I am trying to find a solution for the error that the c++ compiler (codeblocks) keeps showing to me,I searched for answers on the net but none of them seemed to be helpful.
I have a txt file in which there are numbers,each of them is written in a line.
What i want is to read each number(line by line)and store it in a variable after converting it from string to float.
This my code
#include <fstream>
#include <sstream>
#include <string>
#include <iostream>
float z;
int j=0;
stringstream s;
const ifstream fich1;
fich1.open("test.txt",ios::in);
if(!fich1)
{
cerr<<"could not open the file";
EXIT_FAILURE;
};
else const string ligne1;
while((!fich1.eof())
{
if(j!=i)
{
j++;
getline(fich1,l1); // the first error is: no matching function for call to ‘getline(const ifstream&, const string&)
else if(j == i)
{
fich1>>s; // the second error is: ambiguous overload for ‘operator>>’ (operand types are ‘const ifstream {aka const std::basic_ifstream<char>}’ and ‘std::stringstream {aka std::basic_stringstream<char>}’)
z=(float)s; // I couldn't find a clear answer to convert from string to float
}
}
if anyone wants to ask any question about the code to make it clearer may ask it please,I am waiting for your answers as well as your questions :)
After the edit, I am able to read some code, but still I am suggesting the example I have below, since I am see scary things, like an EOF inside a loop!
Oh also, if you have C++11 support, then you could use std::stof.
You could try this (since your post is not readable), which reads a file line by line and stores every float number in a vector:
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <cstdlib>
int main() {
std::vector<float> v;
std::string rec_file = "test.txt";
std::string line;
std::ifstream myfile(rec_file.c_str());
if(myfile.is_open()) {
while(std::getline(myfile,line)) {
if(!line.empty())
v.push_back(atof(line.c_str()));
}
myfile.close();
} else {
std::cerr << "Unable to open file";
}
for(unsigned int i = 0; i < v.size(); ++i)
std::cout << v[i] << std::endl;
return 0;
}
with the test.txt to be:
1.2
2.3
3.4
4.5
5.6
6.7
and the output:
1.2
2.3
3.4
4.5
5.6
6.7
Your ifstream shouldn't be constant, as getline alters the ifstream.
To convert from char array to float use atof(chararray)
To convert from string to float you could use atof(string.cstr())
You cannot use const here. But that's not your only problem.
#include <fstream>
#include <sstream>
#include <string>
#include <iostream>
Looks good.
float z;
int j=0;
stringstream s;
There should be some int main() { here, because that is the function called by the runtime when a C++ executable starts. This isn't Bash or Perl where execution simply picks up at the first statement.
And you either need to write using namespace std; or prefix identifiers like stringstream as std::stringstream.
const ifstream fich1;
This won't work. ifstream must not be declared const. (This is actually the explanation for the errors you encountered, but you made many more.)
An input file stream is an object with complex inner state, like where in the file you're currently at, error flags etc.; this inner state is changing so const ifstream simply won't work.
fich1.open("test.txt",ios::in);
This can't work either due to const, just like all the other operations on fich1 you're doing further down.
if(!fich1)
{ cerr<<"could not open the file";
EXIT_FAILURE;
};
EXIT_FAILURE is a symbolic constant for a return value. As-is, you could just as well have written 0;. If you expected this to end your program, you're wrong.
;
The semicolon ends the if.
else
Since the if has ended, this is a syntax error.
const string ligne1;
If you declare a string const, you cannot assign to it. Besides, even if the else were correct, that semicolon would have ended the else, because you didn't add braces (as you should always do, even with one-line blocks, because it's so easy to make mistakes otherwise).
The way the code continues, I very much doubt this was your intention. (Some discipline with the indenting makes this kind of mistakes really easy to spot.)
while((!fich1.eof())
How to read until EOF from cin.
{ if(j!=i)
{j++;
Do some proper indenting, will you? Besides, no i has been declared at this point.
getline(fich1,l1);
I assume l1 is the string you declared earlier as ligne1. (Since that happened inside the else, ligne1 is no longer in scope, but I'll let that one pass since you obviously intended the while to be inside the else block.)
Anyway, this can't work because both your ifstream and your string are constant, i.e. cannot be changed. This does not make sense, and thus getline() was not defined for const parameters.
else if(j == i)
Since you didn't close the brace of the if statement above, this is a syntax error as well. Again, proper indenting discipline would have made this immediately obvious.
{ fich1>>s;
I very much doubt there exists an operator>>() for const ifstream, so I am not surprised you get an error here.
z=(float)s;
You're trying to cast (C style, too...) a stringstream object to a float number? What do you expect the result might be? It will definitely not be the 3.14 or whatever you've written into test.txt...
