), and a more complex format for the
file: one suggestion would be to separate records by an empty
line (which means that the input cannot contain an empty line),
or to use a record header along the lines of: "%Y-%m-%d
%H:%M:%S line_count\n". (Subversion uses
a variant of this for it's commit messages. With a bit more
information, however, but the timestamp and the number of lines
are there.)
use std::string and std::getline, both from the <string> header
If you are using c++ then std::string is best.
std::string abc="";
I want to store one string. But I don't know the length of the string.
Then you need to use std::string and not a preallocated array of chars.
struct user_record
{
time_t rt; //which stores the time the record was entered by the user
std::string one_string;
};
Help me to write this record as a class object into a binary ".dat" file.
There are a number of serialisation options available to you. Perhaps the simplest is to write this as plain text using the standard stream operations:
std::ostream& operator <<(std::ostream& os, user_record const& ur)
{
return os << ur.rt << ' ' << ur.one_string;
}
std::istream& operator >>(std::istream& is, user_record& ur)
{
return is >> ur.rt >> ur.one_string;
}
For anything more involved than a single-line string, then perhaps you should investigate Boost's serialisation library.
Related
I am trying to read data from a text file using C++ & store the strings at each line into wchar_t [] or LPCWSTR.
(These 2 datatypes are the constraints of the application on which I am working. That's why I have to store the data in these datatypes)
The format of data in the .txt file is, for example:
abc\\def\\ghi 10
jkl\\mnopq\\rstq 20
aqq\\sdsds\\qc 30
I am trying to read data line by line & save each line as a map's key-value pair, where key is of type LPCWSTR or wchar_t[] type & value is of int type
There is no issue in extracting int, but the issue comes in reading the strings
Here is my code:
#include<iostream>
#include<fstream>
#include<windows.h>
#include<cstdlib>
using namespace std;
int main()
{
wchar_t test1[260];
const char* s = "Hello\\ABC\\DEF";
mbstowcs(test1, s, strlen(s));
wcout<<test1<<endl;
wchar_t gr[260];
string gr_temp;
int percentage;
ifstream ifs;
ifs.open("data.txt", ifstream::in);
if (ifs.is_open()) {
while (ifs >> gr_temp >> percentage){
const char* source = gr_temp.c_str();
mbstowcs(gr, source, strlen(source));
wcout<<gr<<L" ";
cout<<percentage<<endl;
}
ifs.close();
}
return 0;
}
However, it is giving the following output:
Hello\ABC\DEFa
abc\\def\\ghi 10
jkl\\mnopq\\rstq 20
aqq\\sdsds\\qc 30
I did not understand why that tiny 'a' appeared out of nowhere in the first line of output
I want the code to instead automatically process those double slashes, i.e. I want the output as:
Hello\ABC\DEF
abc\def\ghi 10
jkl\mnopq\rstq 20
aqq\sdsds\qc 30
It would be even best if I could instead write the entries in the .txt file without double slashes & they get automatically processed without checking for any escape sequences. However, since the issue as in point no. 1) above is there, so I am not sure if it is even possible
Even if add cout<<gr_temp<<endl; as the first line in the while loop, even that also outputs the string with double backward slashes.
What am I missing or doing wrong?
Update:
Also, when I add these key-value pairs to a std::map<LPCWSTR,int> m1 using the statement m1[gr] = percentage; at the end of each while loop, then with the print statement, it only shows one single element in the map.
My updated code is:
#include<iostream>
#include<fstream>
#include<windows.h>
#include<cstdlib>
#include<map>
using namespace std;
std::unordered_map<LPCWSTR, int> m1;
int main()
{
wchar_t test1[260];
const char* s = "Hello\\ABC\\DEF";
mbstowcs(test1, s, strlen(s));
wcout<<test1<<endl;
wchar_t gr[260];
string gr_temp;
int percentage;
ifstream ifs;
ifs.open("data.txt", ifstream::in);
if (ifs.is_open()) {
while (ifs >> gr_temp >> percentage){
const char* source = gr_temp.c_str();
mbstowcs(gr, source, strlen(source));
m1[gr] = percentage;
}
ifs.close();
}
for (auto i = m1.begin(); i != m1.end(); i++) {
wcout<< i->first << L" ";
cout<< i->second << endl;
}
return 0;
}
This code is only adding 1 element in the map & that is the most recent added element.
