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  1. /* 
  2.  * We save the spiral in an n x n Matrix using a 2D array
  3.  * The number at coordinates (x, y) is in the x-th row and the y-th column
  4.  *
  5.  * /  -  -  -  y
  6.  * |  1  2  3  4
  7.  * | 12 13 14  5
  8.  * | 11 16 15  6
  9.  * x 10  9  8  7
  10.  *
  11.  */
  12.  
  13. #include <stdio.h>
  14. #include <stdbool.h>
  15. #include <math.h>
  16.  
  17. void print(int n, int spiral[n][n]) {
  18.     int square = n * n;
  19.  
  20.     int numDigits = floor(log10(square)) + 1;
  21.  
  22.     for (int i = 0; i < n; ++i) {
  23.         for (int j = 0; j < n; ++j) {
  24.             printf("%*d ", numDigits, spiral[i][j]);
  25.         }
  26.         printf("\n");
  27.     }
  28.  
  29.     printf("\n");
  30. }
  31.  
  32. int main(int argc, char *argv[]) {
  33.     bool reverse = argc > 1;
  34.  
  35.     while(1) {
  36.         int num = 0;
  37.         scanf("%d", &num);
  38.  
  39.         if(num <= 0) {
  40.             // Abort on non positive numbers
  41.             break;
  42.         }
  43.  
  44.         int spiral[num][num];
  45.  
  46.         /*
  47.          * The coordinates in the spiral we are currently at
  48.          */
  49.         int x = 0, y = 0;
  50.  
  51.         /*
  52.          * For each number we travel on in the direction given by these two variables
  53.          * With these initial values we travel from the top left to the right
  54.          */
  55.         int xDiff = 0;
  56.         int yDiff = 1;
  57.  
  58.         /*
  59.          * When walking the spiral this counts how many turns are left until we completed one layer of the spiral
  60.          * In the beginning there are three turns after that there are always two turns
  61.          */
  62.         int turns = 3;
  63.  
  64.         /*
  65.          * Length of edges in the current layer
  66.          * The number of steps we can take on an edge before we need to turn depends on what layer we are currently in
  67.          * in the beginning it is one less than the side length of the spiral and decreases by one for each layer
  68.          */
  69.         int len = num - 1;
  70.  
  71.         /*
  72.          * Counts how many steps we have remaining before we need to turn
  73.          */
  74.         int lenRemaining = len;
  75.  
  76.         for (int i = 1; i <= num * num; ++i) {
  77.             /*
  78.              * Just in case
  79.              */
  80.             if(x >= num || y >= num || x < 0 || y < 0) {
  81.                 fprintf(stderr, "An error occured while walking the spiral\n");
  82.                 continue;
  83.             }
  84.  
  85.             spiral[x][y] = i;
  86.  
  87.             if(lenRemaining == 0) {
  88.                 /*
  89.                  * We need to turn
  90.                  */
  91.  
  92.                 turns--;
  93.  
  94.                 /*
  95.                  * Set the new movement vectors
  96.                  */
  97.                 int xDiffOld = xDiff;
  98.                 xDiff = yDiff;
  99.                 yDiff = -xDiffOld;
  100.  
  101.                 /*
  102.                  * If we have no turns left, we have reached a new layer
  103.                  */
  104.                 if(turns == 0) {
  105.                     len--;
  106.                     turns = 2;
  107.                 }
  108.  
  109.                 /*
  110.                  * Reset lenRemaining
  111.                  */
  112.                 lenRemaining = len;
  113.             }
  114.  
  115.             /*
  116.              * Apply movement vector and decrease remaining steps
  117.              */
  118.             x += reverse? yDiff : xDiff;
  119.             y += reverse? xDiff : yDiff;
  120.             lenRemaining--;
  121.         }
  122.  
  123.         print(num, spiral);
  124.     }
  125.     return 0;
  126. }
Success #stdin #stdout 0s 9432KB
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stdin
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5
4
stdout
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 1  2  3  4  5 
16 17 18 19  6 
15 24 25 20  7 
14 23 22 21  8 
13 12 11 10  9 

 1  2  3  4 
12 13 14  5 
11 16 15  6 
10  9  8  7
https://ideone.com/NOStEF
language:
C (gcc 8.3)
created:
6 years ago
visibility:
public

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