[C]Foundation of programming

I am learning the foundation of programming bridging class for my university, I have done some small projects on my own with C to demonstrate some logics.

Programming is a skill to use computer to solve problems, before writing down the code, it is good to do an algorithm to demonstrate a logic.

/******************************************************************************

This program demonstrates the use of:
1. one dimension array.
2. iteration and selection.
3. functions.
4. To include %*c at the end of the format specifier of scanf() in 
while or do...while loop.
*******************************************************************************/
#include <stdio.h>
#include <ctype.h>//required for using tolower()
#include <stdbool.h>//required for boolean operator

void menu (void);
void prime_prompt(void);
void validate_prime_num(int);
void triangle_prompt(void);
void triangle_type(int,int,int);
bool validate_triangle(int,int,int);
void calculator_prompt(void);
void draw_lines(int,char);

/*
Use an array to store the characters.
Just for drawing lines on the menu.
*/
void draw_lines(int j,char pattern)
{
    int i;
    char lines[j];
    for(i=0;iside3 && side2+side3>side1 && side1+side3>side2)
    {
        is_it_a_triangle = 1; //1 for yes.
    }
    else
    {
        is_it_a_triangle=0; //0 for no.
    }
    return is_it_a_triangle;
}


/*
This function is for identifying the type of triangles.
Before identifying, validate_triangle function is called to
validate if it is an invalid triangle.
*/
void triangle_type(int side1, int side2, int side3)
{
    if(validate_triangle(side1,side2,side3)==0)
    {
        printf("Invalid triangle.\n");
    }
    else if(side1==side2 && side1==side3 && side2==side3)
    {
        printf("This is an equilateral triangle\n");
    }
    else if(side1==side2 || side1==side3 || side2==side3)
    {
        printf("This is an isoceles triangle\n");
    }
    else
    {
        printf("This is a scalene triangle\n");
    }
}

/*
Simply a place to execute the triangle_type function for validation
and triangle type identification.
*/
void triangle_prompt()
{
    int side1,side2,side3;
    
    printf("Enter 3 sides of a triangle:");//user to define 3 sides.
    scanf("%d%d%d%*c",&side1,&side2,&side3);
    triangle_type(side1,side2,side3);
}

/*
Prime number is a number that can only be divided by 1 or itself.
The solution is to exclude 1 and the number itself to do division testing.
If any number in the range of numbers between 2 and its own can be divisible
i.e. there is no remainder, the number is not a prime number.
*/
void validate_prime_num(int num)
{
    int i;
    bool flag=1;
    for(i=2;i<num;i++)
    {
        if(num%i==0)
        {
            flag=0; //0 for not prime number.
            break;
        }
    }
    if(flag==1)
    {
        printf("%d is a prime number.\n",num);
    }
    else
    {
        printf("%d is not a prime number.\n",num);
    }
}

//A simple prompt to call the validate_prime_num function.
//Notice %*c is included in the scanf() to ignore Enter.
void prime_prompt()
{
    int num;
    printf("Enter a number to validate if it is prime number:\n");
    scanf("%d%*c",&num);
    validate_prime_num(num);
}

//A simple menu.
void menu()
{
  char response,lines[37];
  int i;
  do
    {
      draw_lines(37,'*');  
      printf ("*Enter three option, press q to quit:\n");
      printf ("*Enter a for validating prime numbers.\n");
      printf ("*Enter b for simple calculator.\n");
      printf ("*Enter c for triangle.\n");
      draw_lines(37,'*');
      printf ("Your choice:");
      scanf ("%c%*c", &response);//%*c is necessary in this loop to escape the Enter key.
      response=tolower(response);
      switch (response)
	    {
	        case 'a':
	            prime_prompt();
	            break;
	        case 'b':
	            calculator_prompt();
	            break;
	        case 'c':
	            triangle_prompt();
	            break;
	        case 'q':
	            break;
	        default:
	            printf ("You have entered an invalid choice.\n");
	    }
    }while(response!='q');
  }


  int main ()
  {
    menu();
    return 0;
  }