}
}
Lost track of your braces. This code is FUBAR, I didn't even check if it makes sense semantically once all the errors are fixed, and I suspect you're pulling some elaborate troll prank here. If that isn't the case, I suggest you take another good look at whatever textbook you're using to learn C++, because you got a good many things very wrong.
Purely style-related advice: fich1, ligne1... don't use localized (non-English) identifiers. It just adds another problem when communicating about your code. (And this is coming from a non-native English speaker.)
Related
I am trying to store binary data that should have the type of a std::complex< float > into a vector, through iterating over each element of the stream buffer. However I keep getting an error saying
no matching function for call to ‘std::istreambuf_iterator<std::complex<float> >::istreambuf_iterator(std::ifstream&)’
std::for_each(std::istreambuf_iterator<std::complex<float> >(i_f1),
I've tried searching for a solution but cannot find anything that would work. I am also trying to follow an example given in How to read entire stream into a std::vector? . Furthermore I'm compiling using g++ and -std=c++11.
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <cmath>
#include <boost/tuple/tuple.hpp>
#include <algorithm>
#include <iterator>
int main(){
//path to files
std::string data_path= "/$HOME/some_path/";
//file to be opened
std::string f_name1 = "ch1_d2.dat";
std::ifstream i_f1(data_path + f_name1, std::ios::binary);
if (!i_f1){
std::cout << "Error occurred reading file "<<f_name1 <<std::endl; std::cout << "Exiting" << std::endl;
return 0;
}
//Place buffer contents into vector
std::vector<std::complex<float> > data1;
std::for_each(std::istreambuf_iterator<std::complex<float> >(i_f1),
std::istreambuf_iterator<std::complex<float> >(),
[&data1](std::complex<float> vd){
data1.push_back(vd);
});
// Test to see if vector was read in correctly
for (auto i = data1.begin(); i != data1.end(); i++){
std::cout << *i << " ";
}
i_f1.close();
return 0;
}
I am quite lost at what I'm doing wrong, and am thus wondering why the
std::istreambuf_iterator()
does not accept the stream I am giving it as parameter?
Also the error message is confusing me as it seems to imply that I am calling the function in a wrong way, or a function that is non-existent.
Thanks
You want to read std::complex from i_f1 (which is a std::ifstream) using operator>> for std::complex, so you need a std::istream_iterator instead of std::istreambuf_iterator1:
std::for_each(std::istream_iterator<std::complex<float> >(i_f1),
std::istream_iterator<std::complex<float> >(),
[&data1](std::complex<float> vd){
data1.push_back(vd);
});
Your code can actually be simplified to:
std::vector<std::complex<float>> data1{
std::istream_iterator<std::complex<float>>(i_f1),
std::istream_iterator<std::complex<float>>()};
1 std::istreambuf_iterator is used to iterate character per character on, e.g., a std::basic_istream, not to iterate over it using overloads of operator>>.
You're probably using the wrong tool for the job.
You're trying to use a buffer iterator, which iterates over the constituent parts of a stream's buffer. But you're telling your computer that the buffer is one of complex<float>s … it isn't. An ifstream's buffer is of chars. Hence the constructor you're trying to use (one that takes an ifstream with a buffer of complex<float>) does not exist.
You can use an istream_iterator to perform a formatted iteration, i.e. to use the stream's magical powers (in this case, lexically interpreting input as complex<float>s) rather than directly accessing its underlying bytes.
You can read more on the previous question "the difference betwen istreambuf_iterator and istream_iterator".
The example you linked to does also go some way to explaining this.
It's quite clear from the code below that I am attempting to convert an int to a string.
#include <sstream>
#include <string>
#include <iostream>
int num = 1;
ostringstream convert;
convert << num;
string str = convert.str();
However, I get the error message
Line 7: error: expected constructor, destructor, or type conversion
before '<<' token
What I am doing wrong? This is basically the same snippet of code that everyone recommends to convert an int to a string.
There's 2 issues here, first you are missing main so subsequently this code is not valid at the top-level (eg outside main/functions/etc). When you compile your program the compiler looks for main then starts executing the code from that point onwards. There's a few things that are allowed before main but this expression is not one of them. The reason is because you are trying to compute something but the program flow never actually goes there, so how can the compiler decide when to execute that code? It matters what order that happens in and before main that order is not defined. That statement is not side-effect free so that's what the error message you posted is complaining about. The compiler looks for main as that's where code will start executing from so you want to put your code in main for that reason (I know this is more to this and it's not 100% accurate but I think this is a good starting point/heuristic for new programmers to get better understanding). You might want to read this question Is main() really start of a C++ program?
Secondly there's an issue with namespaces. ostringstream is in the std namespace, try std::ostringstream instead. The situation with string is similar, use std::string for that.
