Week 6 Lecture Code

// Pantea Aria

// playing with pointers


#include <stdio.h>

int main(void) {
    int a = 5;
    int b = 10;

    int *p = &a;
    int *q = &b;

    printf("Before:\n");
    printf("a = %d, b = %d\n", a, b);

    // what is happening here?
    // let's draw diagrams
    *p = *p + *q;   
    *q = *p - *q;   
    *p = *p - *q;   

    //*p = 100 - *q
    // *q = *p + *q - 90;
    // int y = *p / 10;

    printf("After:\n");
    printf("a = %d, b = %d\n", a, b);    //what will we see???

    // printf("  ....   ");

    return 0;
}

// Pantea Aria

// pointers

// Write a program that:
// Declares and initializes an array of 5 integers (e.g., {1, 2, 3, 4, 5}).
// Uses the name of the array as a pointer and 
// doubles the value of each element using pointer arithmetic 
// Prints the array before and after doubling the values, using the pointer (not array indexing).


#include <stdio.h>

int main() {
    int numbers[5] = {1, 2, 3, 4, 5};   // {2, 4, 6, 8, 10}
   
    int i = 0;
    while (i < 5) {
        //numbers[i] = numbers[i] * 2;
        *(numbers + i) = *(numbers + i) * 2;
        printf ("%d\n", *(numbers + i));
        i++;
    }


    return 0;
}        
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
        
    
    
    
    
    

// Pantea Aria

// pointers

// Write a program that:
// Defines an integer variable in main.
// uses function square_number to square the value (multiply it by itself)
// Print the result in main.


#include <stdio.h>
void square_number (int *pointer);
int main() {
    
    int number = 100;
    printf("number before function is %d\n", number);

    // call function square_number
    square_number(&number);

    printf("number after function is %d\n", number);

    return 0;
}
void square_number(int *p) {
  *p = *p * (*p);
}
 
  // referncing means, getting the address of a variable
  //derefrencing means getting the value stored in an address      

    
    
    

// Pantea Aria
// pointers

// Write a program that:
// Reads two integers from the user.
// Passes their addresses to a function called add.
// The function should add the values and return the result;
// Print the result

#include <stdio.h>
int add(int *p, int *q);
int main() {

    // Reads two integers from the user.
    printf ("Enter two integers:");
    int num1, num2;
    scanf("%d %d", &num1, &num2);
    // Passes their addresses to a function called add.
    // add the values and return the result
    int result = add(&num1, &num2);
    // Print the result
    printf("Result is %d\n", result);

    return 0;

}
// function definition/body
int add(int *p, int *q) {
    int r = *p + *q;
    return r;
}

// Pantea Aria

// pointers

// Write a program that:
// Reads two integers from the user.
// write ONE function that adds AND multiplies the values and print the results in main.
// Print the result in main.

#include <stdio.h>
void add_product(int num1, int num2, int *prod_result, int *add_result);
int main() {
    
    // Reads two integers from the user.
    printf ("Enter two integers:");
    int num1, num2;
    scanf("%d %d", &num1, &num2);
    //write ONE function that adds AND multiplies 
    //the values and print the results in main.
    int prod_result;
    int add_result;
    add_product(num1, num2, &prod_result, &add_result);

    printf ("Add is %d and product is %d\n", add_result, prod_result);

    return 0;
}
void add_product(int n1, int n2, int *p, int *q) {
    *p = n1 * n2;
    *q = n1 + n2;

    return;
}    

// Pantea Aria

// pointers

// Write a program that:
// Declares and initializes an array of 5 integers (e.g., {10, 20, 30, 40, 50}) in main.
// Passes the array and its size to a function called half_array.
// Print the array before and after calling the function.

