I want to solve a programming contest task (C++ with XCode) which has a relatively big input (300 lines). When copying a test input into the console, it doesn't read it all.
So I have written a minimalistic test program to simply read in 300 lines:
#include <iostream>
using namespace std;
int main(int argc, const char * argv[])
{
ios_base::sync_with_stdio(false);
string xxx;
for(int i = 0; i < 300; i++)
cin >> xxx;
return 0;
}
If I execute it and copy 340 lines with "aaaaaaaaaa" into the console, it doesn't end. If I stop it with the debugger, it says i = 92. If I then continue, it quits.
But when I copy pieces of 50 lines into the console, it quits immeadiately as it should...
Can anyone help me with this?
PS: I inserted the 'ios_base::sync_with_stdio(false);', because I read that this would spped the input up.
I want to solve a programming contest task (C++ with XCode) which has a relatively big input (300 lines). When copying a test input into the console, it doesn't read it all
This is possible when you have more than one word per line because cin >> xx will read words as opposed to lines.
You would need to use the getline method to actually read lines.
while (getline(cin, xxx));
If I execute it and copy 340 lines with "aaaaaaaaaa" into the console, it doesn't end. If I stop it with the debugger, it says i = 92.
Yes, this is exactly the same symptom. Even if you had one word per line, you would reach only 300 lines, never 340.
Here is the whole code I would write for your reference:
#include <iostream>
using namespace std;
int main(int argc, const char * argv[])
{
ios_base::sync_with_stdio(false);
string xxx;
while (getline(cin, xxx))
cout << xxx << endl;
return 0;
}
cin >> string inputs words, not lines. Try:
string xxx;
while (getline(cin, xxx)) {
;
}
to input a full line at a time into xxx.
I couldn't solve the problem with X-Code but it works with Qt. So I use Qt now.
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.
There is a part of a code I wrote in C++:
int main(int ac, char **av)
{
std::string line;
while (std::getline(std::cin, line)) {
std::cout << "[" << line << "]\n";
}
return (0);
}
Code works properly when it comes to easy examples; it can read multiple lines (single lines in while loop) one by one and display them in terminal:
The problem appears when I try to copy paste two or more lines and print them on the screen; std::getline() blocks and is stuck, unable to read the input till the end (I can guess it's already been read and it's stored somewhere in buffer maybe?).
However, when I click enter it displays the input left, line by line, which is completely fine and desired since it - as the name points - reads one line at time.
Is it possible to unblock std::getline() somehow so it reads entire block of code?
I hope I explained it well, thanks for helping!
I have a bug with my code (the code at the end of the question). The purpose of my C++ executable is to read a file that contains numbers, copy it in a std::vector and
then just print the contents in the stdout? Where is the problem? (atoi?)
I have a simple text file that contains the following numbers (each line has one number)
mini01:algorithms ios$ cat numbers.txt
1
2
3
4
5
When I execute the program I receive one more line:
mini01:algorithms ios$ ./a.out
1
2
3
4
5
0
Why I get the 6th line in the stdout?
#include <iostream>
#include <string>
#include <fstream>
#include <vector>
using namespace std;
void algorithm(std::vector<int>& v) {
for(int i=0; i < v.size(); i++) {
cout << v[i] << endl;
}
}
int main(int argc, char **argv) {
string line;
std::vector<int> vector1;
ifstream myfile("numbers.txt");
if ( myfile.is_open()) {
while( myfile.good() )
{
getline(myfile, line);
vector1.push_back(atoi(line.c_str()));
}
myfile.close();
}
else {
cout << "Unable to open file" << endl;
}
algorithm(vector1);
return 0;
}
You should not use while (myfile.good()), as it will loop once to many.
Instead use
while (getline(...))
The reason you can't use the flags to check for looping, is that they don't get set until after an input/output operation notices the problem (error or end-of-file).
Don't use good() as the condition of your extraction loop. It does not accurately indicate whether the next read will succeed or not. Move your call to getline into the condition:
while(getline(myfile, line))
{
vector1.push_back(atoi(line.c_str()));
}
The reason it is failing in this particular case is because text files typically have an \n at the end of the file (that is not shown by text editors). When the last line is read, this \n is extracted from the stream. Yes, that may be the very last character in the file, but getline doesn't care to look any further than the \n it has extracted. It's done. It does not set the EOF flag or do anything else to cause good() to return false.
