Twelfth Week Examples

Arrays

Sieve of Erathostenes


/* Sieve of Erathostenes */

#include <stdio.h>

#define TRUE  1
#define FALSE 0
#define MAX 1000

int erath[MAX];
int main() {
int i, j, k;

for (i=0 ; i < MAX ; i++) /* initialize array*/
  erath[i] = FALSE;

for (j=2 ; j*j < MAX ; j++ )  /* do sieve */
   if (erath[j-1] == FALSE) /* j is not a composite */
      for (k=2 ; j*k <= MAX; k++ )
          erath[k*j-1] = TRUE; /* multiples of j are composites */


for (i = 1 ; i < MAX ; i++)
   if (erath[i] == FALSE)
      printf("%d is a prime \n", i+1);
}

Distance


/* Distance between points in 3-space */

#include <stdio.h>
#include <math.h>

int main(){
  int i;

  double  x[3], y[3], sum, dist;
  
  printf("The coordinates of the first point: ");
   for ( i = 0 ; i < 3  ; i++ )
     scanf("%lf",&x[i]);

  printf("\nThe coordinates of the second point: ");
   for ( i = 0 ; i < 3  ; i++ )
     scanf("%lf",&y[i]);
   
   sum = 0;

   for( i = 0 ; i < 3 ; i++)
     sum = sum + (x[i] - y[i])*(x[i] - y[i]);

   dist = sqrt(sum);
     
      printf("\nThe distance is %lf.\n",dist);
}

Structures

In next examples we discuss the notion of a structure. The example is a structure called point in the plane. The program we use as an example calculates the distance between two given points.

/* point1.c */

#include <stdio.h>
#include <math.h>

int main() {

float d;

struct {
  float x;
  float y;
} A, B;

printf("The coordinates of the point A are: ");
scanf("%f %f",&A.x,&A.y);

printf("\nThe coordinates of the point B are: ");
scanf("%f %f",&B.x,&B.y);

d = sqrt((A.x - B.x)*(A.x - B.x) + (A.y - B.y)*(A.y - B.y));
printf("\nThe distance between A and B is %f\n",d);

}

In the second example we use a label point.

/* point2.c */

#include <stdio.h>
#include <math.h>

struct point {
  float x;
  float y;
};

int main(){

float d;
struct point  A, B;

printf("The coordinates of the point A are: ");
scanf("%f %f",&A.x,&A.y);

printf("\nThe coordinates of the point B are: ");
scanf("%f %f",&B.x,&B.y);

d = sqrt((A.x - B.x)*(A.x - B.x) + (A.y - B.y)*(A.y - B.y));
printf("\nThe distance between A and B is %f\n",d);

}

Finally, the third example we define a new type point. This is the most flexible use of structures.

/* point3.c */

#include <stdio.h>
#include <math.h>

typedef struct {
  float x;
  float y;
} point ;

float dist( point A, point B) {
  return(sqrt((A.x - B.x)*(A.x - B.x) + (A.y - B.y)*(A.y - B.y)));
}

int main(){

float d;
point A, B;

printf("The coordinates of the point A are: ");
scanf("%f %f",&A.x,&A.y);

printf("\nThe coordinates of the point B are: ");
scanf("%f %f",&B.x,&B.y);

printf("\nThe distance between A and B is %f\n", dist(A,B));
}