I edited the code to use unordered_map, but still the same issue.
I further tried to print the size() of the map. In both these cases, size of map m1 was displayed as 1.
Miles Budnek already stated your problems.
If you look at the documentation of your function (http://www.cplusplus.com/reference/cstdlib/mbstowcs/), you will see that the third parameter does not expect the number of bytes to translate to wchar_t, but much rather the maximum number of characters the buffer you are pointing to can hold.
It will stop once it finds a \0 (which just happens to be what strlen is also looking for).
So just replace the third parameter of your first mbstowcs call with 260 (or sizeof(test1)/sizeof(wchar_t) and you're good on that stray 'a'.
As has also already been stated, there are no 'escape parameters' while reading from a file.
These only exist in source code and represent ASCII codes you cannot type. (https://www.asciitable.com/)
\n for example represents the codesign for 'new line' 0x0A.
So escaping the backslashes in the file is unnecessary and can be skipped.
If you know that your input file will have 'double backslashes' and need to 'unescape' them, you could look at the std::string functions 'find' and 'replace'.
Find "\\\\" (two backslashes in a row) and replace with "\\".
In response to your updated question (which is basically another question):
The problem is the key you chose for the map.
Each map, unordered or not, requires unique keys and in your scenario, you keep using the same key.
LPCWSTR expands to 'Pointer to Wide Char String', so while you probably think you are using 'abc\def\ghi' as key, you are actually using &gr[0], which remains the same during all iterations.
As an additional result, once the program leaves the scope of gr, its content becomes invalid and accessing the map (which maintains the pointer but not the content), will access freed memory which tends to crash your program.
The solution as such is simple enough though: You need to use the content as key, instead of the pointer, for example by using a container object like std::wstring.
I have a small assignment which partly requires me to take inputs from a file in the form of strings and place them into char arrays so I can check if the string contains any '*' character at the end of it.
I have been able to extract the strings from the files successfully, however i have failed to find a way in which to place them in char arrays so i can process them.
I would be very grateful if someone would let me know how to place a string into char arrays using cstring library. Please keep in mind that the strings are taken from a file and not as user input.
some of the ways i tried is the following:
//Try 1
char CstringArray[] = LineFromFile;
//Try 2
char CstringArray[100] = LineFromFile;
//Try 3
ifstream Test("Test.txt");
Test>>CstringArray;
//Try 4
ifstream Test("Test.txt");
Test>>CstringArray[0];
Thank you very much
Since this is an assignment, your professor will probably not be happy with you using all of C++'s functionality, particularly if you don't understand it, but since it's a one liner I figured I'd tell you how I'd print all strings ending in an asterisks. Given that you have successfully opened the file to ifstream Test you can do:
copy_if(istream_iterator<string>(Test), istream_iterator<string>(), ostream_iterator<string>(cout, " "), [](const auto& i) { return !empty(i) && i.back() == '*'; })
EDIT:
I'm using an istream_iterator to read in each string in Test and istream_iterator, I'm operating on these values immediately, but if you needed to start by saving all the strings to a vector<string> you could also do this: vector<string> CstringArray{ istream_iterator<string>(Test), istream_iterator<string>() }
I'm using an ostream_iterator to directly stream out my selected strings rather than storing them
I'm using copy_if to iterate over all the strings that are streamed in, selecting only those that meet a given criteria
I'm using the lambda: [](const auto& i) { return !empty(i) && i.back() == '*'; } to conditionally select non-empty strings which end with an asterisks character
I'm trying to write a program that reads in a CSV file (no need to worry about escaping anything, it's strictly formatted with no quotes) but any numeric item with a value of 0 is instead just left blank. So a normal line would look like:
12,string1,string2,3,,,string3,4.5
instead of
12,string1,string2,3,0,0,string3,4.5
I have some working code using vectors but it's way too slow.
