I've picked up a book on C++ and I'm basically at the very beginning of it (just started). For some of the problems I had to solve within the book I used the input stream cin the following way -->
cin >> insterVariableNameHere;
But then I did some research and found out the cin can cause a lot of problems, and so found out about the function getline() within the header file sstream.
I'm just having some trouble trying to wrap my head around what's happening in the following code. I don't see anything that uses the extraction operator (>>) to store the number value in. Its (my problem) further explained in the comments I left.
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
// Program that allows a user to change the value stored in an element in an array
int main()
{
string input = "";
const int ARRAY_LENGTH = 5;
int MyNumbers[ARRAY_LENGTH] = { 0 };
// WHERE THE CONFUSION STARTS
cout << "Enter index of the element to be changed: ";
int nElementIndex = 0;
while (true) {
getline(cin, input); // Okay so here its extracting data from the input stream cin and storing it in input
stringstream myStream(input); // I have no idea whats happening here, probably where it converts string to number
if (myStream >> nElementIndex) // In no preceding line does it actually extract anything from input and store it in nElementIndex ?
break; // Stops the loop
cout << "Invalid number, try again" << endl;
}
// WHERE THE CONFUSION ENDS
cout << "Enter new value for element " << nElementIndex + 1 << " at index " << nElementIndex << ":";
cin >> MyNumbers[nElementIndex];
cout << "\nThe new value for element " << nElementIndex + 1 << " is " << MyNumbers[nElementIndex] << "\n";
cin.get();
return 0;
}
stringstream myStream(input): Creates a new stream that uses the string in input as "input stream" so to speak.
if(myStream >> nElementIndex) {...): Extracts number from the stringstream created using the line above into nElementIndex and executes ... because the expression returns myStream, which should be non-zero.
You were probably confused by using the extraction as the condition in the if statement. The above should be equivalent to:
myStream>>nElementIndex; // extract nElement Index from myStream
if(myStream)
{
....
}
What you probably wanted was
myStream>>nElementIndex; // extract nElement Index from myStream
if(nElementIndex)
{
....
}
Related
I am not sure where I put the char command declaration where I wont get a "Not defined in this scope" error and it will loop through accepting a new char command the next time the program loops.
I tried putting it inside of the do loop but then it said that char was not defined in that scope, I then put it right after the int main function and when entering A as a command it infinitely loops my add_entry function without allowing user input.
Do I have to pass by reference maybe? Or pass by value?
My file that holds all function definitions
#include "main.h"
using namespace std;
int syntax::add_entry()
{
cout << "Enter a concept name: ";
cin.get(name, SIZE);
cout << endl << "Enter an example of the syntax: ";
cin.get(example,SIZE);
cout << endl << "Enter a description of the syntax: ";
cin.get(desc,SIZE);
cout << endl << "Enter a difficulty rating from 1-10: ";
cin.get(diff,SIZE);
cout << endl << "Enter a usefulness rating from 1-10: ";
cin.get(use,SIZE);
//open and write to the file
ofstream myfile;
myfile.open("data.txt");
myfile << "Name: " << name << endl;
myfile << "Example of syntax: " << example << endl;
myfile << "Description of syntax: " << desc << endl;
myfile << "Difficulty rating from 1-10: " << diff << endl;
myfile << "Usefulness rating from 1-10: " << use << endl;
myfile.close();
return 0;
}
int syntax::display_entry()
{
ifstream myfile("data.txt");
/*
char name[SIZE];
char example[SIZE];
char desc[SIZE];
char diff[SIZE];
char use[SIZE];
*/
if(myfile.is_open())
{
while(myfile >> name >> example >> desc >> diff >> use)
{
std::cout << name << ", " << example << ", " << desc << ", " << diff << ", " << use;
}
myfile.close();
}else
cout << "File is not open" << endl;
std::cin.get();
return 0;
}
my main .cpp file
#include "main.h"
using namespace std;
int main()
{
char command;
syntax c;
do{
cout << "Welcome to the C++ concept syntax user database." << endl;
cout << "Choose one of the following commands: " << endl;
cout << endl << endl;
cout << "A) Add a new entry B) Display all entrys C) Search for difficulty D) Exit: ";
cin >> command;
cout << endl;
if(command == 'A' || command == 'a')
{
c.add_entry();
}
else if(command == 'B' || command == 'b')
{
c.display_entry();
}
else if(command == 'D' || command == 'd')
{
cout << "Quitting program, Thank you for using" << endl;
}
}while(command != 'D' || command != 'd');
return 0;
}
my .h file
#include <iostream>
#include <fstream>
#include <cstring>
#include <cctype>
using namespace std;
class syntax
{
public:
const static int SIZE = 50;
char name[SIZE];
char example[SIZE];
char desc[SIZE];
char diff[SIZE];
char use[SIZE];
int display_entry();
int add_entry();
private:
};
You need to read and understand about "formatted input" and "unformatted input". Please check here.
