<< problem 235 - An Arithmetic Geometric sequence Top Dice - problem 240 >>

# Problem 239: Twenty-two Foolish Primes

A set of disks numbered 1 through 100 are placed in a line in random order.

What is the probability that we have a partial derangement such that exactly 22 prime number discs are found away from their natural positions?
(Any number of non-prime disks may also be found in or out of their natural positions.)

Give your answer rounded to 12 places behind the decimal point in the form 0.abcdefghijkl.

# My Algorithm

Let's simplify this problem:
- there are 25 prime numbers below 100
- exactly 25 - 22 = 3 must remain at their position
- actually it doesn't matter whether those fixed numbers are prime numbers or not !
- therefore I just look at the first 25 numbers (1 .. 25) and require that three of them are fixed

The Wikipedia article on Derangement contains all the needed formulas: en.wikipedia.org/wiki/Derangement
The "subfactorial" (I haven't heard that name before !) is the core concept behind my derangements() function.

derangements() tells me the number of ways such that the first three numbers are fixed.
There are 2300 ways of choosing any three primes out of the 25 available (choose(25,3)).
Now I a total count of deranged sets - dividing it by the number of permutations (it's 100!) gives the probability.

## Alternative Approaches

A different approach can be found on www.numericana.com/answer/counting.htm although they don't go into detail
(except mentioning inclusion-exclusion principle, see en.wikipedia.org/wiki/Inclusion–exclusion principle)

## Note

Unlike most of my solutions, this time choose() and factorial() return double because their results are really big and their last digits don't matter.
There is an opportunity to pre-compute the factorials but my program terminates after less than 0.01 seconds even without this optimization.

I was a bit confused whether my result has to be the "raw" result or a "percentage" (thus multiplied by 100).
Strangely enough, my first attempt was correct - that's usually never happens ...

# Interactive test

You can submit your own input to my program and it will be instantly processed at my server:

Input data (separated by spaces or newlines):
Note: Enter the number of primes moved away from their original position.

This is equivalent to
echo 21 | ./239

Output:

Note: the original problem's input 22 cannot be entered
because just copying results is a soft skill reserved for idiots.

(this interactive test is still under development, computations will be aborted after one second)

# My code

… was written in C++11 and can be compiled with G++, Clang++, Visual C++. You can download it, too.

       #include <iostream>
#include <iomanip>

// ---------- based on similar code in my toolbox ----------

// factorial
// result is not accurate but supports large ranges
double factorial(unsigned int n)
{
double result = 1;
while (n > 1)
result *= n--;
return result;
}

// number of ways to choose n elements from k available
double choose(unsigned int n, unsigned int k)
{
// n! / (n-k)!k!
return factorial(n) / (factorial(n - k) * factorial(k));
}

// ---------- problem-specific code ----------

// count derangement
// note: need double as return type because results will be HUGE
double derangements(unsigned int move, unsigned int dontCare)
{
// don't need to move a prime away from its original position ?
if (move < 1)
return factorial(dontCare); // permutation of all remaining numbers

// recursion
move--;
auto result = dontCare * derangements(move,     dontCare);
if (move > 0)
result   += move     * derangements(move - 1, dontCare + 1);

return result;
}

int main()
{
unsigned int disks  = 100;
unsigned int primes =  25;
unsigned int moved  =  22;
std::cin >> moved;

// detect invalid input: for live test only
if (moved > primes)
return 1;

unsigned int unchanged = primes - moved;

// count ways
double result = derangements(moved, disks - primes);

// => 2300 ways to choose 3 primes
result *= choose(primes, unchanged);
// divide by total number of permutations
result /= factorial(disks);

// display result
std::cout << std::fixed << std::setprecision(12) << result << std::endl;
return 0;
}


This solution contains 13 empty lines, 15 comments and 2 preprocessor commands.

# Benchmark

The correct solution to the original Project Euler problem was found in less than 0.01 seconds on a Intel® Core™ i7-2600K CPU @ 3.40GHz.
(compiled for x86_64 / Linux, GCC flags: -O3 -march=native -fno-exceptions -fno-rtti -std=c++11 -DORIGINAL)

See here for a comparison of all solutions.

Note: interactive tests run on a weaker (=slower) computer. Some interactive tests are compiled without -DORIGINAL.

# Changelog

September 9, 2017 submitted solution

# Difficulty

Project Euler ranks this problem at 65% (out of 100%).

# Heatmap

Please click on a problem's number to open my solution to that problem:

 green solutions solve the original Project Euler problem and have a perfect score of 100% at Hackerrank, too yellow solutions score less than 100% at Hackerrank (but still solve the original problem easily) gray problems are already solved but I haven't published my solution yet blue solutions are relevant for Project Euler only: there wasn't a Hackerrank version of it (at the time I solved it) or it differed too much orange problems are solved but exceed the time limit of one minute or the memory limit of 256 MByte red problems are not solved yet but I wrote a simulation to approximate the result or verified at least the given example - usually I sketched a few ideas, too

 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200
 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300
 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400
 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500
 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575
The 270 solved problems (level 10) had an average difficulty of 31.3% at Project Euler and
I scored 13,386 points (out of 15600 possible points, top rank was 17 out of ≈60000 in August 2017) at Hackerrank's Project Euler+.

My username at Project Euler is stephanbrumme while it's stbrumme at Hackerrank.

Look at my progress and performance pages to get more details.

 << problem 235 - An Arithmetic Geometric sequence Top Dice - problem 240 >>
more about me can be found on my homepage, especially in my coding blog.
some names mentioned on this site may be trademarks of their respective owners.
thanks to the KaTeX team for their great typesetting library !