can you use Insert Iterators while reading from a file to put the data into STL container?
for example:
FILE *stream;
fread(back_inserter(std::list), sizeof(int), 1, stream);
C++ streams are not compatible with C stdio streams. In other words, you can't use C++ iterators with FILE* or fread. However, if you use the C++ std::fstream facilities along with istream_iterator, you can use an insertion iterator to insert into a C++ container.
Assuming you have an input file "input.txt" which contains ASCII text numbers separated by whitespace, you can do:
#include <iostream>
#include <fstream>
#include <vector>
#include <iterator>
int main()
{
std::ifstream ifs("input.txt");
std::vector<int> vec;
// read in the numbers from disk
std::copy(std::istream_iterator<int>(ifs), std::istream_iterator<int>(), std::back_inserter(vec));
// now output the integers
std::copy(vec.begin(), vec.end(), std::ostream_iterator<int>(std::cout, "\n"));
}
no you can't. And it's fundamentally not-portable to store int like that. Your code will break if you write your file with a big-endian machine and try to read it with a little-endian machine.
But nobody prevents you to. Just define your own forward iterator that reads binary from a istream. You will likely want to stop using FILE and fread/fopen/fclose function as they are from C era.
then you will be able to write :
std::copy_n(your_custom_forward_iterator, count, back_inserter<std::list<....> >);
Related
I would like to know how I can extract / skip certain columns such as age and weight from a CSV file in C++.
Does it make more sense to extract the desired information after I loaded the entire csv file (if memory is not a problem)?
EDIT: If possible, I would like to have a reading, printing and modification part.
If possible, I want to use only the STL. The content of my test csv file looks as follows:
*test.csv*
name;age;weight;height;test
Bla;32;1.2;4.3;True
Foo;43;2.2;5.3;False
Bar;None;3.8;2.4;True
Ufo;32;1.5;5.4;True
I load the test.csv file with the following C++ program that prints the file's content on the screen:
#include <iostream>
#include <vector>
#include <string>
#include <iomanip>
#include <fstream>
#include <sstream>
void readCSV(std::vector<std::vector<std::string> > &data, std::string filename);
void printCSV(const std::vector<std::vector<std::string>> &data);
int main(int argc, char** argv) {
std::string file_path = "./test.csv";
std::vector<std::vector<std::string> > data;
readCSV(data, file_path);
printCSV(data);
return 0;
}
void readCSV(std::vector<std::vector<std::string> > &data, std::string filename) {
char delimiter = ';';
std::string line;
std::string item;
std::ifstream file(filename);
while (std::getline(file, line)) {
std::vector<std::string> row;
std::stringstream string_stream(line);
while (std::getline(string_stream, item, delimiter)) {
row.push_back(item);
}
data.push_back(row);
}
file.close();
}
void printCSV(const std::vector<std::vector<std::string> > &data) {
for (std::vector<std::string> row: data) {
for (std::string item: row) {
std::cout << item << ' ';
}
std::cout << std::endl;
}
}
Any assistance you can provide would be greatly appreciated.
Basically I answered this question already in a similar thread. But anyway, I will show a ready to use solution with a different approach and some explanation here.
One hint: You should make yourself more familiar with object oriented programming. And think over your design. In your read and write function you create a unneccessary dependency to a file or to std::cout- So, you should not handover a file name and then open the file in the function, but, use streams. Because, in the function that I created, using the C++ IO facilities, it doesn't matter, if we read from a file or a std::istringstream or write to std::cout or a file stream.
All will be handled via the (overloaded) extractor and inserter operators.
So, and because I wanted the code a little bit more flexible, I made my struct a template, to be able to put in the selected Columns and reuse the same struct for other column combinations.
If you want to have fixed selected columns then you can delete the line with template and can replace std::vector<size_t> selectedFields{ {Colums...} }; with std::vector<size_t> selectedFields{ {1,2} };
Later we use a using for the template to allow easier handling and understanding:
// Define Dataype for selected columns age and weight
using AgeAndWeight = SelectedColumns<1, 2>;
OK, let's first see the source code and then try to understand.
