I have a situation where I loop through the first 64 lines of a file and save each line into a string. The rest of the file is unknown. It may be a single line or many.
I know that there will be 64 lines at the beginning of the file but I do not know their size.
How can I save the entirety of the rest of the file to a string?
This is what I currently have:
std::ifstream signatureFile(fileName);
for (int i = 0; i < 64; ++i) {
std::string tempString;
//read the line
signatureFile >> tempString;
//do other processing of string
}
std::string restOfFile;
//save the rest of the file into restOfFile
Thanks to the responses this is how I got it working:
std::ifstream signatureFile(fileName);
for (int i = 0; i < 64; ++i) {
std::string tempString;
//read the line
//using getline prevents extra line break when reading the rest of file
std::getline(signatureFile, tempString);
//do other processing of string
}
//save the rest of the file into restOfFile
std::string restOfFile{ std::istreambuf_iterator<char>{signatureFile},
std::istreambuf_iterator<char>{} };
signatureFile.close();
One of std::string's constructors is a template that takes two iterators as parameters, a beginning and an ending iterator, and constructs a string from the sequence defined by the iterators.
It just so happens that std::istreambuf_iterator provides a suitable input iterator for iterating over the contents of an input stream:
std::string restOfFile{std::istreambuf_iterator<char>{signatureFile},
std::istreambuf_iterator<char>{}};
You can use string buffer.
#include <sstream>
// ...
stringbuf buf;
signatureFile.get(buf);
string restOfFile = buf.str();
Related
So let's say I have a vector of ints and a text file which looks like this:
1|2|3|4|5
How can I add the numbers to the vector?
First, you would open the file using std::ifstream. There are a few ways you could then read these out, but one example would be to use std::getline with a custom "end of line" character, being your | in this case:
std::vector<int> myVect;
std::ifstream reader("./file.txt"); //Replace with path to your file
for(int i = 0; i < 5; i++) {
std::string item;
std::getline(reader, item, '|'); //The third argument tells it to read until a '|' char
int item = std::stoi(item); //Convert from string to int
myVect.push_back(number);
}
This example relies on you knowing how many elements you want to get, but can be modified to work with an unknown size.
I have a txt file with a lot of things in it.
The lines have this pattern: 6 spaces then 1 int, 1 space, then a string.
Also, the 1st line has the amount of lines that the txt has.
I want to put the integers in an array of ints and the string on an array of strings.
I can read it and put it into an array , but only if I'm considering the ints as chars and putting into one array of strings.When I try to separate things I have no idea on how I'd do it. Any ideas?
The code I used for putting everything in an array was this:
int size()
{
ifstream sizeX;
int x;
sizeX.open("cities.txt");
sizeX>>x;
return x;
};
int main(void)
{
int size = size();
string words[size];
ifstream file("cities.txt");
file.ignore(100000,'\n');
if(file.is_open())
{
for(int i=0; i<size; i++)
{
getline(file,words[i]);
}
}
}
Just to start I'm going to provide some tips about your code:
int size = size();
Why do you need to open the file, read the first line and then close it? That process can be done opening the file just once.
The code string words[size]; is absolutely not legal C++. You cannot instantiate a variable-length-array in C++. That C feature has been not included in C++ standard (some ref). I suggest you to replace with std::vector, which is more C++ code.
Here I write a snippet of function which perform what you need.
int parse_file(const std::string& filename,
std::vector<std::string>* out_strings,
std::vector<int>* out_integers) {
assert(out_strings != nullptr);
assert(out_integers != nullptr);
std::ifstream file;
file.open(filename, std::ios_base::in);
if (file.fail()) {
// handle the error
return -1;
}
// Local variables
int num_rows;
std::string line;
// parse the first line
std::getline(file, line);
if (line.size() == 0) {
// file empty, handle the error
return -1;
}
num_rows = std::stoi(line);
// reserve memory
out_strings->clear();
out_strings->reserve(num_rows);
out_integers->clear();
out_integers->reserve(num_rows);
for (int row = 0; row < num_rows; ++row) {
// read the line
std::getline(file, line);
if (line.size() == 0) {
// unexpected end of line, handle it
return -1;
}
// get the integer
out_integers->push_back(
std::stoi(line.substr(6, line.find(' ', 6) - 6)));
// get the string
out_strings->push_back(
line.substr(line.find(' ', 6) + 1, std::string::npos));
}
file.close();
return 0;
}
You can definitely improved it, but I think it's a good point where to start.