So at the next iteration, good() is still true, the loop continues and getline attempts to extract from the file. However, now there's nothing left to extract and you just get line set to an empty string. This then gets converted to an int and pushed into the vector1, giving you the extra value.
In fact, the only robust way to check if there is a problem with extraction is to check the stream's status bits after extracting. The easiest way to do this is to make the extraction itself the condition.
You read one too many lines, since the condition while is false AFTER you had a "bad read".
Welcome to the wonderful world of C++. Before we go to the bug first, I would advise you to drop the std:: namespace resolution before defining or declaring a vector as you already have
using namespace::std;
A second advise would be to use the pre increment operator ++i instead of i++ wherever feasible. You can see more details on that here.
Coming to your problem in itself, the issue is an empty new line being read at the end of file. A simple way to avoid this would be to check the length of line before using it.
getline(myfile, line);
if (line.size()) {
vector1.push_back(atoi(line.c_str()));
}
This would enable your program now to read a file interspersed with empty lines. To be further foolproof you can check the line read for presence of any non numeric characters before using atoi on it. However the best solution as mentioned would be use to read the line read to the loop evaluation.
im struggling with this part of code , no matter what i try i cant get it to read into a record after two lines
the text file contains
Mickey Mouse
12121
Goofy
24680
Andy Capp
01928
Quasi Modo
00041
end
and the code is
#include<iostream>
#include<string.h>
#include <stdio.h>
#include <windows.h>
#include<iomanip>
#include<conio.h>
#include<fstream>
#include<string>
using namespace std;
struct record
{
char name[20];
int number;
};
void main()
{
record credentials[30];
int row=0;
fstream textfile;//fstream variable
textfile.open("credentials.txt",ios::in);
textfile.getline (credentials[row].name,30);
//begin reading from test file, untill it reads end
while(0!=strcmp(credentials[row].name,"end"))
{
textfile>>credentials[row].number;
row++;
//read next name ....if its "end" loop will stop
textfile.getline (credentials[row].name,30);
}
textfile.close();
}
the record is only taking the first two lines and the rest is empty
any ideas ??
The problem is that:
textfile>>credentials[row].number;
while not consume the newline character. The subsequent call to textfile.getline() reads a blank line and the next:
textfile>>credentials[row].number;
attempts to read "Goofy" into an int which fails and set the failbit of the textfile stream meaning all further attempts to read fail. Check the return value to detect failure:
if (textfile >> credentials[row].number)
{
// Success.
}
I am not entirely sure how the program ends as "end" will never be read but I suspect it ends abnormally as there is no mechanism to prevent overruning the end of the credentials array (i.e no row < 30 as part of the loop terminating condition).
Other:
Instead of using a fixed sized char[] to read the names into you can use std::getline():
#include <string>
struct record
{
std::string name;
int number;
};
if (std::getline(textfile, credentials[row].name))
{
}
Instead of using a fixed sized record[] you could use a std::vector<record> which will grow as required.
I need to make a program in C++ that must read and write text files line by line with an specific format, but the problem is that in my PC I work in Windows, and in College they have Linux and I am having problems because of line endings are different in these OS.
I am new to C++ and don't know could I make my program able read the files no matter if they were written in Linux or Windows. Can anybody give me some hints? thanks!
The input is like this:
James White 34 45.5 10 black
Miguel Chavez 29 48.7 9 red
David McGuire 31 45.8 10 blue
Each line being a record of a struct of 6 variables.
Using the std::getline overload without the last (i.e. delimiter) parameter should take care of the end-of-line conversions automatically:
std::ifstream in("TheFile.txt");
std::string line;
while (std::getline(in, line)) {
// Do something with 'line'.
}
Here's a simple way to strip string of an extra "\r":
std::ifstream in("TheFile.txt");
std::string line;
std::getline(input, line));
if (line[line.size() - 1] == '\r')
line.resize(line.size() - 1);
If you can already read the files, just check for all of the newline characters like "\n" and "\r". I'm pretty sure that linux uses "\r\n" as the newline character.