#include <iostream>
#include <string>
#include <vector>
#include <regex>
#include <fstream>
#include <initializer_list>
#include <iterator>
#include <algorithm>
std::regex re{ ";" };
// Proxy for reading an splitting a line and extracting certain fields and some simple output
template<size_t ... Colums>
struct SelectedColumns {
std::vector<std::string> data{};
std::vector<size_t> selectedFields{ {Colums...} };
// Overwrite extractor operator
friend std::istream& operator >> (std::istream& is, SelectedColumns& sl) {
// Read a complete line and check, if it could be read
if (std::string line{}; std::getline(is, line)) {
// Now split the line into tokens
std::vector tokens(std::sregex_token_iterator(line.begin(), line.end(), re, -1), {});
// Clear old data
sl.data.clear();
// So, and now copy the selected columns into our data vector
for (const size_t& column : sl.selectedFields)
if (column < tokens.size()) sl.data.push_back(tokens[column]);
}
return is;
}
// Simple extractor
friend std::ostream& operator << (std::ostream & os, const SelectedColumns & sl) {
std::copy(sl.data.begin(), sl.data.end(), std::ostream_iterator<std::string>(os, "\t"));
return os;
}
};
// Define Dataype for selected columns age and weight
using AgeAndWeight = SelectedColumns<1U, 2U>;
const std::string fileName{ "./test.csv" };
int main() {
// Open the csv file and check, if it is open
if (std::ifstream csvFileStream{ fileName }; csvFileStream) {
// Read complete csv file and extract age and weight columns
std::vector sc(std::istream_iterator<AgeAndWeight>(csvFileStream), {});
// Now all data is available in this vector sc Do something
sc[3].data[0] = "77";
// Show some debug out put
std::copy(sc.begin(), sc.end(), std::ostream_iterator<AgeAndWeight>(std::cout, "\n"));
// By the way, you could also write the 2 lines above in one line.
//std::copy(std::istream_iterator<AgeAndWeight>(csvFileStream), {}, std::ostream_iterator<AgeAndWeight>(std::cout, "\n"));
}
else std::cerr << "\n*** Error: Could not open source file\n\n";
return 0;
}
One major task here is to split a line with CSV Data into its tokens. Let us have a look at this.
Splitting a string into tokens:
What do people expect from the function, when they read
getline ?
Most people would say, Hm, I guess it will read a complete line from somewhere. And guess what, that was the basic intention for this function. Read a line from a stream and put it into a string.
But, as you can see here std::getline has some additional functionality.
And this lead to a major misuse of this function for splitting up std::strings into tokens.
Splitting strings into tokens is a very old task. In very early C there was the function strtok, which still exists, even in C++. Here std::strtok. Please see the std::strtok-example
std::vector<std::string> data{};
for (char* token = std::strtok(const_cast<char *>(line.data()), ","); token != nullptr; token = std::strtok(nullptr, ","))
data.push_back(token);
Simple, right?
But because of the additional functionality of std::getline is has been heavily misused for tokenizing strings. If you look on the top question/answer regarding how to parse a CSV file (please see here), then you will see what I mean.
People are using std::getline to read a text line, a string, from the original stream, then stuffing it into an std::istringstream and use std::getline with delimiter again to parse the string into tokens. Weird.
But, since many many years, we have a dedicated, special function for tokenizing strings, especially and explicitly designed for that purpose. It is the
std::sregex_token_iterator
And since we have such a dedicated function, we should simply use it.
This thing is an iterator. For iterating over a string, hence the function name is starting with an s. The begin part defines, on what range of input we shall operate, the end part is default constructed, and then there is a std::regex for what should be matched / or what should not be matched in the input string. The type of matching strategy is given with last parameter.
0 --> give me the stuff that I defined in the regex and (optional)
-1 --> give me that what is NOT matched based on the regex.
We can use this iterator for storing the tokens in a std::vector. The std::vector has a range constructor, which takes 2 iterators as parameter, and copies the data between the first iterator and 2nd iterator to the std::vector. The statement
std::vector tokens(std::sregex_token_iterator(s.begin(), s.end(), re, -1), {});
defines a variable “tokens” as a std::vector and uses the so called range-constructor of the std::vector. Please note: I am using C++17 and can define the std::vector without template argument. The compiler can deduce the argument from the given function parameters. This feature is called CTAD ("class template argument deduction").
