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Squiffyk7

C++ skips a statement in it's first run

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Hello, i've been learning how to write in C++ using the tutorials on www.cplusplus.com for currently 3 days, so ofcourse my code will look very ammaturish. As I'm getting to grips with the syntax, i'm working on a simple program that lets you name how many films are your favourite, and specify what those films are called. Obviously its rather pointless, but as I said I'm practicing what i've learnt from the C++ syntax so far.

The error in my code is not preventing execution, it's just not doing what I want it to do.
 
// A program that lets you name "n" films.

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main ()
{
    int n, i;
    string quant;

    cout << "How many favourite films do you have?" << endl;
    cin >> quant;
    stringstream(quant) >> n;
    cout << "What are your " << n << " favourite films?" << endl;
    string n_movie[n];

    for (i = 0; i < n; i ++)
    {
        getline(cin, n_movie[i]);
        cout << "Movie " << i << " is: " << n_movie[i] << endl;
    }


    return 0;
}
Once I run the code, I can enter my number but the console straight away also prints "Movie 0 is: " without allowing me to specify what movie movie 0 is. I can enter all movies up to movie n-1 as i should be able to.
Thanks in advance.

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Short answer: change cin >> quant; to getline(cin, quant);. Using >> to extract from the stream leaves a carriage return hanging around in the stream that the first getline() tries to read as the first movie title.

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Short answer: change cin >> quant; to getline(cin, quant);. Using >> to extract from the stream leaves a carriage return hanging around in the stream that the first getline() tries to read as the first movie title.


And it works! Thanks, although I'm not to sure what's changed, I think I need to look further into what streams are before I move on.

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Short answer: change cin >> quant; to getline(cin, quant);. Using >> to extract from the stream leaves a carriage return hanging around in the stream that the first getline() tries to read as the first movie title.


And it works! Thanks, although I'm not to sure what's changed, I think I need to look further into what streams are before I move on.

 

This documentation/tutorial can be a good read to refresh on the basic knowledge of what is happening: http://www.cplusplus.com/doc/tutorial/basic_io/

 

Basically what is happen is that cin>> extraction stops as soon as it finds a carriage return or even a blank space.  That is left inside the stream.  Then the next time you get input getline will see that still in the stream.  So it reads it in.

 

So the program is technically doing what it is "supposed" to do.  Just not what you expected it to do.  It does go through the for loop properly.  It just so happens that there is something left in the input stream the first time through.

Edited by Chad Smith

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In languages like C/C++, I/O is always one of the hairier things to mess with.  It's always worthwhile to check the nuances of the particular functions you're using in that area.

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This must be a common issue... I had the same problem with C#'s Console.Read() method... 

 

cin>>

reads <b>UP TO</b> the first enter/return, leaving the return itself in the input stream, to be read by your next input function.

 

getLine()

reads <b>and includes</b>the first enter/return, removing it from the input stream, so your next input function will have an empty input stream.

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string n_movie[n];
 
Or perhaps.....  
 
vector<string> n_movie;
n_movie.resize(n);
 
??


What's the question? Is there one? Yes I'd use a vector but it is likely the OP hasn't studied vectors yet so an array is fine with him learning.

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He's probably trying to indicate that initializing an array with local storage with a non-const array extent is not legal standard C++. It would be easier to tell if he employed the communication device commonly known as the complete sentence.

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