The last suggest I can give you, in order to improve the robustness of your code, you can match each line with a regular expression. In this way you can be sure your line is formatted exactly how you need.
For example:
std::regex line_pattern("\\s{6}[0-9]+\\s[^\\n]+");
if (std::regex_match(line, line_pattern) == false) {
// ups... the line is not formatted how you need
// this is an error
}
I've a task to copy elements from .txt file[direct access file] to .bin file[fixed length record file] (homework).
.txt file holds strings. Every line has one word.
I came up with code below, but I'm not sure if that's what is needed and even slighly correct. Any help will be useful! (I'm new to C++)
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
const int buffer_size = 30;
class Word{
char name[buffer_size];
public:
void setName () // Trying to get every word from a line
{
string STRING;
ifstream infile;
infile.open ("text.txt");
while(!infile.eof()) // To get you all the lines.
{
getline(infile,STRING); // Saves the line in STRING.
}
infile.close();
}
};
void write_record()
{
ofstream outFile;
outFile.open("binFILE.bin", ios::binary | ios::app);
Word obj;
obj.setName();
outFile.write((char*)&obj, sizeof(obj));
outFile.close();
}
int main()
{
write_record();
return 0;
}
NEW APPROACH:
class Word
{
char name[buffer_size];
public:
Word(string = "");
void setName ( string );
string getName() const;
};
void readWriteToFile(){
// Read .txt file content and write into .bin file
string temp;
Word object;
ofstream outFile("out.dat", ios::binary);
fstream fin ("text.txt", ios::in);
getline(fin, temp);
while(fin)
{
object.setName(temp);
outFile.write( reinterpret_cast< const char* >( &object ),sizeof(Word) );
getline(fin, temp);
}
fin.close();
outFile.close();
}
int main()
{
readWriteToFile();
return 0;
}
Word::Word(string nameValue)
{
setName(nameValue);
}
void Word::setName( string nameString )
{
// Max 30 char copy
const char *nameValue = nameString.data();
int len = strlen(nameValue);
len = ( len < 31 ? len : 30);
strncpy(name, nameValue, len);
name[len] = '\0';
}
string Word::getName() const
{
return name;
}
Quick commentary and walk through
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
Avoid using namespace std; while you are learning. It can lead to some really nasty, hard to pin-down bugs as your functions may be silently replaced by functions with the same name in the standard library.
const int buffer_size = 30;
class Word
{
char name[buffer_size];
Since it looks like you are allowed to use std::string why not use it here?
public:
void setName() // Trying to get every word from a line
Really bad name for a function that apparently is supposed to // Trying to get every word from a line
{
string STRING;
ifstream infile;
infile.open("text.txt");
while (!infile.eof()) // To get you all the lines.
{
getline(infile, STRING); // Saves the line in STRING.
}
Few things wrong here. One is the epic Why is iostream::eof inside a loop condition considered wrong?
Next is while the code reads each line, it doesn't do anything with the line. STRING is never stored anywhere.
Finally in a class that sounds as though it should contain and manage a single word, it reads all the words in the file. There may be a case for turning this function into a static factory that churns out a std::vector of Words.
infile.close();
}
};
void write_record()
{
ofstream outFile;
outFile.open("binFILE.bin", ios::binary | ios::app);
ios::app will add onto an existing file. This doesn't sound like what was described in the assignment description.
Word obj;
obj.setName();
We've already coverred the failings of the Word class.
outFile.write((char*) &obj, sizeof(obj));
Squirting an object into a stream without defining a data protocol or using any serialization is dangerous. It makes the file non-portable. You will find that some classes, vector and string prominent among these, do not contain their data. Writing a string to a file may get you nothing more than a count and an address that is almost certainly not valid when the file is loaded.
In this case all the object contains is an array of characters and that should write to file cleanly, but it will always write exactly 30 bytes and that may not be what you want.
outFile.close();
}
int main()
{
write_record();
return 0;
}
Since this is homework I'm not writing this sucker for you, but here are a few suggestions:
Read file line by line will get you started on the file reader. Your case is simpler because there is only one word on each line. Your teacher may throw a curveball and add more stuff onto a line, so you may want to test for that.
Read the words from the file into a std::vector. vector will make your job so easy that you might have time for other homework.
A very simplistic implementation is:
std::vector<std::string> words;
while (getline(infile, STRING)) // To get you all the lines.
{
words.push_back(STRING);
}
For writing the file back out in binary, I suggest going Pascal style. First write the length of the string in binary. Use a known, fixed width unsigned integer (no such thing as a negative string) and watch out for endian. Once the length is written, write only the number of characters you need to write.