Formatted input is done using the extractor operator >>. It reads characters from a stream and formats them to the expected variable type. If you write int x; std::cin >> x and you enter the number 12, so the digits/characters '1' and '2', your input will be formatted / converted to an integer value 12.
It is important to understand that formatted input
ignores leading white space
stops any conversion when encountering white space (but does not extract it from the stream)
Meaning, if you enter 12 and then press the enter-key, the characters '1' and '2' will be extracted from the stream, but the newline 'n' will not be consumed or extracted from the screen and is still available.
This default behavior can be addapted by setting certain flags.
Now, if we look at "unformatted input" functions, like get, it will read all kind of characters, including spaces and so on until it hits the specified delimiter, which is '\n' per default. For the get function, the delimiter '\n' will not be extracted. So, it is still in the stream. This is in contrast to the getline function which would extract the '\n' from the stream (but not store it).
All this you can read in the linked description.
And now, the root cause for all you problems, is also written in the description:
If no characters were extracted, calls setstate(failbit)
Then, let us look on the order of events
You enter a 'a', becuase you want to add an entry
The 'a' will be extracted and the '\n' is still in the stream
In function "add_entry" you call "get"
Get will try to read charcters, until it finds a newline '\n'
But, as a leftover from the previous operation, it will immediately see the '\n' , and hence store no other data at all, and consequently sets the failbit of the stream. All the following calls to std::cin will do nothing, because the failbit of the stream is set.
The functions returns to main and the failbit is still set
The next call cin >> command; will do nothing and will especially not modify the "command" variable. This will still contain an 'a'
And then the loop runs forever
You have an additional bug in the "while" condition. This must be corrected to: ´while (command != 'D' && command != 'd');´
Now, what to do.
First, and very important, for any IO-function you need to check, if it worked or not. There are functions to read the iostate of the stream. But c++ makes life easier. The bool-operator and the not-operator are overwritten and will return state information. If you want to know, if any IO operation was successful, the you can write something like if (std::cin) ....
Very convenient. But must be used.
And since IO operations return mostly a reference to the stream for which they were called, you can write if (cin >> command) . . . . This will first call the extraction operator. This will return a reference to the stream and for that you can use an if statement, because of the overwritten bool-operator.
But how to overcome the nasty problem with the '\n' in the stream, which is often there? There are basically 2 functionalities:
Function ignore. Will ignore all/a number of characters, until a delimiter is hit.
Function/manipulator std::ws. Will eat all white spaces.
I recommend to add one time cin >> std::ws; at the top of your "add_entry" routine and then you must change all get functions to getline. If not, you would need to add std::ws before each get statement.
And again, for each IO function, check the status! For example if (!cin.getline(example, SIZE)) .... do something, show error
And in the future. For any transition from formatted to unformatted input, use std::ws
And, do never forget to read the documentation carefully.
Have fun!
I'm trying to learn C++ to help my sibling with their assignment. So I'm attempting the assignment. It's a simple program to load a dictionary test file with words, their type, and definition to an array of Word type objects. I was able to get started with a normal string array instead of an object array as requested. But as soon as I defined the Word class and the array the code builds without an issue. When I try to run the code the cursor simply blinks for a few seconds and returns to the normal terminal.
Am I doing something wrong with my Class constructor ??