#include <stdio.h>
void half_array(int *numbers, int size);
int main() {
    int numbers[5] = {10, 20, 30, 40, 50};

    printf("Original array: ");
    for (int i = 0; i < 5; i++) {
        printf("%d ", numbers[i]);
    }
    printf("\n");
    
    // print out the address of array (the name of the array)
    // 1. using the name of the array
    printf ("The address of array is %p\n", numbers);
    // 2. the address of the first element in the array
    printf ("The address of array is %p\n", &numbers[0]);
    
    // call function half_array
    half_array(numbers, 5);
    
    
    printf("array after calling function: ");
    for (int i = 0; i < 5; i++) {
        printf("%d ", numbers[i]);
    }
    printf("\n");
    return 0;
}
void half_array(int *numbers, int size) {
    int i = 0;
    while (i < size) {
        numbers[i] = numbers[i] / 2;
        i++;
    }
    return;
}




    
 // print out the address of the array from the function too   
    
    
    
    

// Pantea Aria

// pointers and strings

#include <stdio.h>

int main() {
    char str[] = "Hello World";
    // make a pointer *s to this string
    char *s = str;
    // char *s = &str[0];
    
    // print out each character
    printf("str is %s\n", str);
    printf ("str char by char:");
    int i = 0;
    while(s[i]) {
        putchar(s[i]);   //putchar(s[i]);
        i++;
    }
    printf("\n");

    printf("using the pointer %s", s);

    printf("derefrencing char by char:");
    i = 0;
    while (*(s + i)) {
        putchar(*(s + i));   // putchar(*s[i]);
        i++;
    } 
    printf("\n");
    // do not change str
        
    
    // print out the address of the string 
    printf("the address of str is %p\n", str);
    printf("The address of str is %p\n", s);
    
    // print out the address of where 'W' is stored
    printf ("Address of W is %p and the value is %c\n", &str[6], str[6]);
    
    // can you move s to point to 'W' and not 'H'? how?
    s = s + 6;
    printf ("s is now %p\n", s);
    
    
    // Can I move str to point to 'W' and not 'H'???  NO pls don't
            
    return 0;
}    
    
    
    
    
    
    
    
    
    
    
    
    

// Pantea Aria

// pointers

// Write a program that:
// Declares a string in main (e.g., "Programming is fun").
// Passes the string to a function called count_vowels.
// The count_vowels function uses a pointer to traverse the string and 
// count how many vowels (a, e, i, o, u) are in it.
// main then prints the total number of vowels.


#include <stdio.h>
int count_vowels(char *s);   // int count_vowels(char s[]);
int is_vowel(char ch);
int main() {
    char sentence[] = "Programming is fun";
    int result = count_vowels(sentence);

    printf("Number of vowels: %d\n", result);

    // your turn

    return 0;
}
int count_vowels(char *s) {
    int i = 0;
    int count = 0;
    while(s[i]) {
        if (is_vowel(s[i])) {
            count++;
        }
        i++;
    }
    return count;
}
int is_vowel(char ch) {
    return (ch == 'a' ||
             ch =='e' || 
             ch == 'i' || 
             ch == 'o' || 
             ch == 'u');
}

// Pantea Aria

// introduction to pointers


#include <stdio.h>

int main() {
    int x = 90;
    // what is the address (or reference) of x
    printf ("the address of x in memory is
    printf ("the value of x is
    
    // I can store the address of x in another variable
    // that variable is called a pointer 
    // & is called reference/address operator 



    // dereference
    
    
    // change the value of x indirectly
    
    
    

    return 0;
}    




















// When we declare a variable in C, where does it live?
// (This leads to the idea of memory locations.)

// What do you think this operator & does in C?
// int x = 10;

// printf("%p\n", &x); // What does this print?  shows some hex value 

// Can a variable store another variable’s memory address?

// What is the value of this variable, and what is its address?
// int a = 5;   value is 5, some hex value &a with %p

// How can we print the address of a? printf ("%p", &a);
// value: printf ("%d", a);

// Can we store this address in another variable? yes, I must declare a pointer

// If we store the address of a in a variable, how can we use it to get the value back?

// int a = 10;
// int *p = &a;
// *p = 20;
// printf("%d\n", a); // What will this print?

// Predict the output and explain:
// int x = 3;
// int y = 4;
// int *p = &x;    //declare a pointer to x, p has the address of x, referencing
// *p = y;   //deferencing
// printf("%d\n", x);
// What’s the difference between p = &x; and *p = x;?