Ignoring endian, you should have something like this:
uint32_t length = word.length(); // length will always be 32 bits
out.write((char*)&length, sizeof(length));
out.write(word.c_str(), length);
When you are done writing the writer, write a reader function so that you can test that the writer works correctly. Always test your code, and I recommend not writing anything until you know how you'll test it. Very often coming at a program from the test side first will find problems before they even have a chance to start.
I want to be able to read a full line into a character array using a function.
example input text is:
Schmidt, Helga
Alvarez, Ruben
Zowkowski, Aaron
Huang, Sun Lee
Einstein, Beverly
However, im not sure how to read a full line of characters into the array. I know the delimiter for >> is whitespace, but I'm not sure if I change that delimiter to '\n' if it'd work?
void buildList(char (*array)[25], ifstream& inputFile){
string line;
for (int i = 0; i < 5; i++)
getline(inputFile, line);
array[i] = line.c_str();
}
Currently this only reads either a last name or first name into my input instead of the whole line. I'm not sure how I can go about changing this. Thanks.
First, you definitely want to use std::string here. Once you
do that, you can use std::getline:
std::vector<std::string>
buildList( istream& input )
{
std::vector<std::string> results;
std::string line
while ( std::getline( input, line ) ) {
results.push_back( line );
}
}
This will make for much simpler and more robust code.
If you have to use such a broken interface, there is a member
function getline:
for ( int i = 0; i != 5; ++ i ) {
input.getline( array[i], maxLength );
}
Also: a function should never take an std::ifstream& as
argument (unless it is going to open or close the file). An
std::istream& should be used.
Use this-
void buildList(char (*array)[25], ifstream& inputFile){
for (int i = 0; i < 5; i++)
std::inputFile.getline(array[i],50);
}
The second parameter of getline is the maximum number of characters to write to the character array.
I need to read the nth line of a text file (e.g. textfile.findline(0) would find the first line of the text file loaded with ifstream textfile). Is this possible?
I don't need to put the contents of the file in an array/vector, I need to just assign a specific line of the text file to a varible (specifically a int).
P.S. I am looking for the simplest solution that would not require me to use any big external library (e.g. Boost)
Thanks in advance.
How about this?
std::string ReadNthLine(const std::string& filename, int N)
{
std::ifstream in(filename.c_str());
std::string s;
//for performance
s.reserve(some_reasonable_max_line_length);
//skip N lines
for(int i = 0; i < N; ++i)
std::getline(in, s);
std::getline(in,s);
return s;
}
If you want to read the start of the nth line, you can use stdin::ignore to skip over the first n-1 lines, then read from the next line to assign to the variable.
template<typename T>
void readNthLine(istream& in, int n, T& value) {
for (int i = 0; i < n-1; ++i) {
in.ignore(numeric_limits<streamsize>::max(), '\n');
}
in >> value;
}
Armen's solution is the correct answer, but I thought I'd throw out an alternative, based on jweyrich's caching idea. For better or for worse, this reads in the entire file at construction, but only saves the newline positions (doesn't store the entire file, so it plays nice with massive files.) Then you can simply call ReadNthLine, and it will immediately jump to that line, and read in the one line you want. On the other hand, this is only optimal if you want to get only a fraction of the lines at a time, and the line numbers are not known at compile time.
class TextFile {
std::ifstream file_stream;
std::vector<std::ifstream::streampos> linebegins;
TextFile& operator=(TextFile& b) = delete;
public;
TextFile(std::string filename)
:file_stream(filename)
{
//this chunk stolen from Armen's,
std::string s;
//for performance
s.reserve(some_reasonable_max_line_length);
while(file_stream) {
linebegins.push_back(file_stream.tellg());
std::getline(file_stream, s);
}
}
TextFile(TextFile&& b)
:file_stream(std::move(b.file_stream)),
:linebegins(std::move(b.linebegins))
{}
TextFile& operator=(TextFile&& b)
{
file_stream = std::move(b.file_stream);
linebegins = std::move(b.linebegins);
}
std::string ReadNthLine(int N) {
if (N >= linebegins.size()-1)
throw std::runtime_error("File doesn't have that many lines!");
std::string s;
// clear EOF and error flags
file_stream.clear();
file_stream.seekg(linebegins[N]);
std::getline(file_stream, s);
return s;
}
};
It's certainly possible. There are (n-1) '\n' characters preceding the nth line. Read lines until you reach the one you're looking for. You can do this on the fly without storing anything except the current line being considered.