int main(int argc, char** argv)
{
string filename("path\\to\\file.csv");
string outname("path\\to\\outfile.csv");
ifstream infile(filename.c_str());
if(!infile)
{
cerr << "Couldn't open file " << filename.c_str();
return 1;
}
vector<vector<string>> records;
string line;
while( getline(infile, line) )
{
vector<string> row;
string item;
istringstream ss(line);
while(getline(ss, item, ','))
{
row.push_back(item);
}
records.push_back(row);
}
return 0;
}
Is it possible to overload operator<< of ostream similar to How to use C++ to read in a .csv file and output in another form? when fields can be blank?
Would that improve the performance?
Or is there anything else I can do to get this to run faster?
Thanks
The time spent reading the string data from the file is greater than the time spent parsing it. You won't make significant time savings in the parsing of the string.
To make your program run faster, read bigger "chunks" into memory; get more data per read. Research on memory mapped files.
One alternative way to handle this to get better performance is to read the whole file into a buffer. Then go through the buffer and set pointers to where the values start, if you find a , or end of line put in a \0.
e.g. https://code.google.com/p/csv-routine/
I am maintaining a C++ method which one of my clients is hitting an issue with. The method is supposed to write out a series of identifiers to a file delimited by a new line. However on their machine somehow the method is writing a series of NULL's out to the file. Opening the file in a binary editor shows that it contains all zeros.
I can't understand why this is happening. I've tried assigning empty strings and strings with the first character set to 0. There is no problem creating the file, just writing the identifiers to it.
Here is the method:
void writeIdentifiers(std::vector<std::string> IDs, std::string filename)
{
std::ofstream out (filename.c_str(), std::ofstream::binary);
if (out.is_open())
{
for (std::vector<std::string>::iterator it = IDs.begin();
it != IDs.end();
it++)
{
out << *it << "\n";
}
}
out.close();
}
My questions: is there any possible input you can provide that method which will create a file which has NULL values in it?
Yeah, the following code quite clearly writes a series of NULL bytes:
std::vector<std::string> ids;
std::string nullstring;
nullstring.assign("\0\0\0\0\0\0\0\0\0\0", 10);
ids.push_back(nullstring);
writeIdentifiers(ids, "test.dat");
Because the std::string container stores the string length, it can't necessarily be used in the same way as an ordinary C (null-terminated) string. Here, I assign a string containing 10 NULL bytes. Those are then output because the string length is 10.
I need to make a program in C++ that must read and write text files line by line with an specific format, but the problem is that in my PC I work in Windows, and in College they have Linux and I am having problems because of line endings are different in these OS.
I am new to C++ and don't know could I make my program able read the files no matter if they were written in Linux or Windows. Can anybody give me some hints? thanks!
The input is like this:
James White 34 45.5 10 black
Miguel Chavez 29 48.7 9 red
David McGuire 31 45.8 10 blue
Each line being a record of a struct of 6 variables.
Using the std::getline overload without the last (i.e. delimiter) parameter should take care of the end-of-line conversions automatically:
std::ifstream in("TheFile.txt");
std::string line;
while (std::getline(in, line)) {
// Do something with 'line'.
}
Here's a simple way to strip string of an extra "\r":
std::ifstream in("TheFile.txt");
std::string line;
std::getline(input, line));
if (line[line.size() - 1] == '\r')
line.resize(line.size() - 1);
If you can already read the files, just check for all of the newline characters like "\n" and "\r". I'm pretty sure that linux uses "\r\n" as the newline character.