Additionally, you can see that I do not use the "end()"-iterator explicitly.
This iterator will be constructed from the empty brace-enclosed default initializer with the correct type, because it will be deduced to be the same as the type of the first argument due to the std::vector constructor requiring that.
You can read any number of tokens in a line and put it into the std::vector
But you can do even more. You can validate your input. If you use 0 as last parameter, you define a std::regex that even validates your input. And you get only valid tokens.
Overall, the usage of a dedicated functionality is superior over the misused std::getline and people should simple use it.
Some people complain about the function overhead, and, they are right, but how many of them are using big data. And even then, the approach would be probably then to use string.findand string.substring or std::stringviews or whatever.
So, now to further topics.
In the extractor, we first read a complete line from the source stream and check, if that worked. Or, if we have and end of file or any other error.
Then we tokenize that just read string as described above.
And then, we will copy only selected columns from the tokens into our resulting data. This is done in a simple for loop. Here we also check the boundaries, because somebody could specify invalid selected columns, or, a line could have less tokens than expected.
So the body of the extractor is vey simple. Just 5 line of code . . .
Then, again,
You should start using object-oriented features in C++. In C++ you can put data and methods that operate on these data into one object. The reason is that the outside world should not care about objects internals. For example, your readCSV and printCSV function should be part of a struct (or class).
And as next step, we will not use your “read” and “print” functions. We will use the dedicated function for Stream-IO, the extractor operator >> and the inserter operator <<. And we will overwrite the standard IO-functions in our struct.
In function main we will open the the source file and check, if the open was successful. BTW. All input output functions shall be checked, if they were successful.
Then, we use the next iterator, the std::istream_iterator. And this together with our “AgeAndWeight”-type and the input file stream. Also here we use CTAD and the default constructed end-iterator. The std::istream_iterator will repeatedly call the AgeAndWeight extractor operator, until all lines of the source file are read.
For output, we will use the std::ostream_iterator. This will call the inserter operator for "AgeAndWeight" until all data are written.
I'm trying to read in information from a tab separated value file with the format:
<string> <int> <string>
Example:
Seaking 119 Azumao
Mr. Mime 122 Barrierd
Weedle 13 Beedle
This is currently how I'm doing it:
string americanName;
int pokedexNumber;
string japaneseName;
inFile >> americanName;
inFile >> pokedexNumber
inFile >> japaneseName;
My issue stems from the space in the "Mr. Mime" as the strings can contain spaces.
I would like to know how to read the file in properly.
Standard library uses such things as locales to determine the categories of different symbols and other locale-dependent things depending on your system locale. Standard streams use that to determine what is a space because of various unicode issues.
You can use this fact to control the meaning of ' ' in your case:
#include <iostream>
#include <locale>
#include <algorithm>
struct tsv_ws : std::ctype<char>
{
mask t[table_size]; // classification table, stores category for each character
tsv_ws() : ctype(t) // ctype will use our table to check character type
{
// copy all default values to our table;
std::copy_n(classic_table(), table_size, t);
// here we tell, that ' ' is a punctuation, but not a space :)
t[' '] = punct;
}
};
int main() {
std::string s;
std::cin.imbue(std::locale(std::cin.getloc(), new tsv_ws)); // using our locale, will work for any stream
while (std::cin >> s) {
std::cout << "read: '" << s << "'\n";
}
}
Here we make ' ' a punctuation symbol, but not a space symbol, so streams don't consider it a separator anymore. The exact category isn't important, but it mustn't be space.
That's quite powerful technique. For example, you could redefine ',' to be a space to read in CSV format.
You can use std::getline to extract strings with non-tab whitespace.
std::getline(inFile, americanName, '\t'); // read up to first tab
inFile >> pokedexNumber >> std::ws; // read number then second tab
std::getline(inFile, japaneseName); // read up to first newline
Seems like you want to read csv data or in your case tsv data. But let's stick to the common term "csv". This is a standard task and I will give you detailed explanations. In the end all the reading will be done in a one-liner.
I would recommend to use "modern" C++ approach.