// What is happening here?
// int a = 100;
// int *ptr1 = &a;
// int *ptr2 = ptr1;
// printf("%d\n", *ptr2);

// Pantea Aria

// playing with pointers


#include <stdio.h>

int main(void) {
    int a = 5;
    int b = 10;

    int *p = &a;
    int *q = &b;

    printf("Before:\n");
    printf("a = %d, b = %d\n", a, b);

    // what is happening here?
    // let's draw diagrams
    *p = *p + *q;   
    *q = *p - *q;   
    *p = *p - *q;   

    printf("After:\n");
    printf("a = %d, b = %d\n", a, b);    //what will we see???

    return 0;
}

// Pantea Aria
// 20/10/2025
// pointers

// Write a program that:
// Declares and initializes an array of 5 integers (e.g., {1, 2, 3, 4, 5}).
// Uses the name of the array as a pointer and 
// doubles the value of each element using pointer arithmetic 
// Prints the array before and after doubling the values, using the pointer (not array indexing).


#include <stdio.h>

int main() {
    int numbers[5] = {1, 2, 3, 4, 5};   // {2, 4, 6, 8, 10}


    return 0;
}        
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
        
    
    
    
    
    

// Pantea Aria
// 20/10/2025
// pointers

// Write a program that:
// Defines an integer variable in main.
// uses function square_number to square the value (multiply it by itself)
// Print the result in main.


#include <stdio.h>
void square (int *pointer);
int main() {
    
    int number = 100;
    printf("number before function is %d\n", number);

    // call function square_number

    printf("number after function is %d\n", number);

    return 0;
}
 
  // referncing means, getting the address of a variable
  //derefrencing means getting the value stored in an address      

    
    
    

// Pantea Aria
// pointers

// Write a program that:
// Reads two integers from the user.
// Passes their addresses to a function called add.
// The function should add the values and return the result;
// Print the result

#include <stdio.h>

int main() {


}

// Pantea Aria

// pointers

// Write a program that:
// Reads two integers from the user.
// write ONE function that adds AND multiplies the values and print the results in main.
// Print the result in main.

#include <stdio.h>

int main() {
    

// Pantea Aria

// pointers

// Write a program that:
// Declares and initializes an array of 5 integers (e.g., {10, 20, 30, 40, 50}) in main.
// Passes the array and its size to a function called half_array.
// Print the array before and after calling the function.

#include <stdio.h>

int main() {
    int numbers[5] = {10, 20, 30, 40, 50};

    printf("Original array: ");
    for (int i = 0; i < 5; i++) {
        printf("%d ", numbers[i]);
    }
    printf("\n");
    
    // print out the address of array (the name of the array)
    
    // print out the address of array (the address of the first element)

    // call function half_array
    
    
    printf("array after calling function: ");
    for (int i = 0; i < 5; i++) {
        printf("%d ", numbers[i]);
    }
    printf("\n");
    return 0;
}

    
 // print out the address of the array from the function too   
    
    
    
    

// Pantea Aria

// pointers and strings

#include <stdio.h>

int main() {
    char str[] = "Hello World";
    // make a pointer *s to this string
    
    // print out each character
    
    // print out the address of the string 
    
    // print out the address of the where 'W' is stored
    
    // can you move s to point to 'W' and not 'H'? how?
    
    
    // Can I move str to point to 'W' and not 'H'??? 
    
    
    
        
    return 0;
}    
    
    
    
    
    
    
    
    
    
    
    
    

// Pantea Aria

// pointers

// Write a program that:
// Declares a string in main (e.g., "Programming is fun").
// Passes the string to a function called count_vowels.
// The count_vowels function uses a pointer to traverse the string and 
// count how many vowels (a, e, i, o, u) are in it.
// main then prints the total number of vowels.


#include <stdio.h>
int main() {
    char sentence[] = "Programming is fun";
    int result = count_vowels(sentence);

    printf("Number of vowels: %d\n", result);

    // your turn

    return 0;
}

// Pantea Aria

// pointers recap

// Write a program that:
// send two variables to a function
// swap them
// print them in the main


#include <stdio.h>
void swap(int *num1, int *num2);
int main() {
    int num1, num2;
    printf("Enter two integers:");
    scanf("%d %d", &num1, &num2);

    printf("\nnum1 = %d and num2 = %d before swap", num1, num2);

    // call swap
    swap(&num1, &num2);

    printf("\nnum1 = %d and num2 = %d after swap\n", num1, num2);

    return 0;
}    
void swap(int *num1, int *num2) {

    int temp = *num1;
    *num1 = *num2;
    *num2 = temp;
    printf("\nnum1 = %d and num2 = %d in swap", *num1, *num2);
}