Ok, so its been a while since i messed with reading and writing file and i have just about forgot everything i learned. So, i am currently just trying to figure out how to read specific lines from a text file and output that said line into the command prompt. Here is my code that i am having issues with:
#include <iostream>
#include <fstream>
using namespace std;
int main(){
ifstream input;
int lineN=0;
string line[lineN];
input.open("input.txt");
getline(input, line[lineN]);
cout << line[lineN];
}
As it currently is, it will read the first line of the text file no problem. However, if i change the variable lineN(which stands for line number) to 1 to read the second line, it crashes the prompt. I have no idea what it is i am doing wrong. I have tried researching this problem, but everyone's answer is too vague (That or i'm just too dumb). If you could help me out that would great.
The problem is that you define here an empty array of strings and arrays are not dynamic:
int lineN=0;
string line[lineN];
When you change lineN to 1, nothing changes in the array, and you'll get out of bound !
The bettter way would be to use vectors:
vector<string> line;
Read in a temporary string:
string current_line;
getline(input, current_line);
and add it to your vector:
line.push_back(current_line);
Putting all this in a nice loop would be more useful:
string current_line;
while (getline(input, current_line)) {
line.push_back(current_line);
}
You may access any line later, by using line[i] exactly with your array, as long as i< line.size(). Or you may iterate easily throug all its content:
for (string x : line) { // means for every x in line[]
cout<< x<<endl;
}
you allocate a array of size 0 ...
you will find answer of what will happen can be found here:
C++ new int[0] -- will it allocate memory?
Related
I began learning strings yesterday and wanted to manipulate it around by filling it with a text from a text file. However, upon filling it the cstring array only prints out the last word of the text file. I am a complete beginner, so I hope you can keep this beginner friendly. The lines I want to print from the file are:
"Hello World from UAE" - First line
"I like to program" - Second line
Now I did look around and eventually found a way and that is to use std::skipary or something like that but that did not print it the way I had envisioned, it prints letter by letter and skips each line in doing so.
here is my code:
#include <fstream>
#include <iostream>
#include <cstring>
#include <cctype>
using namespace std;
int main() {
ifstream myfile;
myfile.open("output.txt");
int vowels = 0, spaces = 0, upper = 0, lower = 0;
//check for error
if (myfile.fail()) {
cout << "Error opening file: ";
exit(1);
}
char statement[100];
while (!myfile.eof()) {
myfile >> statement;
}
for (int i = 0; i < 30; ++i) {
cout << statement << " ";
}
I'm not exactly sure what you try to do with output.txt's contents, but a clean way to read through a file's contents using C++ Strings goes like this:
if (std::ifstream in("output.txt"); in.good()) {
for (std::string line; std::getline(in, line); ) {
// do something with line
std::cout << line << '\n';
}
}
You wouldn't want to use char[] for that, in fact raw char arrays are hardly ever useful in modern C++.
Also - As you can see, it's much more concise to check if the stream is good than checking for std::ifstream::fail() and std::ifstream::eof(). Be optimistic! :)
Whenever you encounter output issues - either wrong or no output, the best practise is to add print (cout) statements wherever data change is occurring.
So I first modified your code as follows:
while (!myfile.eof()) {
myfile >> statement;
std::cout<<statement;
}
This way, the output I got was - all lines are printed but the last line gets printed twice.
So,
We understood that data is being read correctly and stored in statement.
This raises 2 questions. One is your question, other is why last line is printed twice.
To answer your question exactly, in every loop iteration, you're reading the text completely into statement. You're overwriting existing value. So whatever value you read last is only stored.
Once you fix that, you might come across the second question. It's very common and I myself came across that issue long back. So I'm gonna answer that as well.
Let's say your file has 3 lines:
line1
line2
line3
Initially your file control (pointer) is at the beginning, exactly where line 1 starts. After iterations when it comes to line3, we know it's last line as we input the data. But the loop control doesn't know that. For all it knows, there could be a million more lines. Only after it enters the loop condition THE NEXT TIME will it come to know that the file has ended. So the final value will be printed twice.