After searching for "reading csv data", people are still are linking to How can I read and parse CSV files in C++?, the questions is from 2009 and now over 10 years old. Most answers are also old and very complicated. So, maybe its time for a change.
In modern C++ you have algorithms that iterate over ranges. You will often see something like "someAlgoritm(container.begin(), container.end(), someLambda)". The idea is that we iterate over some similar elements.
In your case we iterate over tokens in your input string, and create substrings. This is called tokenizing.
And for exactly that purpose, we have the std::sregex_token_iterator. And because we have something that has been defined for such purpose, we should use it.
This thing is an iterator. For iterating over a string, hence sregex. The begin part defines, on what range of input we shall operate, then there is a std::regex for what should be matched / or what should not be matched in the input string. The type of matching strategy is given with last parameter.
1 --> give me the stuff that I defined in the regex and
-1 --> give me that what is NOT matched based on the regex.
So, now that we understand the iterator, we can std::copy the tokens from the iterator to our target, a std::vector of std::string. And since we do not know, how may columns we have, we will use the std::back_inserter as a target. This will add all tokens that we get from the std::sregex_token_iterator and append it ot our std::vector<std::string>>. It does'nt matter how many columns we have.
Good. Such a statement could look like
std::copy( // We want to copy something
std::sregex_token_iterator // The iterator begin, the sregex_token_iterator. Give back first token
(
line.begin(), // Evaluate the input string from the beginning
line.end(), // to the end
re, // Add match a comma
-1 // But give me back not the comma but everything else
),
std::sregex_token_iterator(), // iterator end for sregex_token_iterator, last token + 1
std::back_inserter(cp.columns) // Append everything to the target container
);
Now we can understand, how this copy operation works.
Next step. We want to read from a file. The file conatins also some kind of same data. The same data are rows.
And as for above, we can iterate of similar data. If it is the file input or whatever. For this purpose C++ has the std::istream_iterator. This is a template and as a template parameter it gets the type of data that it should read and, as a constructor parameter it gets a reference to an input stream. It doesnt't matter, if the input stream is a std::cin, or a std::ifstream or a std::istringstream. The behaviour is identical for all kinds of streams.
And since we do not have files an SO, I use (in the below example) a std::istringstream to store the input csv file. But of course you can open a file, by defining a std::ifstream testCsv(filename). No problem.
And with std::istream_iterator, we iterate over the input and read similar data. In our case one problem is that we want to iterate over special data and not over some build in data type.
To solve this, we define a Proxy class, which does the internal work for us (we do not want to know how, that should be encapsulated in the proxy). In the proxy we overwrite the type cast operator, to get the result to our expected type for the std::istream_iterator.
And the last important step. A std::vector has a range constructor. It has also a lot of other constructors that we can use in the definition of a variable of type std::vector. But for our purposes this constructor fits best.
So we define a variable csv and use its range constructor and give it a begin of a range and an end of a range. And, in our specific example, we use the begin and end iterator of std::istream_iterator.
If we combine all the above, reading the complete CSV file is a one-liner, it is the definition of a variable with calling its constructor.
Please see the resulting code:
#include <iostream>
#include <sstream>
#include <fstream>
#include <string>
#include <vector>
#include <iterator>
#include <regex>
#include <algorithm>
std::istringstream testCsv{ R"(Seaking 119 Azumao
Mr. Mime 122 Barrierd
Weedle 13 Beedle)" };
// Define Alias for easier Reading
using Columns = std::vector<std::string>;
using CSV = std::vector<Columns>;
// Proxy for the input Iterator
struct ColumnProxy {
// Overload extractor. Read a complete line
friend std::istream& operator>>(std::istream& is, ColumnProxy& cp) {
// Read a line
std::string line; cp.columns.clear();
if (std::getline(is, line)) {
// The delimiter
const std::regex re("\t");
// Split values and copy into resulting vector
std::copy(std::sregex_token_iterator(line.begin(), line.end(), re, -1),
std::sregex_token_iterator(),
std::back_inserter(cp.columns));
}
return is;
}
// Type cast operator overload. Cast the type 'Columns' to std::vector<std::string>
operator std::vector<std::string>() const { return columns; }
protected:
// Temporary to hold the read vector
Columns columns{};
};
int main()
{
// Define variable CSV with its range constructor. Read complete CSV in this statement, So, one liner
CSV csv{ std::istream_iterator<ColumnProxy>(testCsv), std::istream_iterator<ColumnProxy>() };
// Print result. Go through all lines and then copy line elements to std::cout
std::for_each(csv.begin(), csv.end(), [](Columns & c) {
std::copy(c.begin(), c.end(), std::ostream_iterator<std::string>(std::cout, " ")); std::cout << "\n"; });
}
I hope the explanation was detailed enough to give you an idea, what you can do with modern C++.
