Input from text file into char *array[9]

Question

I have a file with 9 words and i have to store each word into the char array of 9 pointers but i keep getting an error message. I cannot use vectors!

#include <iostream>
#include <fstream>


using namespace std;


int main()
{
   char *words[9];
   ifstream inStream;
   inStream.open("sentence.txt");
   if (inStream.fail())
   {
       cout << "Input file opening failed.\n";
       exit(1);
   }

   for ( int i = 0; i < 10; i++)
   {
       inStream >> words[i];
   }

       inStream.close();

   return 0;
}

Answer 1

The declaration

char *words[9];

declares a raw array of pointers. This array is not initialized so the pointers have indeterminate values. Using any of them would be Undefined Behavior.

Instead you want

vector<string> words;

where vector is std::vector from the <vector> header, and string is std::string from the <string> header.

Use the push_back member function to add strings to the end of the vector.

Also you need to move the close call out of the loop. Otherwise it will close the file in the first iteration.

This approach gives the code (off the cuff, disclaimer...)

#include <fstream>
#include <iostream>
#include <vector>
#include <string>
using namespace std;

int main()
{
   vector<string> words;
   ifstream inStream;
   inStream.open("sentence.txt");

   for ( int i = 0; i < 10; i++)
   {
       string word;
       if( inStream >> word )
          words.push_back( word );
   }
   inStream.close();
}

---

If you can't use std::string and std::vector then you need to initialize the array of pointers, and make sure that you don't read more into the buffers than there's room for.

The main problem here is that >> is unsafe for reading into a raw array given by a pointer. It doesn't know how large that array is. It can easily lead to a buffer overrun, with dire consequences.

And so this gets a bit complicated, but it can look like this:

#include <ctype.h>          // isspace
#include <fstream>
#include <iostream>
#include <locale.h>         // setlocale, LC_ALL
#include <stdlib.h>         // EXIT_FAILURE
using namespace std;

void fail( char const* const message )
{
    cerr << "! " << message << "\n";
    exit( EXIT_FAILURE );
}

void readWordFrom( istream& stream, char* const p_buffer, int const buffer_size )
{
    int charCode;

    // Skip whitespace:
    while( (charCode = stream.get()) != EOF and isspace( charCode ) ) {}

    int n_read = 0;
    char* p = p_buffer;
    while( n_read < buffer_size - 1 and charCode != EOF and not isspace( charCode ) )
    {
        *p = charCode;  ++p;
        ++n_read;
        charCode = stream.get();
    }
    *p = '\0';      // Terminating null-byte.

    if( charCode != EOF )
    {
        stream.putback( charCode );
        if( not isspace( charCode ) )
        {
            assert( n_read == buffer_size - 1 );    // We exceeded buffer size.
            stream.setstate( ios::failbit );
        }
    }
}

int main()
{
    static int const n_words            = 9;
    static int const max_word_length    = 80;
    static int const buffer_size        = max_word_length + 1;  // For end byte.

    char *words[n_words];
    for( auto& p_word : words ) { p_word = new char[buffer_size]; }

    ifstream inStream{ "sentence.txt" };
    if( inStream.fail() ) { fail( "Input file opening failed." ); }

    setlocale( LC_ALL, "" );            // Pedantically necessary for `isspace`.
    for( auto const p_word : words )
    {
        readWordFrom( inStream, p_word, buffer_size );
        if( inStream.fail() ) { fail( "Reading a word failed." ); }
    }

    for( auto const p_word : words ) { cout << p_word << "\n"; }

    for( auto const p_word : words ) { delete[] p_word; }
}

Answer 2

You never allocate any memory for your char* pointers kept in the array.

The idiomatic way to write a c++ code would be:

#include <iostream>
#include <fstream>
#include <vector>

int main() {
   std::vector<std::string> words(9);
   std::ifstream inStream;
   inStream.open("sentence.txt");

   for ( int i = 0; inStream && i < 9; i++) {
       inStream >> words[i];
   }
}

The inStream.close() isn't necessary, and even wrong inside the loop. The std::istream will be closed automatically as soon the variable goes out of scope.

Answer 3

There are a few problems with your code.

char *words[9]; This allocates space for 9 pointers, not nine strings. Since you don't know how big the strings are you have two choices. You can either "guess" how much you'll need and limit the inputs accordingly, or you can use dynamic memory allocation (malloc or new) to create the space you need to store the strings. Dynamic memory would be my choice.

for ( int i = 0; i < 10; i++) This loop will execute on words[0] through words[9]. However, there is no words[9] (that would be the tenth word) so you'll overwrite memory that you have not allocated

inStream >> words[i]; This will send your input stream to memory that you don't "own". You need to allocate space for the words to live before capturing them from the input stream. To do this correctly, you'll need to know how much space each word will need so you can allocate it.

you could try something like this:

int main()
{
    char *words[9];
    char tempInput[256]; // space to capture the input, up to a maximum size of 256 chars
    ifstream inStream;
    inStream.open("sentence.txt");
    if (inStream.fail())
    {
        cout << "Input file opening failed.\n";
        exit(1);
    }

    for ( int i = 0; i < 9; i++)
    {
        //Clear the input buffer
        memset(tempInput, 0, 256);

        //Capture the next word
        inStream >> tempInput;

        //allocate space to save the word
        words[i] = new char(strlen(tempInput));

        //Copy the word to its final location
        strcpy(words[i], tempInput)
    }

    inStream.close();

    return 0;
}