Thread: Cannot understand 'char' array text problem.

Code:

char a[]="";
char b[]="";

Try printing out the size of those arrays and see how big they are.

Thankyou @syzy and @hk_

@syzy: I tried your code but its giving kind of the same problem. Here is the code with your edit and its output::::

Code:

#include <iostream>
#include <string>

using namespace std;
//Initializing two empty (?) char arrays to store the alternate chars from
//the test string.
char a[] = "";
char b[] = "";

int main()
{
    // Initialization of the test srting. using no space to make it simple for now,
    // but spaces also work.
    char test[] = "HelloWorldHowAreYou";
    cout << "The string is " << test << " of size " << sizeof(test) << endl;

    // Loop test to output the string "test" using loop.
    // Used unsigned int to avoid any warnings in comparisson.
    cout << "\nLoop Test: \n";
    for (unsigned int j=0; j<sizeof(test); j++)
    {
        cout << test[j] << " ";
    }
    cout << "\n\n\n";
/*
    // The main program to store alternate chars in two different char arrays.
    // Loop using unsigned int. The output of this process is correct!
    for (unsigned int i=0;i<sizeof(test);i++)
    {
        a[i]=test[(2*i)];
        b[i]=test[(2*i)+1];

        cout << "a " << i << " is: " << a[i] << endl;
        cout << "b " << i << " is: " << b[i] << endl;

    }
    cout << "\n\n\n";

*/

    //// Your code @syzy::::::

    cout << "The new process and output after the storing of alternate chars in a,b. \n\n";

    int sizeA, sizeB;
    sizeA = sizeB = 0;
    for (unsigned int i=0; i<sizeof(test); i++)
    {
        if( i % 2 == 0 )
        {
            a[sizeA] = test[i];
            sizeA++;
        }
        else
        {
            b[sizeB] = test[i];
            sizeB++;
        }
    }


    //The display of arrays. Here using siezeof "test" just to be sure. Otherwise sizeofA = 10 (half of 20, this much is correct) 
    //and sizeof "a" is 4 (?!)

    for (unsigned int k=0;k<sizeof(test);k++)
    {
        cout << "a " << k << " is: " << a[k] << endl;
        cout << "b " << k << " is: " << b[k] << endl;
    }

    //cout << "\n" << a << b << endl;


    return 0;
}


The Output:

The string is HelloWorldHowAreYou of size 20

Loop Test:
H e l l o W o r l d H o w A r e Y o u


The new process and output after the storing of alternate chars in a,b.

a 0 is: H
b 0 is: l
a 1 is: l
b 1 is: o
a 2 is: o
b 2 is: o
a 3 is: o
b 3 is: l
a 4 is: l
b 4 is: H
a 5 is: H
b 5 is: w
a 6 is: w
b 6 is: r
a 7 is: r
b 7 is: Y
a 8 is: Y
b 8 is: u
a 9 is: u
b 9 is:
a 10 is:
b 10 is:
a 11 is:
b 11 is:
a 12 is:
b 12 is:
a 13 is:
b 13 is:
a 14 is:
b 14 is:
a 15 is:
b 15 is:
a 16 is:
b 16 is:
a 17 is:
b 17 is:
a 18 is:
b 18 is:
a 19 is:
b 19 is: ▒

Process returned 0 (0x0)   execution time : 0.167 s
Press any key to continue.

@hk_
Yea, just after initialization of arrays a and b, their size is just 1. I guess(!) this is the size of the first element of their pointed value since nothing is stored in them. I am slowly beginning to understand the working of pointers but this program I wrote still baffles me.

Glad you got it. The reason it doesn't work when you go less than 10 is probably the same reason why it wasn't working when you declared your arrays without a fixed length. You are trying to write past the bounds of the memory that you told the compiler to allocate. This will cause all kinds of problems.

If you say you want char a[10], then the program sets up 10 consecutive bytes of memory for you to use, which you access through the variable 'a'. It's technically a pointer to the beginning of that sequence of memory.

You have to be careful when accessing arrays, because they basically are pointers. If you tell it you want what is at a[1024] but you only made a 12 bytes long, then you are accessing unknown memory. For instance, your last for loop that displays the two arrays a and b, counts up too high. 'test' is longer than a or b so by the time you reach the end of test, it's already gone past the bounds of a and b (assuming you declared them with a length shorter than test).

There is a way to allocate memory dynamically, but that will be for later. I'm sure you will learn about it soon, it deals with using the new operator and pointers.

Edit:

If you want to read up on dynamic allocations: http://www.cprogramming.com/tutorial...llocation.html
I'm sure it will explain things better than I can.