I've been scouring the forums and internet trying to find a solution to my problem but I can't seem to find anything that relates to me specifically. As a disclaimer, while this relates to homework, the question I'm asking isn't how to do it, it's how to fix a problem in what I already have done.
Anyway, this project is to get an indeterminate amount of values from a file, store them in variables, and output them in a "receipt" layout. I decided the best way to do this would be structs and vectors
struct Item {
string itemName;
double itemPrice;
bool taxable;
}
vector<Item> itemList;
Now that I have my struct and my vector set up in my program, I'm trying to read from the file using getlines.
int vectorNumber = 0;
while(!inFile.eof()){
itemList.resize(vectorNumber+1);
string tempDouble;
string tempBool;
getline(inFile, itemList[vectorNumber].itemName);
getline(inFile, tempDouble);
itemList[vectorNumber].itemPrice = stod(tempDouble);
getline(inFile, tempBool);
itemList[vectorNumber].taxable = tempBool == "Y" ? true : false;
vectorNumber++;
}
The reason I'm doing itemList.resize(vectorNumber + 1) is because I was getting a vector subscript out of range on the first getLine. I figured it was because the vector was blank, but I don't know for sure. Anyway, after I added that line, the program was able to move on, but now on the line with the stod, I'm getting an Unhandled exception at [hexValue] in [exe]: std::invalid_argument at memory location [hexValue].
I added a temporary cout << tempDouble before the faulty stod line to see if it was actually pulling a value, and it is. So my question is why am I getting this weird error?
If I comment out everything from the second getline to the ?: line, the program runs successfully, so it's just this one thing I need help fixing. What am I doing wrong? The value it's pulling from the file (I checked it using cout) is 2.49, with an endline character after it. I know getline discards the endline character, but I figured I'd just tell you.
Thank you!
EDIT: I've been asked to see a minimal version of the code that still gives me problems, so here we are:
// TestApplication.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <vector>
#include <string>
using namespace std;
struct Item {
string itemName;
double itemPrice;
};
vector<Item> itemList;
int main()
{
ifstream inFile;
inFile.open("HW3_Data.txt");
int vectorNumber = 0;
while (!inFile.eof()) {
itemList.resize(vectorNumber + 1);
string tempDouble;
string tempBool;
getline(inFile, itemList[vectorNumber].itemName);
getline(inFile, tempDouble);
itemList[vectorNumber].itemPrice = stod(tempDouble);
vectorNumber++;
}
}
Because I have a feeling this has to do with the file itself and not the program, here is the file:
Bread
2.49
Y
Milk
1.89
N
Eggs, dozen
0.97
N
Apples
4.75
Y
Bananas
1.69
Y
Peanut Butter
2.49
Y
I copied and pasted the file straight into here. There is another line underneath the Y in the last line of the file, that is not an extra return I added in.
I'm trying to write my own vocabulary with a test for my little brother, but I have a problem when I want to read data from file into two arrays - first with English words, and second with Polish words. File looks alike
black - czarny
red - czerwony etc.
And my function:
void VOC::readout()
{
fstream file;
VOC *arr = new VOC;
string line;
file.open("slowka.txt");
if(file.good())
{
int i=0;
while(!file.eof())
{
getline(file, line);
size_t pos = line.find(" - ");
int position = static_cast<int>(pos);
file>>arr[i].en;
file>>arr[i].pl;
++i;
}
}
}
I thought it could be a good idea to insert a line into first array until the function finds " - ", and after that insert the rest of line into second array, but I have some problems with that. Could someone help me? I know I can solve it by using std::vector but I care to do that by using arrays.
If you insist on using plain arrays, you'll first have to count the number of lines in your file and then allocate enough memory. Arrays -- unlike std::vector objects -- won't grow automatically but have a fixed size.
