cin in do while loop

Question

l3vi 4 Newbie Poster

14 Years Ago

Been developing for years under web based services like PHP and SQL, and I thought it would be a good idea that I come back and learn C++. Been enjoying it, but I got this one thing that is just driving me nuts.

In the do while loop after the cin is set on the first loop it goes into a death spin. In the 2nd example posted below it works just fine. Its as if the cin does not take a 2nd input after the first loop and just assumes its the same as the first input.

#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;

class customerClass {
	string masterArray[2][255];

	public:
	void getActions(int);
	void saveInArray(int, string);
	string readArray(int,int);
};
string customerClass::readArray(int x, int y) {
	cout << masterArray[x][y];
}
void customerClass::saveInArray(int a, string sx) {
	switch(a) {
		case 1:
		masterArray[a][0] = sx;
		break;
	}
}
void customerClass::getActions(int actions) {
	string uInput;
	switch(actions) {
		case 1:
			cout << "Options: (2) Add Name, (3) Add Address, (0) Exit: ";
		break;
		case 2:
			cout << "Customer Name ";
			getline(cin, uInput);
			saveInArray(1,uInput);
		break;
		case 3:
			cout << "Customer Address: ";
			getline(cin,uInput);
		break;
		default:
			// exit(1);
		break;
	}
}
int main() {
	customerClass rect;
	int n;
	rect.getActions(1);
	do {	
		cin >> n;
		rect.getActions(n);
	}while(n!=0);
return 0;
}

2nd code test that works fine:

// number echoer

#include <iostream>
using namespace std;

int main ()
{
  unsigned long n;
  do {
    cout << "Enter number (0 to end): ";
    cin >> n;
    cout << "You entered: " << n << "\n";
  } while (n != 0);
  return 0;
}

c++

4 Contributors
6 Replies
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Latest Post 14 Years Ago Latest Post by l3vi

mvmalderen commented: Well done on the code tags ! +4

All 6 Replies

Alex Edwards 321 Posting Shark

14 Years Ago

This may help--

#include <iostream>
#include <string>
#include <cstdlib>
#include <sstream>

using namespace std;

class customerClass {
	string masterArray[2][255];

	public:
	void getActions(int);
	void saveInArray(int, string);
	string readArray(int,int);
};
string customerClass::readArray(int x, int y) {
	cout << masterArray[x][y];
}
void customerClass::saveInArray(int a, string sx) {
	switch(a) {
		case 1:
		masterArray[a][0] = sx;
		break;
	}
}
void customerClass::getActions(int actions) {
    stringstream ss (stringstream::in | stringstream::out);
	string uInput;
	switch(actions) {
		case 1:
			cout << "Options: (2) Add Name, (3) Add Address, (0) Exit: ";
		break;
		case 2:
			cout << "Customer Name ";
			cin >> uInput;
            ss << uInput;
            saveInArray(1,ss.str());
		break;
		case 3:
			cout << "Customer Address: ";
			cin >> uInput;
			ss << uInput;
		break;
		default:
			// exit(1);
		break;
	}
}
int main() {
	customerClass rect;
	int n;
	
	do {
        rect.getActions(1);
		cin >> n;
		
		if(cin.fail()){
             cout << "bad input!" << endl;
             cin.clear();
             cin.ignore(INT_MAX, '\n');
             continue;
        }
		
		rect.getActions(n);
	}while(n!=0);
return 0;
}

You're probably wondering why I bothered with the sstream?

Consider the potential problem with bad input for the input buffer, cin.

You can pass information from cin to another buffer. That way if
you encounter extraction problems, its no longer cin's responsibility
to handle it - its that of the buffer that you are performing extraction on!

I personally, never like messing with cin directly. Using temporary buffers
can simplify problems with the standard input buffer at the cost of space
in your program.

mvmalderen 2,072 Postaholic

14 Years Ago

>This may help
Yeah, Alex, it will surely help him, but you don't explain him what you've changed to make his code work ...

To the OP:
It's because of the getline function, so you'll have to flush the input stream each time to make your code work as expected, there's a thread about flushing the input stream, you can find it here :)

Alex Edwards commented: My apologies and you are exactly right. +5

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Alex Edwards 321 Posting Shark · Answer 1 · 2009-05-08T21:40:19+00:00

>This may help
Yeah, Alex, it will surely help him, but you don't explain him what you've changed to make his code work ...
To the OP:
It's because of the getline function, so you'll have to flush the input stream each time to make your code work as expected, there's a thread about flushing the input stream, you can find it here :)

Yeah, after realizing how tired I was I also realized that I never answered the original question.

the getline function is accepting characters for your string but it doesn't seem like you're specifying a delimeter for the getline (by default it should be the newline char) nor are you flushing the input buffer after you push characters in it, so whan cin>>n is processed, characters are still in the buffer and then cin gets placed in a fail state.

Actually, Narue's post about this is far more intuitive than I could possibly explain. It wouldn't hurt to take the time to read it.

In this portion of code, after getting input from the user (such as the number for the option) I immediately ignore any characters that were put in the buffer then make a request to get characters in the input buffer until the newline delimiter is met then the characters are stored in the string.

#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;

class customerClass {
	string masterArray[2][255];

	public:
	void getActions(int);
	void saveInArray(int, string);
	string readArray(int,int);
};
string customerClass::readArray(int x, int y) {
	cout << masterArray[x][y];
}
void customerClass::saveInArray(int a, string sx) {
	switch(a) {
		case 1:
		masterArray[a][0] = sx;
		break;
	}
}
void customerClass::getActions(int actions) {
    
	string uInput;
	switch(actions) {
		case 1:
			cout << "Options: (2) Add Name, (3) Add Address, (0) Exit: ";
		break;
		case 2:
			cout << "Customer Name ";
			cin.ignore(INT_MAX, '\n');
			getline(cin, uInput, '\n');
            saveInArray(1,uInput);
		break;
		case 3:
			cout << "Customer Address: ";
			cin.ignore(INT_MAX, '\n');
			getline(cin, uInput, '\n');
		break;
		default:
			// exit(1);
		break;
	}
}
int main() {
	customerClass rect;
	int n;
	
	do {
        rect.getActions(1);
		cin >> n;
		
		if(cin.fail()){
             cout << "bad input!" << endl;
             cin.clear();
             cin.ignore(INT_MAX, '\n');
             continue;
        }
		
		rect.getActions(n);
	}while(n!=0);
return 0;
}

NathanOliver 429 Veteran Poster Featured Poster · Answer 2 · 2009-05-08T23:58:36+00:00

just for your information you do not need to implicitly define the delimiter as a newline in getline() it is already set there and if you want to change it the you must specify what you want. at least that is according to the msdn for visual c++ 6.0 professional

mvmalderen 2,072 Postaholic · Answer 3 · 2009-05-09T00:24:58+00:00

just for your information you do not need to implicitly define the delimiter as a newline in getline() it is already set there and if you want to change it the you must specify what you want. at least that is according to the msdn for visual c++ 6.0 professional

Yup, that's true: http://www.cplusplus.com/reference/iostream/istream/getline/
:)

l3vi 4 Newbie Poster · Answer 4 · 2009-05-09T10:57:33+00:00

Thanks for the info and link. Alex; if I'm fallowing you right, using stringstream and passing uInput into ss it's moving the data out of the cin buffer into a buffer in ss stringstream?

cin in do while loop

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