#include <fstream>
#include <string>
using namespace std;
class Word {
public:
string WordEntry;
string Type;
string Definition;
//constructor
Word(string word, string type, string definition){
WordEntry=word;
Type=type;
Definition=definition;
}
};
int main(){
cout << "Test1";
Word *wordArray[318555];
int count=0;
string word, type, definition,blank;
cout << "TEST" << count << "\n";
ifstream file("dictionary2021 (1).txt");
if (file.is_open()){
cout << "File dictionary2021.txt has been opened \n";
while (!file.eof()){
getline(file,word);
getline(file,type);
getline(file,definition);
getline(file,blank);
wordArray[count]= new Word(word,type,definition);
count++;
}
file.close();
cout << "File dictionary2021.txt has " << count/3 << " entries\n";
}
cout << "TEST" << count << endl;
cout << cc;
int selection;
string input;
cout << "Function List - Please hit Enter after your selection \n";
cout << " 1. Word Search \n 2. Repetitive z search \n 3. Wild Card Search\n";
cout << "Selection:";
cin >> selection;
if(selection=1){
cout << "Enter word:\n";
cin >> input;
string str("a");
for (int i = 0; i < 12; i+3)
{
cout << "1";
if (input.compare(str)== 0)
{
cout << wordArray[i+1];
return 0;
}
cout << "2";
}
}
}```
Word* wordArray[318555]; is a huge value and we're talking about 2548440 bytes (or roughly 2.4MB). This might be too large for a single stack frame and can easily be inefficient.
What I suggest is to use std::vector to store the word array and use std::vector<>::push_back() method to insert data to it.
Note: In your code snippet your not deallocating the Word object pointers once everything is done. Either explicitly delete those pointers using delete or use a smart pointer like std::unique_ptr.
Disclaimer: I am a beginner to programming, so what I say might sound really stupid
I have to make a "Telephone Directory" for school. The program isn't complete, but there are some things that I need to fix before moving on. The array TelephoneNumbers either isn't storing the numbers from the file correctly, or isn't displaying them. For the SeaerchRecords function, the first number in the file is displayed correctly, the second is displayed as "2147483647," and the rest of the numbers display as "0." The modify function also doesn't change the number, and I confirmed this with the while in the function. The string array works perfectly fine, however. May someone explain what I'm doing incorrectly?
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
string TelephoneNames[100];
int TelephoneNumbers[100];
void ModifyRecords(); //Function to Modify Records
void SearchRecords(); //Function to Search Records
void DeleteRecords(); //Function to Delete Records
int main()
{
fstream inputFile;
fstream outputFile;
char choice;
inputFile.open("Telephone Names.txt"); //To store
for (int count=0;count<100;count++) //file names
{ //into a
inputFile >> TelephoneNames[count]; //string
}
inputFile.close();
inputFile.open("Telephone Numbers.txt");//To store
for (int count=0;count<100;count++) //file #'s
{ //into a
inputFile >> TelephoneNumbers[count];//string
}
inputFile.close();
//Display options available
cout << " Hello, do you want to:\n";
cout << " ======================\n";
cout << "-Modify Records|Enter M\n";
cout << "-Search Records|Enter S\n";
cout << "-Delete Records|Enter D\n";
//Store choice
cin >> choice;
//Send to different function
if (choice=='M'||choice=='m')
{
ModifyRecords();
}
if (choice=='S'||choice=='s')
{
SearchRecords();
}
return 0;
}
void ModifyRecords()
{
string name;
string newname;
int newnumber;
int count=0;
cout << "Enter the name of the person: ";
cin >> name;
for (count=0;TelephoneNames[count]!=name;count++)//To determine where in the strings the new numbers need to be
{
}
cout << "Enter the new name of the person: ";
cin >> newname;
cout << "Enter the new number of the person: ";
cin >> newnumber;
TelephoneNames[count]={newname};
TelephoneNumbers[count]={newnumber};
count=0;
while (count<6)
{
cout << TelephoneNames[count] << endl;
cout << TelephoneNumbers[count] << endl;
cout << endl;
count++;
}
}
void SearchRecords()
{
string name;
int count=0;
cout << "Enter the name of the person you would like to find: ";
cin >> name;
for (count=0;TelephoneNames[count]!=name;count++)//To determine where in the strings the new numbers need to be
{
}
cout << "Name: " << TelephoneNames[count] << endl;
cout << "Number: " << TelephoneNumbers[count] << endl;
}
Since there is no any answer still and I don't see exactly the problem at this point I'll provide some suggestions how you can find a problem in your code.