That being said, note that using !file.eof() is not the best way to read a stream until the end is reached. You can use the simpler
std::string line;
while (std::getline(file, line)) {
// ...
}
idiom instead, which also takes care of error conditions. See this question (and corresponding answers) for more information on that.
I have 4 days of training in C++, so bear with me.
Two data files are required to evaluate a multiple-choice examination. The first file
(booklet.dat) contains the correct answers. The total number of questions is 50. A sample
file is given below:
ACBAADDBCBDDAACDBACCABDCABCCBDDABCACABABABCBDBAABD
The second file (answer.dat) contains the students’ answers. Each line has one student
record that contains the following information:
The student’s answers (a total of 50 answers) in the same format as above (* indicates no answer)., followed by Student ID and Student Name. Example:
AACCBDBC*DBCBDAAABDBCBDBAA*BCBDD*BABDBCDAABDCBDBDA 6555 MAHMUT
CBBDBC*BDBDBDBABABABBBBBABBABBBBD*BBBCBBDBABBBDC** 6448 SINAN
ACB*ADDBCBDDAACDBACCABDCABCCBDDABCACABABABCBDBAABD 6559 CAGIL
I have a homework assignment to write a C++ program that counts the total number of correct answers by each student and outputs this information to another file called report.dat. In this file, the student’s IDs, names and scores must be given. Each correct answer is worth 1 point. For the sample files given above, the output should be as follows:
6555 MAHMUT 10
6448 SINAN 12
6550 CAGIL 49
Here's what I have so far:
include <iostream>
include <fstream>
using namespace std;
int main()
{
char booklet[50] answers[50]
int counter
// Link answers with booklet.dat
booklet = ifstream
input_file("booklet.dat");
return 0;
// Link answers with answers.dat
answers = ifstream
input_file("answer.dat");
return 0;
while (booklet==answers)
{
counter++
cout << "The student had">>counter>> "answers right";
}
}
I'm not even sure I am in the correct direction. I know I need to create an array from the file booklet.dat and another one from the file answer.dat. Then the comparison has to be made and the matches between the two have to be counted.
I don't expect anyone to do the assignment for me, i just need a nudge in the right direction.
1.) On your Syntax:
a) Each line in C++ has to end with an ";". There are some lines in your excample which don't. (Normally your compile should point at this or the following line with an error)
b) Multiple variable definitions need a "," in between two different variables.
2.) I would recommend you to use something like that:
(have a look at C++ Reference fstream)
EDIT: just a little outline, which is not complete in this form, just to give you and idea ;-)
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
int main()
{
int nr_of_students = 1000; /* Or any number you'd like to analyze */
int stud_nr[nr_of_students];
string stud_name[nr_of_students];
int stud_count[nr_of_students];
fstream in_out;
in_out.open("filename.dat",fstream::in); // fstream::in for reading from file
// fstream::out for writing to this file
if(in_out.is_open())
{
for(lines=0;(in_out>>answers && lines<nr_of_students);lines++)
{
in_out >> stud_nr[lines]; /* EDIT: sorry hat some index confusions here... */
in_out >> stud_name[lines];
stud_count[lines]=0;
for(int i=0;i<50;i++)
{
/* comparison between the booklet_array and the answers_array */
/* Count up the stud_count[lines] for each right comparison */
}
}
/* some simmilar code for the output-file */
}
else cout << "Error reading " << "filename.dat" << endl;
return 1;
}
3.) Your code would also get more performance with vectors.
A good Tutorial would be: Tutorial part I
and you find part 2 in the comments there
4.) you can achieve a more dynamic code with argc and argv**, just google for that
I hope these comments help you a little bit to carry on ;)
You are already on the right direction. Basically you want to load the answer key into an array for fast comparison and then you need to check the answers of each student and each time they get a correct answer you increment a counter and write the ID, name and score for each student. There are problems with your code such as missing semicolons.
Also please note that returning exits a function and that no statements after an unconditional return are executed, returning from main terminates your program.