You can read this page: http://en.wikipedia.org/wiki/Newline
and here is a list of all the ascii codes including the newline characters:
http://www.asciitable.com/
Edit: Linux uses "\n", Windows uses "\r\n", Mac uses "\r". Thanks to Seth Carnegie
Since the result will be CR LF, I would add something like the following to consume the extras if they exist. So once your have read you record call this before trying to read the next.
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
If you know the number of values you are going to read for each record you could simply use the ">>" method. For example:
fstream f("input.txt" std::ios::in);
string tempStr;
double tempVal;
for (number of records) {
// read the first name
f >> tempStr;
// read the last name
f >> tempStr;
// read the number
f >> tempVal;
// and so on.
}
Shouldn't that suffice ?
Hi I will give you the answer in stages. Please go trough in order to understand the code.
Stage 1: Design our program:
Our program based on the requirements should...:
...include a definition of a data type that would hold the data. i.e. our
structure of 6 variables.
...provide user interaction i.e. the user should be able to
provide the program, the file name and its location.
...be able to
open the chosen file.
...be able to read the file data and
write/save them into our structure.
...be able to close the file
after the data is read.
...be able to print out of the saved data.
Usually you should split your code into functions representing the above.
Stage 2: Create an array of the chosen structure to hold the data
...
#define MAX 10
...
strPersonData sTextData[MAX];
...
Stage 3: Enable user to give in both the file location and its name:
.......
string sFileName;
cout << "Enter a file name: ";
getline(cin,sFileName);
ifstream inFile(sFileName.c_str(),ios::in);
.....
->Note 1 for stage 3. The accepted format provided then by the user should be:
c:\\SomeFolder\\someTextFile.txt
We use two \ backslashes instead of one \, because we wish it to be treated as literal backslash.
->Note 2 for stage 3. We use ifstream i.e. input file stream because we want to read data from file. This
is expecting the file name as c-type string instead of a c++ string. For this reason we use:
..sFileName.c_str()..
Stage 4: Read all data of the chosen file:
...
while (!inFile.eof()) { //we loop while there is still data in the file to read
...
}
...
So finally the code is as follows:
#include <iostream>
#include <fstream>
#include <cstring>
#define MAX 10
using namespace std;
int main()
{
string sFileName;
struct strPersonData {
char c1stName[25];
char c2ndName[30];
int iAge;
double dSomeData1; //i had no idea what the next 2 numbers represent in your code :D
int iSomeDate2;
char cColor[20]; //i dont remember the lenghts of the different colors.. :D
};
strPersonData sTextData[MAX];
cout << "Enter a file name: ";
getline(cin,sFileName);
ifstream inFile(sFileName.c_str(),ios::in);
int i=0;
while (!inFile.eof()) { //loop while there is still data in the file
inFile >>sTextData[i].c1stName>>sTextData[i].c2ndName>>sTextData[i].iAge
>>sTextData[i].dSomeData1>>sTextData[i].iSomeDate2>>sTextData[i].cColor;
++i;
}
inFile.close();
cout << "Reading the file finished. See it yourself: \n"<< endl;
for (int j=0;j<i;j++) {
cout<<sTextData[j].c1stName<<"\t"<<sTextData[j].c2ndName
<<"\t"<<sTextData[j].iAge<<"\t"<<sTextData[j].dSomeData1
<<"\t"<<sTextData[j].iSomeDate2<<"\t"<<sTextData[j].cColor<<endl;
}
return 0;
}
I am going to give you some exercises now :D :D
1) In the last loop:
for (int j=0;j<i;j++) {
cout<<sTextData[j].c1stName<<"\t"<<sTextData[j].c2ndName
<<"\t"<<sTextData[j].iAge<<"\t"<<sTextData[j].dSomeData1
<<"\t"<<sTextData[j].iSomeDate2<<"\t"<<sTextData[j].cColor<<endl;}
Why do I use variable i instead of lets say MAX???
2) Could u change the program based on stage 1 on sth like:
int main(){
function1()
function2()
...
functionX()
...return 0;
}
I hope i helped...