With these changes the code will end up looking something like this:
int main(){
int num = 1;
std::ostringstream convert;
convert << num; //This isn't allowed outside of main
std::string str = convert.str();
std::cout << str;
return 0;
}
#include <string>
#include <sstream> // std::stringstream
#include <iostream>
int main()
{
int num = 1;
std::stringstream convert;
convert << num;
std::string str = convert.str();
std::cout << str ;
return 0;
}
I'm wondering if there's a C++ way of opening a file and reading the input line by line.
I encountered the following code that accomplishes the task:
#include <iostream>
#include <fstream>
using namespace std;
int main () {
ifstream myfile;
myfile.open ("example.txt");
return 0;
}
I'm encouraged to not use any C functions or commands.
The thing is, my "example.txt" is in the form of a string, and using str.c_str() is a C function, so I guess I have two ways to solve the issue.
Is there another way to read input from a file line by line? Perhaps using something that will accept a string as a parameter for the filepath? Is there a C++ way of doing things? :)
Or, is there another way to convert the string in to a const char *, which is what the myfile.open() function needs?
Many thanks in advance!
EDIT: My lack of practivity and research led me to think c_str() was a C function, and it isn't. My apologies. Since it isn't I have found my answer.
C++11's fstream constructor accepts string. In most cases, you want to use fstream's constructor, rather than .open() - you save one line and one function call.
For reading the file line-by-line, you should use std::getline().
Also note that string::c_str() is still C++ function, not C one, as well as fstream's constructor taking const char *. Most of (if not all, I'm not 100% sure) C standard library function are also included in C++ standard.
Since the issue about str.c_str() is already answered, I'm just gonna add a bit about getting inputs line by line. for example, you wanna take 2 ints input per line, extract them, and put it into a vector.
fstream fs(filename.c_str(), ios_base::in);
string line;
stringstream ss;
int a,b;
vector<int> d;
int numlines;
int i;
for (i = 0; getline(fs, line); i++) {
for (ss.str(line); ss >> a >> b; d.push_back(a), d.push_back(b)) {}
ss.clear();
}
numlines = i;
Hope you get the idea of using getline() and fstream()
It's going to look very similar. You'll want an ifstream instead of an ofstream, you'll want the >> operator, and assuming your file has more than one line, you'll need a loop and the ifstream::feof() function.
EDIT - there are no blank lines.
EDIT 2 - my bad, it seems I was wrong all along. There is a blank line somewhere, although the csv file indicates otherwise. It's working now.
Okay so here is the entire (short) program:
#include <iostream>
#include <cctype>
#include <string>
#include <fstream>
using namespace std;
int main(){
char a;
string stringmanip;
fstream myfile("readthisfile.csv");
if (myfile.is_open()){
while ( myfile.good() ){
getline(myfile,stringmanip);
a=stringmanip[0];
}
myfile.close();
}
else cout<< "Unable to open file";
return 0;
}
It makes no sense to me. I can copy stringmanip, I can cout stringmanip, I can use .substr with stringmanip. If I define a regular string I can use the [] operation just fine. I've tried .at as well, but that only leads to another error. (Out of range).
Any help would be GREATLY appreciated. Sorry I'm such a beginner, as I'm sure you can tell.
Thanks,
Ben
If readthisfile.csv has an empty line at the end of the file (or anywhere in the file), then you will get back an empty string. You can't dereference the 0th character of an empty string. string::operator[] only accepts indices from 0 to string::length() - 1. If you have an empty string, any call to string::operator[] will result in undefined behavior.
This is so stupid. I've been stuck literally for an hour trying to read in a .txt file of numbers that are separated by a single whitespace. The while loops only gets executed once for some reason!
#include <iostream>
#include <string>
#include <fstream>
#include <stack>
using namespace std;
int main(int argc, char* argv[])
{
string line;
string str(argv[1]);
ifstream myfile((str).c_str());
int num;
stack<int> x;
while (myfile >> num);
{
x.push(num);
}
return(0);
}
Hmm, look at this line more closely:
while (myfile >> num);
Eventually, you'll notice the semi-colon. The compiler thinks this means you want a loop that does nothing (the semi-colon here indicates a single, empty statement). So, the loop reads in all the numbers, but does nothing with them.
The next section is interpreted separately as a statement in its own scope (denoted by the braces), to be executed after the loop:
{
x.push(num);
}
All that does is push the last number read onto the stack, leading you to think the loop only executes once.
Remove the ; and you're OK! Once bitten by this, you'll never forget ;-)
On an unrelated note, it's a bit silly to take argv[1] (a C-style string), put it into a string object, then use c_str() to turn that back into a C-string for the ifstream constructor. Just use argv[1] directly, since you're not doing anything else with it. Also, it would be a good idea to check argc first and make sure that a filename was passed in. Finally, you should check that the file was successfully opened instead of assuming it -- at the very least make your assumption explicit with an assert(myfile.is_open());. Oh, and you don't use the line variable at all.