The normal approach to open a file for reading is:
#include<fstream>
#include<string>
int main()
{
std::ifstream input_file("inputfilename");
// since the answer key is one line
// and each students answer , id and name are also one line
// getting that line using std::getline() would be sufficient
std::string line;
std::getline(input_file, line);
// line would now contain the entire first line except the newline character
std::getline(input_file, line);
//now line would now contain the second line in the file
return 0;
}
Writing to a file is similar we use ofstream to open a file for writing.
Like so:
#include<fstream>
int main()
{
std::ofstream output_file("outputfilename");
// lets say we have a string and an int that we want to write
std::string line_to_write("Hello File");
int number = 42;
output_file << line_to_write << number; // writes the string and then 42 on the same line
output_file << '\n'; // writes the newline character so that next writes would appear on another line
return 0;
}
For references to the standard library and C++ in general when you need to know the available functions to do something I recommend cppreference here are the specific pages on ifstream and ofstream.
I’m getting system error when I try to compile the code below on Visual C++ 2008 Express. What I’m trying to do is to initialize array of objects with data read from file. I think there is something wrong inside the while loop, because when I initialize these objects manually without the while loop it seems to work. Here is the code and text file:
#include <iostream>
#include <string>
#include "Book.h"
using namespace std;
int main()
{
const int arraySize = 3;
int indexOfArray = 0;
Book bookList[arraySize];
double tempPrice;//temporary stores price
string tempStr;//temporary stores author, title
fstream fileIn( "books.txt" );
while ( !fileIn.eof( ))
{
getline(fileIn,tempStr);
bookList[indexOfArray].setAuthor(tempStr);
getline(fileIn,tempStr);
bookList[indexOfArray].setTitle(tempStr);
fileIn >> tempPrice;
bookList[indexOfArray].setPrice(tempPrice);
if ( indexOfArray < arraySize ) //shifting array index while not exceeding array size
indexOfArray++;
}
fileIn.close();
return 0;
}
and the text file:
Author1
Book1
23.99
Author2
Book2
10.99
Autho3
Book3
14.56
It looks like you are trying to write to bookList[3] in the loop. You will loop through three times filling your array incrementing indexOfArray each time. This will leave indexOfArray at 3 -- your condition as it is written will allow indexOfAray to be incremented to 3. Then if you have a newline after the "14.56" in your data file you will loop one more time and attempt to pass an empty string to bookList[indexOfArray].setAuthor() leading to a segfault since indexOfArray is past the end of the array.
I would suggest ditching the hard-coded array and using a std::vector instead. At the start of each loop just use push_back() to add a new book to the end of the vector and then use back() to access the new element in the array.
There's another run-time error in your code: You don't read an entire line with the call to fileIn >> tempPrice;. The next call to getline() will read to the end of the line, so you'll get an empty string when you're expecting an author.
You're then off by one line in your text file, and you try to convert a title into a double. That make the fstream signal an error, and after that, you're in trouble.
Brett's right, a vector with push_back is a better solution here.
Brett also correctly pointed out that you could run into errors if your file has extra lines. You can fix that by checking if you successfully read from the file:
if(fileIn >> tempPrice)
{
bookList[indexOfArray].setPrice(tempPrice);
}
else
{
break;
}
if(!getline(fileIn,tempStr))
{
break;
}
The key must be in the contents of
#include "Book.h"
I copy-pasted your code, and replaced the #include with my assumption of what class Book might look like:
class Book
{
std::string auth;
std::string title;
double price;
public:
void setAuthor(std::string& str)
{
auth = str;
}
void setTitle(std::string& t)
{
title = t;
}
void setPrice(double d)
{
d = price;
}
};
and it compiled. Perhaps you could share your Book.h, or look there for any problems? Start with some simple definition from Book (like above) and begin readding code until you've found the lines that cause the problem. Its a crude method of figuring out the issue, but sometimes its the most direct way.