I'm trying to get inputs from a file by overloading the istream operator. For that, I declared it as friend of a class. Then, I take as input that same class like this:
file >> *(class pointer);
When I'm trying to debug the part of my code that need this to work, it goes as expected into this:
istream& operator>> (istream& in, MYCLASS& n)
{
string buffer;
while (!in.eof()) { // input is a file
in >> buffer;
// do stuff
}
return in;
}
The problem is that the buffer stays empty ("") and does not take what it's suppose to be taking from the file. Normally, the format of the file should not be a problem since I'm using a similar method elsewhere in my code without a problem, but here it is in case:
* Name Age
* Name Age
* Name Age
...
What should I put inside my istream overload function so i get inputs as intended?
This...
while (!in.eof()) {
...is broken. You should attempt to read and parse data into your variables, then check for failure/eof. Do not assume that you'll necessarily be at the end of file after reading the last MYCLASS. Instead:
string name;
int age;
while (in >> name >> age)
...do stuff...
If you've really got some kind of leading dot on each line, add a char and read into it too:
char dot;
string name;
int age;
while (in >> dot && dot == '.' && in >> name >> age)
...do stuff...
More generally, it's not a very scalable model to assume the rest of the input stream will contain one MYCLASS object. You could instead have a delimiter (e.g. when the first word on a line is not a name, but <END>), that terminates the loop.
Related
I have a custom String class that contains a char array, and I need to overload the >> operator for this class. For the life of me I can't figure out how to do two things.
1 - Read the user input until a ; is reached
2 - Include whitespace from user input
I cannot use namespace or c++'s built-in string. Unfortunately that rules out the use of getline and any of the convenient find functions (I think it does, anyway?). Some things I have tried:
std::istream& operator >> (std::istream& output, String& input) {
output >> input.str;
return output;}
This works but only up until the first whitespace, after which point it stops reading the user input.
std::istream& operator >> (std::istream& output, String& input) {
while (output != ';') {
output >> input.str;
}
return output;}
An istream I guess isn't equivalent to the user input so you cannot compare it to a char like I tried to in my while loop.
So, my questions are, how does one read input until a specified character is encountered, and how does one include all whitespace when using >> ?
The global operator>> for string/character input stops reading when it encounters whitespace, so it is not worthwhile to implement your custom operator>> in terms of the global operator>>.
You ruled out use of std::getline(), but you can use std::istream::getline() instead. Like std::getline(), it also has an optional delim parameter (the default is '\n'), and will read characters - including whitespace - until the delimiter or EOF is reached.
std::istream& operator >> (std::istream& input, String& output)
{
return input.getline(output.str, yourmaxstrsize, ';');
}
Book.h:
#ifndef BOOKDATE
#define BOOKDATE
#include <iostream>
#include <string>
class Book{
friend std::istream& operator>>(std::istream&, Book&);
private:
std::string title, author;
int number;
};
std::istream& operator>>(std::istream&, Book&);
#endif // BOOKDATE
Book.cpp:
#include "BookDate.h"
using namespace std;
istream& operator>>(istream& is, Book& rhs){
getline(is, rhs.title);
getline(is, rhs.author);
is >> rhs.number;
if(!is)
rhs = Book();
return is;
}
I was wondering how exactly I should approach creating the input operator for the Book class. The title and author will be more than one word, so it fits that I need to use getline to receive that data. The issue then with getline is that it may pick up any '\n' left in the stream since cin was last used. For instance;
int x;
cin >> x; //newline is not extracted and left behind
Book a;
cin >> a; //"title" is automatically made empty!
I could instead use cin.ignore(256, '\n') but whose responsibility, the user's or class author's, is it to use this? Does the user use .ignore before he inputs a Book object or does the class author put .ignore at the beginning of the input operation?
It seems that in the former case the user would have to understand an .ignore method is needed but in doing so has to understand the implementation of the Book's input operator, which is not desirable. In the latter case, putting .ignore in the operator means my operator may not adapt to certain circumstances, since it always expects to encounter a newline before processing. For instance reading from an input file with data such as:
book1
author1
1
book2
author2
2
Means book1 gets ignored by cin.ignore(256,'\n').
To make your operator>> behave more like the operators for the built in types, you can use the ws manipulator to skip whitespace before you read your input.
Just use
is >> ws;
at the beginning of your input operator, and the stream will be positioned at the first non-whitespace character after the current position.
