Functions, Arrays and Two Pointers

Functions with Pointers

Let's experiment with a basic function to modify the value of our data. For instance, imagine you need a function that converts kilo-Ohms to Ohms (1 kOhm = 1000 Ohm).

Main


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#include <stdio.h>

void Ohm(double R)
{
    R = R * 1000;
}

int main()
{
    double r = 1.5; // kOhm
  
    printf("The value of resistance before using function: %f\n", r);

    Ohm(r);

    printf("The value of resistance after using function: %f", r);

    return 0;
}

Our attempt to change the value of the r variable was unsuccessful. This is because the function receives a copy of the r variable, not the actual value itself.

To make our program work as intended, we need to pass the address of the r variable to the function. As a result, the Ohm function should accept double* instead of just double.

Main


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#include <stdio.h>

void Ohm(double* R)
{
    // dereferencing the entered address and changing the object it points to
    *R = *R * 1000;
}

int main()
{
    double r = 1.5; // kOhm

    printf("The value of resistance before using function: %f\n", r);

    Ohm(&r);

    printf("The value of resistance after using function: %f\n", r);

    return 0;
}

Note that we reference the r variable twice. After invoking the Ohm function, the value of r is altered. This is because the function received the original address of the variable r, not a mere copy, and then modified the value at that particular address.

Moreover, a function can return a pointer to an object that it has generated:

Main


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#include <stdio.h>
#include <stdlib.h>

int* func()
{
    int* x = (int*)malloc(sizeof(int));

    printf("Address into function: %p\n", x);

    return x;
}

int main()
{
    int* pointerToFunc = func();

    printf("Address after using function: %p\n", pointerToFunc);
   
    return 0;
}

Are Arrays Merely Pointers?

What do you predict will transpire if a number is appended to an address?

Main


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#include <stdio.h>

int main()
{
    int x = 100;

    int* pX = &x;

    printf("Address: %p | Adress + 1: %p", pX, pX + 1);

    return 0;
}

When a number (pX + 1) is added to an address, it yields the address of the subsequent memory cell!

Let's script a loop to navigate the "sequence" of RAM:

Main


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#include <stdio.h>

int main()
{
    int* pX = NULL; // pointer to `int` type (4 bites)

    for (int i = 0; i < 3; i++)
        printf("Address: %p\n", pX + i);

    return 0;
}

We've projected three steps ahead. It's apparent from the derived addresses that there's a clear hierarchy.

Given that the int type occupies 4 bytes, we progress by 4 bytes with each step. This behavior is strikingly reminiscent of an array!

It seems an array is essentially a fixed address (represented by the array's name) coupled with allocated memory. The indices of the elements represent their offset from the address of the initial element!

This notion can be validated with the following program:

Main


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#include <stdio.h>

int main()
{
    int array[] = {1,2,3,4,5};

    printf("Address of array: %p\n", array);

    for(int i = 0; i < 5; i++)
        printf("Value: %d | Address of element with index %d: %p\n", *(array + i), i , &array[i]);

    return 0;
}

As observed, we don't traverse directly through the array. We solely utilize its address, specifically the address of its initial element.

Everything was clear?

Section 6. Chapter 5