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2010-05-25T10:22:10Z

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==A prime example using binary files==

<source lang="cpp">
/*
Beginning C, Third Edition
By Ivor Horton
ISBN: 1-59059-253-0
Published: Apr 2004
Publisher: apress
*/
#include <stdio.h>
#include <stdlib.h>
#include <math.h > /* For square root function sqrt() */
#define MEM_PRIMES 100 /* Count of number of primes in memory */
/* Function prototypes */
int test_prime(unsigned long N);
void put_primes(void);
int check(unsigned long buffer[], int count, unsigned long N);
/* Global variables */
char myfile[] = "C:\\myfile.bin"; /* Physical file name */
FILE *pfile = NULL; /* File pointer */
unsigned long primes[MEM_PRIMES] = { 2UL,3UL,5UL }; /* Array to store primes */
int index = 3; /* Index of free location in array primes */
int nrec = 0; /* Number of file records */
void main()
{
unsigned long trial = 5UL; /* Prime candidate */
long num_primes = 3L; /* Prime count */
long total = 0L; /* Total required */
int i = 0; /* Loop counter */
printf("How many primes would you like? ");
scanf("%d", &total); /* Total is how many we need to find */
total = total<4 ? 4:total; /* Make sure it is at least 4 */
/* Prime finding and storing loop */
while(num_primes < total) /* Loop until we get total required */
{
trial += 2; /* Next value for checking */
if(test_prime(trial)) /* Check if trial is prime */
{ /* Positive value means prime */
primes[index++] = trial; /* so store it */
num_primes++; /* Increment total number of primes */
if(index == MEM_PRIMES) /* Check if array is full */
{
/* File opened OK? */
if((pfile = fopen(myfile, "ab")) == NULL)
{ /* No, so explain and end the program */
printf("\nUnable to open %s to append\n", myfile);
abort();
}
/* Write the array */
fwrite(primes, sizeof(long), MEM_PRIMES, pfile);
fclose(pfile); /* Close the file */
index = 0; /* Reset count of primes in memory */
nrec++; /* Increment file record count */
}
}
}
if(total>MEM_PRIMES) /* If we wrote some to file */
put_primes(); /* Display the contents of the file */
if(index) /* Display any left in memory */
for(i = 0; i<index ; i++)
{
if(i%5 == 0)
printf("\n"); /* Newline after five */
printf("%12lu", primes[i]); /* Output a prime */
}
if(total>MEM_PRIMES) /* Did we need a file? */
if(remove(myfile)) /* then delete it. */
printf("\nFailed to delete %s\n", myfile); /* Delete failed */
else
printf("\nFile %s deleted.\n",myfile); /* Delete OK */
}
/************
* Function to test if a number, N, is prime using primes in *
* memory and on file *
* First parameter N ?value to be tested *
* Return value - a positive value for a prime, zero otherwise *
************/
int test_prime(unsigned long N)
{
unsigned long buffer[MEM_PRIMES]; /* local buffer for primes from the file */

int i = 0; /* Loop counter */
int k = 0;
if(nrec > 0) /* Have we written records? */
{
if((pfile = fopen(myfile, "rb")) == NULL) /* Then open the file */
{
printf("\nUnable to open %s to read\n", myfile);
abort();
}
for(i = 0; i < nrec ; i++)
{ /* Check against primes in the file first */
/* Read primes */
fread(buffer, sizeof(long), MEM_PRIMES, pfile);
if((k = check(buffer, MEM_PRIMES, N)) >= 0) /* Prime or not? */
{
fclose(pfile); /* Yes, so close the file */
return k; /* 1 for prime, 0 for not */
}
}
fclose(pfile); /* Close the file */
}
return check(primes, index, N); /* Check against primes in memory */
}
/************
* Function to check whether an integer, N, is divisble by any *
* of the elements in the array pbuffer up to the square root of N.*
* First parameter buffer ?an array of primes *
* second parameter count ?number of elements in pbuffer *
* Third parameter N ?the value to be checked *
* Return value - 1 if N is prime, zero if N is not a prime, *
* -1 for more checks *
************/
int check(unsigned long buffer[], int count, unsigned long N)
{
int i = 0; /* Loop counter */
/* Upper limit */
unsigned long root_N = (unsigned long)(1.0 + sqrt((double)N));
for(i = 0 ; i<count ; i++)
{
if(N % buffer[i] == 0UL ) /* Exact division? */
return 0; /* Then not a prime */
if(buffer[i] > root_N) /* Divisor exceeds square root? */
return 1; /* Then must be a prime */
}
return 1; /* More checks necessary... */
}
/* Function to output primes from the file */
void put_primes(void)
{
unsigned long buffer[MEM_PRIMES]; /* Buffer for a block of primes */
int i = 0; /* Loop counter */
int j = 0; /* Loop counter */

if((pfile = fopen( myfile, "rb"))==NULL) /* Open the file */
{
printf("\nUnable to open %s to read primes for output\n", myfile);
abort();
}
for (i = 0 ; i< nrec ; i++)
{
/* Read a block of primes */
fread(buffer, sizeof(long), MEM_PRIMES, pfile);
for(j = 0 ; j<MEM_PRIMES ; j++) /* Display the primes */
{
if(j%5 == 0) /* Five to a line */
printf("\n");
printf("%12lu", buffer[j]); /* Output a prime */
}
}
fclose(pfile); /* Close the file */
}

</source>

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