C++ Tutorials - Absolute n00b spoonfeed

(I hope i'm not breaking a rule posting here)

Okay, so I came across this problem while coding, and i though it might be useful showing it to everyone else.

So tell me what the problem is, and how to fix it.

HINT: The program compiles fine, so it couldn't tell you where the problem is )

EDIT:Before, i accidently solved my problem, but now its broken, like it should be. Before looking at c++'s solution, just take a look at the code first, and guess, then look.

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#include "stdafx.h"
#include <iostream>
#include <string>
using namespace std;

int main ()
{
   string name;
   int age;
   string address;

cout << "\nEnter your name\n";
   cin >> name;

cout << "\nNow enter your age\n";
   cin >> age;

cout << "\n Good, now enter your address, including street number\n";
cout << "(Example. 23 Maple Lane)\n";
   getline( cin, address);
   cout << "\nYou, " << name << ", are " << age << " years old, and you live on " << address << ".";

   char f;
   cin >> f;
}

If you really can't get it, try running the program

I came across the same problem while I was learning. The problem is the combined use of cin and getline. cin does not remove whitespaces or newlines from the input buffer when it encounters one. Getline however will stop at the first newline it encounters. Btw... I didn't encounter the problem in your example because you use getline twice. The first getline will simply read the newline only, while the second one will read the entire string up until the newline.

After inputting name a newline or a part of the name will be still in the input buffer. It's better to use getline for getting a name, since cin will stop when it encounters a space. The newline gets ignored and removed from the input buffer when cin scans the buffer for an entered int value. However requesting int type of input from cin is dangerous. If a user inputs a string, or the second part of the name is still in the buffer, the results will be unpredictable. Therefore you check in a loop if cin is going into error mode. You then simply clear the error state and clean the max length of the buffer of any chars cin won't swallow.

However after cin is done putting a valid int value in age, there's still a newline in the buffer that gets ignored. A simple cin.ignore() will remove a single unprocessed char from the input buffer. This will prevent the first getline from processing nothing but a newline. The first newline should therefore be replaced by a cin.ignore() since it's more efficient.

Code: Select all

#include <iostream>
#include <string>

using namespace std;

int main ()
{
     string name;
     int age;
     string address;

     cout << "\nEnter your name\n";
     getline(cin, name);
     
     cout << "\nNow enter your age\n";
     while (!(cin >> age))
          {
               cin.clear();
               cin.ignore(numeric_limits<streamsize>::max(),'\n');
          }

     cout << "\n Good, now enter your address, including street number\n";
     cout << "(Example. 23 Maple Lane)\n";
     cin.ignore();
     getline(cin, address);
     cout << "\nYou, " << name << ", are " << age << " years old, and you live on " << address << ".";
     
     char f;
     cin >> f;
}


ah.... i fail at life... lol. (I "fixed" my code) I added the second getline in afterwards, to make sure it still works with it added in.... But forgot to take it out, and accidently solved it for you....

Anyways, adding in two
getline(cin, name);
getline(cin, name);

seems like the easiest solution for me. However, c++'s method is fool proof, and means it won't crash jsut because of some damn runtime error.

My metaphorical explanation of why it works is that the first getline takes the metaphorical bullet for the next getline (bullet = newline). The second getline does its job correctly.


EDIT: Hmm, thats interesting, i tried copy pasting your code into my compiler, and it didn't work..... Hmmm

Doctor Salt wrote:ah.... i fail at life... lol. (I "fixed" my code) I added the second getline in afterwards, to make sure it still works with it added in.... But forgot to take it out, and accidently solved it for you....

Anyways, adding in two
getline(cin, name);
getline(cin, name);

seems like the easiest solution for me. However, c++'s method is fool proof, and means it won't crash jsut because of some damn runtime error.

My metaphorical explanation of why it works is that the first getline takes the metaphorical bullet for the next getline (bullet = newline). The second getline does its job correctly.


EDIT: Hmm, thats interesting, i tried copy pasting your code into my compiler, and it didn't work..... Hmmm

Well unfortunately you have to have a failsafe in user input when it comes to numbers. A program can crash if the unstable result is used for further stuff. It has to be idiot proof The cin.ignore() is more efficient to use than a second getline. A getline requires more operations than a simple char that gets cleared from the input buffer.

Why isn't it working? Do you get a compiler error? I noticed that you use Visual Studo C++ with precompiled header support. I removed the include because I'm using Linux with the GNU comiler. Be sure to replace it again or remove the precompiled header support.

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#include "stdafx.h"


Yeah. The Cin.Ignore peace of code is a lifesaver. I came across this problem once, and it would just skip my getline() function. I swore it wasn't my fault and the computer was retarded, but then I found out about the whole ignore function a while back and it was great.

Hmm, well so i tried compiling it, and it basically said..

error C2780: 'const _Ty &std::max(const _Ty &,const _Ty &)' : expects 2 arguments - 0 provided

see declaration of 'std::max'

error C2065: 'numeric_limits' : undeclared identifier
error C2275: 'std::streamsize' : illegal use of this type as an expression

see declaration of 'std::streamsize'
error C2780: 'const _Ty &std::max(const _Ty &,const _Ty &,_Pr)' : expects 3 arguments - 0 provided

Am I missing some type of heading or something? I'm using visual express, on vista....

Doctor Salt wrote:Hmm, well so i tried compiling it, and it basically said..

error C2780: 'const _Ty &std::max(const _Ty &,const _Ty &)' : expects 2 arguments - 0 provided

see declaration of 'std::max'

error C2065: 'numeric_limits' : undeclared identifier
error C2275: 'std::streamsize' : illegal use of this type as an expression

see declaration of 'std::streamsize'
error C2780: 'const _Ty &std::max(const _Ty &,const _Ty &,_Pr)' : expects 3 arguments - 0 provided

Am I missing some type of heading or something? I'm using visual express, on vista....

I think you need to include limits.

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#include <limits>


Ah, that did the trick. Just wondering, how would one use that code, the

Code: Select all

while (!(cin >> variable))
          {
               cin.clear();
               cin.ignore(numeric_limits<streamsize>::max(),'\n');
          }


and have it so when it encounters an error, it also gives you a simple message?

cout << "You did not enter a number! You die now infidel!\n";

Well, i guess i mean where would you put the cout statement.

Doctor Salt wrote:Ah, that did the trick. Just wondering, how would one use that code, the

Code: Select all

while (!(cin >> variable))
          {
               cin.clear();
               cin.ignore(numeric_limits<streamsize>::max(),'\n');
          }


and have it so when it encounters an error, it also gives you a simple message?

cout << "You did not enter a number! You die now infidel!\n";

Well, i guess i mean where would you put the cout statement.

Code: Select all

while (!(cin >> variable))
          {
               cout << "You did not enter a number! You die now infidel!\n";
               cin.clear();
               cin.ignore(numeric_limits<streamsize>::max(),'\n');
          }


Now everytime you enter a non numerical value, you will get an error message. If cin encounters a char in the input stream it can't translate as a digit, then the cin state goes into error state.

Thanks, that should work