Pointer

June 5, 2019

One of the most fundamentals in programming is understanding a pointer. It is confusing even for developers. You can also face it in some programming interview.

Let's get back in C. This old programming language is going to help us in understanding a pointer. I'm going to write some examples of it.

A pointer is a variable that stores an address of a variable in memory.

int main() {
    int number = 10;
    int *number_pointer = &number; // Pointer assigned to an integer

    printf("%d\n", *number_pointer); // 10
    printf("%p", number_pointer); // 0x7ffc5302df28

    return 0;
}

The code above is a general use of pointers. The result of a memory address could be different. You could try C language at https://repl.it/languages/c.

int main() {
    int number = 10;
    int *number_pointer = &number;
    int *change_number = number_pointer;

    *change_number = 99;

    printf("%d\n", number); // The result would be 99.

    return 0;
}

We can change the value of number because the variable of change_number contains the address of number.

String

String is an array of characters.

int main() {
    char *string = "Hello World";

    while (*string != '\0') {
        printf("%c", *string);
        string++;
    }

    return 0;
}

We just printed the characters or string by incrementing the pointer.

void reverse(char *string) 
{ 
    int length = 0;
    char *start, *end, temporary;

    start = string;
    end = string;

    while (*end != '\0') {
        if (length > 0) end++;

        length++;
    }

    end--;
    length--;

    for (int index = 0; index < length / 2; index++) {  
        temporary = *end; 
        *end = *start; 
        *start = temporary; 

        start++; 
        end--; 
    } 
}

int main() {
    char string[] = "Hello World";

    printf("%s\n", string);

    reverse(string);

    printf("%s", string);

    return 0;
}

Reverse string. The most common question in the programming interview.

Linked List

Linked List is a linear collection of data elements.

struct node {
    int data;
    struct node *next;
};

We built a struct to contain data of integer and a pointer to next struct.

struct node a, b, c, d;

a.data = 1;
a.next = &b;

b.data = 2;
b.next = &c;

c.data = 3;
c.next = &d;

d.data = 4;
d.next = NULL;

That is how we built the linked list.

int get_length(struct node *list) {
    int length = 0;
    
    while (list != NULL) {
        length++;
        list = list->next;
    }
    
    return length;
}

Count the length of the linked list. Almost similar to string.

struct node *reverse(struct node *list) {
    if (
        list == NULL ||
        list->next == NULL
    ) {
        return list;
    }
    
    struct node *current = list->next;
    struct node *previous = list;
    
    previous->next = NULL;
    
    while (current != NULL) {
        struct node *next = current->next;
    
        current->next = previous;
        previous = current;
        current = next;
    }
    
    return previous;
}

Reverse Linked List. One of the most common questions in a programming interview about Linked List.

That is all I want to cover. A simple definition then followed by common examples. The most important is understanding how the pointer works.

To understand more is to practice more. There is a lot of common questions in the programming interview about pointer that I didn't cover here. Feel free to solve them.

References:

Pointer

String

Linked List

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