In any programming situation when you can't find a bug, first task is to locate it as much precisely as you can and check all input data and assumptions. Usually, debugger is used for such purposes, but you can just output text in console before creating final version of your program.
To start with, you must check that you really received names and telephones from your file:
inputFile.open("Telephone Names.txt"); //To store
for (int count=0;count<100;count++) //file names
{ //into a
inputFile >> TelephoneNames[count]; //string
cout << TelephoneNames[count] << endl; //WE MUST SEE WHAT IS REALLY STORED IN TelephoneNames
}
inputFile.close();
inputFile.open("Telephone Numbers.txt");//To store
for (int count=0;count<100;count++) //file #'s
{ //into a
inputFile >> TelephoneNumbers[count];//string
cout << TelephoneNumbers[count] << endl; //WE MUST SEE WHAT IS REALLY STORED IN TelephoneNumbers
}
inputFile.close();
Ok, when it is checked and you are defenitely sure there is no problem in your data we can move to SeaerchRecords function doing the same procedure. We must check what is happening while you are searching:
for (count=0;TelephoneNames[count]!=name;count++)//To determine where in the strings the new numbers need to be
{
cout << "Search step: " << count << " name " << name << " found name " << TelephoneNames[count] << " number " << TelephoneNumbers[count] << endl;
}
Doing so you will locate your bug rather quickly. The problem can be in input files format, in difference of "name" and stored names format etc.
I'll provide several additional suggestion how you can improve your code.
1) Try to use const declarations for such commonly used things as number of records (const int NUMBER_OF_RECORDS = 100; insted of just putting '100' everywhere), it will reduce the amout of work and possible bugs. 2) Try to check all possible problems that you program can encounter if someting is wrong with data. What will happen if you have less than 100 records in your files now? Program crush or silent reading of unappropriate data which is even worse. Check that you haven't reach file end on any step of reading along with current check that you've reached you number of records and do something in case of unappropriate data.
3) Check the possible problems with conditions in your cycles not to run them infinite number of times. Now your condition for(count=0;TelephoneNames[count]!=name;count++)
will execute forever if there is no such name or just crush the program on count 100 or more. You should check that count doesn't exceed that value. Good luck!
I am trying to read data from a text file formatted similarly to this:
knife, object, 0
bag, object, 15
kitchen, room, 400
Into an array composed of structures. Here is what I have so far, but it only reads the first element then returns garbage.
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
struct itemlist
{
string type;
string selltype;
int price;
int diditsell=0;
};
int main()
{
string filename;
cout << "Please enter a file name. " << endl;
cin >> filename;
ifstream in(filename);
itemlist c[100];
for (int i=0;i<100;i++)
{
in >> c[i].type >> c[i].selltype >> c[i].price;
cout << c[i].type << endl;
cout << c[i].selltype << endl;
cout << c[i].price << endl;
}
}
I have tried to find examples that specifically suit what I am trying to do but implementing them has not fixed the problem. Any help would be greatly appreciated.
The crux of the visible problem is that with
for (int i=0;i<100;i++)
the entire 100 element array will be printed out whether there was data in the file to be loaded into the array or not.
Probably the easiest way to do this is with a std::vector. It's a dynamically sized array. As you add to it it gets bigger so you don't have to worry about it overflowing. We'll get back to it at the end.
The next thing you have to do is make sure you're reading the file successfully. Streams can be tested to see if they are valid.
if (in)
{
cout << "in is good!" << endl;
}
and the >> operator returns a reference to the stream so you can
if (in >> data)
{
cout << "data is good!" << endl;
}
If the stream is still good after reading data, you know that at the very least the file read something into data that was of the correct type or could be converted into the correct type. You owe it to yourself to check the value read after reading it in to make sure the user didn't typo or go out of their way to crash the program. If you want to loop through a lot of stuff, like a file, you wind up with something like this:
while (in >> c[i].type >> c[i].selltype >> c[i].price)
If any of the reads failed the the stream will return false when tested and the loop will exit.
