Scanning a 2d array for prime numbers in C - c

Question

I have a 20x20 array randomly generated. I need a function to scan through the array, find all the prime numbers within the array, and add them to a new array. The array was generated like this:
#define TOTAL_ROWS 20
#define TOTAL_COLUMNS 20
void fillMatrix(int A[TOTAL_ROWS][TOTAL_COLUMNS], int *set_rows, int *set_columns)
{
int rows = 20, columns = 20;
for(int i = 0; i < rows; i++)
for(int j = 0; j < columns; j++)
A[i][j] = rand() % 500;
*set_rows = rows;
*set_columns = columns;
}
I also have it sorted, with this function:
void sortMatrix(int A[TOTAL_ROWS][TOTAL_COLUMNS], int rowsize, int colsize)
{
for(int r = 0; r < rowsize; r++)
qsort(A[r], colsize, sizeof(int), compare);
}
There is also a display function. Here is main which calls the functions:
int main(void)
{
int array[TOTAL_ROWS][TOTAL_COLUMNS] = { 0 };
int rows, columns;
fillMatrix(array, &rows, &columns);
sortMatrix(array, rows, columns);
displayArray(array, rows, columns);
return 0;
}
An example of the generated array:
0 41 145 169 205 224 281 327 327 334 358 436 442 461 462 464 467 478 491 495
35 38 104 153 167 216 218 226 271 292 299 369 382 391 394 395 402 412 421 447
29 37 47 141 144 162 164 173 203 211 223 241 253 257 278 311 322 333 359 368
6 35 40 48 101 106 148 190 229 264 288 305 316 342 350 370 390 393 442 446
37 38 41 82 84 118 123 126 129 256 308 323 340 376 429 431 439 444 454 466
12 21 72 73 97 115 139 158 173 204 245 270 277 306 329 333 386 424 430 477
7 31 52 74 107 136 150 155 161 191 224 267 290 337 350 355 430 441 466 486
6 30 88 91 155 168 209 221 253 258 262 287 383 400 409 413 422 445 446 457
20 21 37 41 48 95 96 102 124 168 199 291 336 348 350 359 374 410 483 484
14 53 117 127 148 228 234 281 288 307 310 313 393 400 418 421 438 467 483 499
8 19 56 93 100 109 116 195 202 224 249 298 303 309 343 344 435 451 485 489
3 9 23 38 80 87 89 118 122 157 200 281 292 296 298 298 314 448 458 472
11 38 55 134 146 156 157 179 190 191 202 272 315 328 362 375 386 388 433 458
3 10 21 57 75 100 142 151 199 212 322 344 369 369 381 389 392 416 476 498
2 85 88 117 154 169 182 188 189 221 255 257 285 289 332 361 401 423 426 432
12 29 49 55 60 139 145 187 192 218 253 279 329 368 423 425 434 441 476 496
49 105 114 171 193 195 201 234 263 282 286 286 297 316 366 416 437 449 455 488
53 58 129 144 146 161 185 196 222 256 308 313 313 321 332 355 412 445 481 482
10 18 22 24 35 44 149 154 159 168 173 186 245 253 292 313 439 450 466 474
18 70 125 159 177 202 287 297 314 324 333 334 372 374 391 405 414 458 477 487
I need an isPrime() function to go in main that will find each prime in the arrays and add them to a new array. I'm just not sure how to.

Answer 1

You can use this.
int is_prime(int num)
{
if (num <= 1) return 0;
if (num % 2 == 0 && num > 2) return 0;
for(int i = 3; i < num / 2; i+= 2)
{
if (num % i == 0)
return 0;
}
return 1;
}
int prime_numbers_array[10000];
int array_length = 0;
int i,j;
for(i = 0; i < rows; i++){
for(j = 0 ;j < cols; j++){
if(is_prime(array[i][j])){
prime_numbers_array[array_length++] = array[i][j]
}
}
}

Answer 2

The fastest way is to run a sieve of Eratosthenes:
bool prime[max_number + 1];
void sieve()
{
memset(prime, true, sizeof(prime));
for (int i=2; i*i<=max_number; i++)
{
if (prime[i] == true)
{
for (int j=i*i; j<=max_number; j += i)
prime[j] = false;
}
}
}
And use the prime array as a lookup to check if a given number p is prime in constant time just by checking if prime[p] == true. Here max_number should be the maximum number that you will be having as an input.