You can read this page: http://en.wikipedia.org/wiki/Newline
and here is a list of all the ascii codes including the newline characters:
http://www.asciitable.com/
Edit: Linux uses "\n", Windows uses "\r\n", Mac uses "\r". Thanks to Seth Carnegie
Since the result will be CR LF, I would add something like the following to consume the extras if they exist. So once your have read you record call this before trying to read the next.
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
If you know the number of values you are going to read for each record you could simply use the ">>" method. For example:
fstream f("input.txt" std::ios::in);
string tempStr;
double tempVal;
for (number of records) {
// read the first name
f >> tempStr;
// read the last name
f >> tempStr;
// read the number
f >> tempVal;
// and so on.
}
Shouldn't that suffice ?
Hi I will give you the answer in stages. Please go trough in order to understand the code.
Stage 1: Design our program:
Our program based on the requirements should...:
...include a definition of a data type that would hold the data. i.e. our
structure of 6 variables.
...provide user interaction i.e. the user should be able to
provide the program, the file name and its location.
...be able to
open the chosen file.
...be able to read the file data and
write/save them into our structure.
...be able to close the file
after the data is read.
...be able to print out of the saved data.
Usually you should split your code into functions representing the above.
Stage 2: Create an array of the chosen structure to hold the data
...
#define MAX 10
...
strPersonData sTextData[MAX];
...
Stage 3: Enable user to give in both the file location and its name:
.......
string sFileName;
cout << "Enter a file name: ";
getline(cin,sFileName);
ifstream inFile(sFileName.c_str(),ios::in);
.....
->Note 1 for stage 3. The accepted format provided then by the user should be:
c:\\SomeFolder\\someTextFile.txt
We use two \ backslashes instead of one \, because we wish it to be treated as literal backslash.
->Note 2 for stage 3. We use ifstream i.e. input file stream because we want to read data from file. This
is expecting the file name as c-type string instead of a c++ string. For this reason we use:
..sFileName.c_str()..
Stage 4: Read all data of the chosen file:
...
while (!inFile.eof()) { //we loop while there is still data in the file to read
...
}
...
So finally the code is as follows:
#include <iostream>
#include <fstream>
#include <cstring>
#define MAX 10
using namespace std;
int main()
{
string sFileName;
struct strPersonData {
char c1stName[25];
char c2ndName[30];
int iAge;
double dSomeData1; //i had no idea what the next 2 numbers represent in your code :D
int iSomeDate2;
char cColor[20]; //i dont remember the lenghts of the different colors.. :D
};
strPersonData sTextData[MAX];
cout << "Enter a file name: ";
getline(cin,sFileName);
ifstream inFile(sFileName.c_str(),ios::in);
int i=0;
while (!inFile.eof()) { //loop while there is still data in the file
inFile >>sTextData[i].c1stName>>sTextData[i].c2ndName>>sTextData[i].iAge
>>sTextData[i].dSomeData1>>sTextData[i].iSomeDate2>>sTextData[i].cColor;
++i;
}
inFile.close();
cout << "Reading the file finished. See it yourself: \n"<< endl;
for (int j=0;j<i;j++) {
cout<<sTextData[j].c1stName<<"\t"<<sTextData[j].c2ndName
<<"\t"<<sTextData[j].iAge<<"\t"<<sTextData[j].dSomeData1
<<"\t"<<sTextData[j].iSomeDate2<<"\t"<<sTextData[j].cColor<<endl;
}
return 0;
}
I am going to give you some exercises now :D :D
1) In the last loop:
for (int j=0;j<i;j++) {
cout<<sTextData[j].c1stName<<"\t"<<sTextData[j].c2ndName
<<"\t"<<sTextData[j].iAge<<"\t"<<sTextData[j].dSomeData1
<<"\t"<<sTextData[j].iSomeDate2<<"\t"<<sTextData[j].cColor<<endl;}
Why do I use variable i instead of lets say MAX???
2) Could u change the program based on stage 1 on sth like:
int main(){
function1()
function2()
...
functionX()
...return 0;
}
I hope i helped...