Looking at your source data you have spaces and commas to deal with. >> only knows how to deal with spaces unless you're going to do a lot of extra work. What you will read in is
knife,
object,
0
and we don't want the comma. Fortunately, it's the last character so dealing with it is easy. A C++11 std::string can be used like a stack and you can just pop the unwanted character off:
c[i].type.pop_back();
c[i].selltype.pop_back();
All together, this gives us a loop that looks like
ifstream in(filename);
itemlist c[100];
int i = 0;
while (in >> c[i].type >> c[i].selltype >> c[i].price)
{
c[i].type.pop_back();
c[i].selltype.pop_back();
cout << c[i].type << endl;
cout << c[i].selltype << endl;
cout << c[i].price << endl;
i++;
}
but this can overrun the end of the 100 element array, so we need to change the while loop slightly:
while (i < 100 && in >> c[i].type >> c[i].selltype >> c[i].price )
If i is greater than or equal to 100, the i < 100 case fails and the loop exits without even trying in >> c[i].type >> c[i].selltype >> c[i].price and writing into the non-existent array slot.
Remember to keep the value of i around because arrays are dumb. They don't know how full they are.
But with a vector you don't need i to count or to keep track of how full it is and you don't need to worry about overflowing the array until you run your computer out of RAM. What we do need is one temporary variable to read into and we're good to go.
vector<itemlist> c;
itemlist temp;
while (in >> temp.type >> temp.selltype >> temp.price)
{
temp.type.pop_back();
temp.selltype.pop_back();
cout << temp.type << endl;
cout << temp.selltype << endl;
cout << temp.price << endl;
c.push_back(temp);
}
I had the same problem.
A debug showed that it was reading the first array element but skipping to the second element and outputting the info. from the first element.
This was fixed by making it read the first element twice.
For example see below.
I had other input in the array for the player also.
After that line was added everything worked great.
I had to do that for every array that I read.
I looked at the text file it was reading from and sure enough
there is a blank line before the start of every array.
I do not know why the program writing the file did that.
I did not put a blank line before the array.
Note: Instead of having it read the first array element twice,
you could probably have it read a blank line instead.
for (int i = 0; i < PLAYER; i++)
{
getline(teamRosterIn, playerName[i]);
cout << playerName[i] << endl;
getline(teamRosterIn, playerName[i]);
cout << playerName[i] << endl;
}
Consider the following code:
#include <sstream>
#include <iostream>
using namespace std;
int main()
{
stringstream ss;
ss << string("12");
int h;
ss >> h;
cout << h << endl;
ss.str(string("")); // clear the content of ss
ss << string("30");
int m;
ss >> m;
cout << m << endl;
return 0;
}
Running the above code yields some random output:
12
0
At some other times, the following output is observed:
12
4
I expected the output to be simply:
12
30
Why did I get the unexpected results?
Also, what should be the best way to parse a string s to int i without necessary C++11 support? Should it be int i = atoi(s.c_str())?
When you extract 12 from the stream, you reach the end of it, which puts it in a bad state. Any further extractions will fail. You need to call ss.clear() around the time you clear its contents.
If you had checked the success of your extractions, you would have avoided this problem. I generally expect to see any extraction from a stream as some kind of condition.
And yes, using string streams to parse strings as integers is a perfectly reasonable way to do it pre-C++11. I would prefer it over using atoi. For anybody who wants to know the C++11 way, use std::stoi.
For those who're here with something similar to the above but not exactly, I found that when you've got a stream in a scenario where you need to re-use it (like in a while loop), the easiest way to avoid head-aches (in addition to ss.clear) is to create a new stream each time. For example:
int GetInteger(){
cout << "Enter an int: " << endl;
string userInput;
while (true){
stringstream ss;
getline(cin,userInput);
ss << userInput;
//Making sure that an int was passed
int result;
if (ss >> result){
//Making sure that there is no extra stuff after
string extra;
if (ss >> extra){
cout << "Unexpected stuff at end of input: " << extra << endl;
} else{
return result;
}
} else {
cout << "Number you entered is not an INT. Please enter an integer" << endl;
}
cout << "Retry: " << endl;
// ss.clear();
}
}
So every time the user enters an invalid input, at the start of the while loop, I create a new stringstream object. While researching my function's undefined behavior, I found this question that has a similar example.