To overload the extraction operator properly you can change your input format to be a sequence of three variables that you want to populate, namely:
(title, author, number)
and modify your operator>> to:
istream& operator>>(istream& is, Book& rhs){
// just a suggestion: it is better if there is no input to do nothing
if(!is) return is;
string title, author;
int number;
char par1, comma, par2;
cin >> skipws >> par1 >> title >> comma >> author>> comma >> number >> par2;
if (par1 != '(' || comma != ',' || par1 != ')'){
// set failbit to indicate invalid input format
is.clear(ios_base::failbit);
}
rhs(title, author, number);
return is;
}
put is.ignore(); before getline(is, rhs.title);
I am trying to finish my lab, however I don't know how to allocate memory to a string. So I keep getting the error
warning: ‘_name’ is used uninitialized in this function [-Wuninitialized]
I don't also understand if my getline line is correct.
std::istream& operator>>(std::istream& is, Grade& RO){
int _mark;
char* _name;
std::cout<<"Subject name: ";
is.ignore();
is.getline(_name, (strlen(_name) + 1));
std::cout<<"Mark :";
is>> _mark;
RO=Grade(_name, _mark);
return is;
}
Ok #Jessica, (to general question and few info) I guess,
Grade is a class with two data members: int mark and string name. And you want to overload the insertion operator >> to populate these values.
(I recommend you leave all the cout expression outside this function). Here is one possible implementation:
istream& operator>> (istream& is, Grade& RO){
// declare local variables to insert the input values
int mark;
string name;
// extract values from input stream
is >> mark >> name;
// assuming you have member functions that set values for the object RO
RO.set_mark(mark);
RO.set_name(name);
return is;
}
I have a file called grades.txt with some lines of text and integers on it that i want to read into an already defined class ive created called Assignment.
int main()
{
ifstream input_file("grades.txt");
Assignment assignment;
input_file >> assignment;
return 0;
}
Above is my main function that will read input_file into the created class assignment.
friend istream& operator >> (istream& is, Assignment& assignment)
{ // function to read in data to class variables
string line;
getline(*****, line);
// to be able to operate on strings
istringstream iss(line);
// set values read in from input file.
iss >> assignment.Assignment_type;
iss >> assignment.Date;
iss >> assignment.Max_score;
iss >> assignment.Actual_score;
// sometimes Assignment Name will have spaces, have to use getline()
getline(is, assignment.Assignment_name);
return is;
}
Heres the class function that will overload the >> operator to read into each of the variables in assignment. the group of stars is what im having problems with, i dont know what to pass to it. I've tried ifstream and ofstream thinking it was that easy but they return the same error code
P01.cpp:34:21: error: expected primary-expression before ‘,’ token
I guess, you defined these operator >> inside the class Assignment. Only declare it as friend, but put the definition somewhere out: it is not a member of Assignment. The error is telling you that Operator >> take only one argument when defined as a member (the right operand, becouse the link is this).
Als "namespace" or global function it accept two arguments: link and right.
this is part of a homework assignment. I don't want an answer just help. I have to make a class called MyInt that can store any sized positive integer. I can only use cstring cctype iomanip and iostream libraries. I really don't understand even where to begin on this.
6) Create an overload of the extraction operator >> for reading integers from an input stream. This operator should ignore any leading white space before the number, then read consecutive digits until a non-digit is encountered (this is the same way that >> for a normal int works, so we want to make ours work the same way). This operator should only extract and store the digits in the object. The "first non-digit" encountered after the number may be part of the next input, so should not be extracted. You may assume that the first non-whitespace character in the input will be a digit. i.e. you do not have to error check for entry of an inappropriate type (like a letter) when you have asked for a number.
Example: Suppose the following code is executed, and the input typed is " 12345 7894H".
MyInt x, y;
char ch;
cin >> x >> y >> ch;
The value of x should now be 12345, the value of y should be 7894 and the value of ch should be 'H'.
The last state of my code is as follows:
istream& operator>>(istream& s, MyInt& N){
N.Resize(5);
N.currentSize=1;
char c;
int i = 0;
s >> c;
N.DigitArray[i++] = C2I(c);
N.currentSize++;
c = s.peek();
while(C2I(c) != -1){
s >> c;
if(N.currentSize >= N.maxSize)
N.Resize(N.maxSize + 5);
N.DigitArray[i] = C2I(c);
i++;
N.currentSize++;
}
}
It almost works! Now it grabs the right number but it doesn't end when I hit enter, I have to enter a letter for it to end.
You can create an operator>> overload for your class this way (as a free function, not inside the class):
std::istream& operator>>(std::istream& lhs, MyInt& rhs) {
// read from lhs into rhs
// then return lhs to allow chaining
return lhs;
}
You can use the members peek and read of istream to read in characters, and isspace to test if a character is a space, and isdigit to check if a character is a number (isspace and isdigit are in the <cctype> header).
First of all, your operator>> should be concerned only with extracting the sequence of chars from the stream and knowing when to stop based on your rules for that. Then, it should defer to a constructor of myInt to actually ingest that string. After all, that class will probably want to expose constructors like:
myInt bigone ("123456123451234123121");
for more general-purpose use, right? And, functions should have a single responsibility.
So your general form will be:
istream& operator>> (istream& is, myInt x)
{
string s = extract_digits_from_stream(is);
x = myInt(s);
return is; // chaining
}
Now how do you extract just digits from a stream and stop at a non-digit? Well, the peek function comes to mind, as does unget. I'd look at source code for the extraction operator for regular integers and see what it does.