This example does basically not care how many rows and columns are in the source text file. It will eat everything.
I have a text file which looks like this:
73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
and etc. 20 lines in total.
What I want to do is to read every digit from the text file and put them into an array of integers(one element = one digit). How can I read only one digit from this text file, not the whole line?
There are several ways to accomplish what you are looking for, in this post I'll describe three different methods. All three of them assume that you open your file using an std::ifstream ifs ("filename.txt") and that your "array" is actually a vector declared as std::vector<int> v.
At the end of this post there is also a little advice on how to speed up insertion into your vector.
I'd like to keep it simple..
The most simple approach is to read one char at a time using operator>> and then subtract '0' from the value returned.
The standard guarantee that '0' through '9' are sequential, and since a char is nothing but a numeric value printed in a different matter it can implicitly be casted to int.
char c;
while (ifs >> c)
v.push_back (c - '0');
I love the STL, and hate writing loops..
This will by many be treated as the "c++ way to do it", espacially if you are talking to STL-fanboys, though it requires a lot more code to write..
#include <algorithm>
#include <functional>
#include <iterator>
...
std::transform (
std::istream_iterator<char> (ifs),
std::istream_iterator<char> (),
std::back_inserter (v),
std::bind2nd (std::minus<int> (), '0')
);
I don't want to write loops, but why not use a lambda? c++11
#include <algorithm>
#include <functional>
#include <iterator>
...
std::transform (
std::istream_iterator<char> (iss),
std::istream_iterator<char> (),
std::back_inserter (v),
[](char c){return c - '0';}
);
Will my std::vector reallocate storage upon each insertion?
Yes, probably. To speed things up you can reserve storage in your vector before you start doing any insertions, as in the below.
ifs.seekg (0, std::ios::end); // seek to the end of your file
v.reserve (ifs.tellg () ); // ifs.tellg () -> number of bytes in it
ifs.seekg (0, std::ios::beg); // seek back to the beginning
char digit;
std::ifstream file("digits.txt");
std::vector<int> digits;
// if you want the ASCII value of the digit.
1- while(file >> digit) digits.push_back(digit);
// if you want the numeric value of the digit.
2- while(file >> digit) digits.push_back(digit - '0');
I'm trying to initialize a vector using iterators and I'm getting a compiler error basically saying that there's no matching function to call.
The code reads from a file with an istream_iterator and ends with an input sentinel. Then I try to initialize the vector with those two iterators.
#include "std_lib_facilities.h"
#include<iterator>
int main()
{
string from, to; // get source and target file names
cin >> from >> to;
ifstream is(from.c_str()); // open input stream
ofstream os(to.c_str()); // open output stream
istream_iterator<string> ii(is); // make input iterator for stream
istream_iterator<string> eos; // input sentinel
ostream_iterator<string> oo(os,"\n");
vector<string> words(ii, eos); // vector initialized from input
sort(words.begin(), words.end()); // sort the buffer
copy(words.begin(), words.end(), oo); // copy buffer to output
}
I know I could use the copy function to copy the input stream into the vector, but I read that it can be done this way as well. Can anyone explain why this is not compiling? Thanks.
Compiler error:
C:\Users\Alex\C++\stream_iterators.cpp|16|error: no matching function for call to `Vector<String>::Vector(std::istream_iterator<String, char, std::char_traits<char>, ptrdiff_t>&, std::istream_iterator<String, char, std::char_traits<char>, ptrdiff_t>&)'|
Edit: It is not a header problem. Std_lib_facilities has all of the needed headers.
vector<string> words(ii, eos);
is an analogue to
vector<string> words;
copy( ii, eos, back_inserter(words) );
vector class has the following constructor:
// initialize with range [First, Last)
template<class InputIterator>
vector(
InputIterator First,
InputIterator Last
);
To make your sample compiling you need to include the following:
#include <sstream>
#include <iostream>
#include <vector>
#include <fstream>
#include <algorithm> // for std::copy
Since your identifiers not fully qualified you should add the following:
using namespace std;
Or fully qualify all STL identifiers.
And to change, I guess,
copy(words.begin(), words.end(), out)
to
copy(words.begin(), words.end(), oo)
Please copy and paste the compiler error. Also you are missing a number of headers such as algorithm and vector. You need a use namespace std declaration or use std:: to access the STL classes.
Once you provide more information we can give you more advice.
Update: why is your error message referring to "Vector" (with capital), not vector (lower case)?
The book header had some kind of compliance issues, so I just included the appropriate headers and it worked.
The Vector class in std_lib_facilities.h defines three constructors, but not one which accepts a pair of iterators. You can go ahead and add that to the header:
template <class Iter>
Vector(Iter from, Iter to): std::vector<T>(from, to) {}
With this header you'll need to take into account that this is for hand-holding. operator[] in a real std::vector is not supposed to do range-checking. (Why not just teach beginners to use vector::at instead, until they get the idea that it might be better to stay in bounds...?)
I'm working this source code:
#include <string>
#include <vector>
#include <iostream>
#include <istream>
#include <ostream>
#include <iterator>
#include <sstream>
#include <algorithm>
int main()
{
std::string str = "The quick brown fox";
// construct a stream from the string
std::stringstream strstr(str);
// use stream iterators to copy the stream to the vector as whitespace separated strings
std::istream_iterator<std::string> it(strstr);
std::istream_iterator<std::string> end;
std::vector<std::string> results(it, end);
// send the vector to stdout.
std::ostream_iterator<std::string> oit(std::cout);
std::copy(results.begin(), results.end(), oit);
}
To, instead of tokenizing a single line and putting it into the vector results, it tokenizes a group of lines taken from this text file and puts the resulting words into a single vector .
Text File:
Munroe states there is no particular meaning to the name and it is simply a four-letter word without a phonetic pronunciation, something he describes as "a treasured and carefully-guarded point in the space of four-character strings." The subjects of the comics themselves vary. Some are statements on life and love (some love strips are simply art with poetry), and some are mathematical or scientific in-jokes.
So far, I'm only clear that I need to use a
while (getline(streamOfText, readTextLine)){}
to get the loop running.
But I don't think this would work:
while (getline(streamOfText, readTextLine)) {
cout << readTextLine << endl;
// construct a stream from the string
std::stringstream strstr(readTextLine);
// use stream iterators to copy the stream to the vector as whitespace separated strings
std::istream_iterator it(strstr);
std::istream_iterator end;
std::vector results(it, end);
/*HOw CAN I MAKE THIS INSIDE THE LOOP WITHOUT RE-DECLARING AND USING THE CONSTRUCTORS FOR THE ITERATORS AND VECTOR? */
// send the vector to stdout.
std::ostream_iterator<std::string> oit(std::cout);
std::copy(results.begin(), results.end(), oit);
}
Yes, then you have one whole line in readTextLine. Is it that what you wanted in that loop? Then instead of constructing the vector from the istream iterators, copy into the vector, and define the vector outside the loop:
std::vector<std::string> results;
while (getline(streamOfText, readTextLine)){
std::istringstream strstr(readTextLine);
std::istream_iterator<std::string> it(strstr), end;
std::copy(it, end, std::back_inserter(results));
}
You actually don't need to read a line into the string first, if all you need is all words from a stream, and no per-line processing. Just read from the other stream directly like you did in your code. It will not only read words from one line, but from the whole stream, until the end-of-file:
std::istream_iterator<std::string> it(streamOfText), end;
std::vector<std::string> results(it, end);
To do all that manually, like you ask for in the comments, do
std::istream_iterator<std::string> it(streamOfText), end;
while(it != end) results.push_back(*it++);
I recommend you to read a good book on this. It will show you much more useful techniques i think. C++ Standard library by Josuttis is a good book.