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Added comments to the code of problems 36, 37, 38, 39 and 40 in C.

C/p036.c

@@ -1,8 +1,16 @@
+/* The decimal number, 585 = 1001001001_2 (binary), is palindromic in both bases.
+ *
+ * Find the sum of all numbers, less than one million, which are palindromic in base 10 and base 2.
+ *
+ * (Please note that the palindromic number, in either base, may not include leading zeros.)*/
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
 #include "projecteuler.h"
 
+#define N 1000000
+
 int main(int argc, char **argv)
 {
    int i, sum = 0;
@@ -11,7 +19,10 @@ int main(int argc, char **argv)
 
    clock_gettime(CLOCK_MONOTONIC, &start);
 
-   for(i = 1; i < 1000000; i += 2)
+   /* Brute force approach. For every number below 1 million, 
+    * check if they're palindrome in base 2 and 10 using the
+    * function implemented in projecteuler.c.*/
+   for(i = 1; i < N; i += 2)
    {
       if(is_palindrome(i, 10) && is_palindrome(i, 2))
       {

C/p037.c

@@ -1,3 +1,10 @@
+/* The number 3797 has an interesting property. Being prime itself, it is possible to continuously remove digits from left to right,
+ * and remain prime at each stage: 3797, 797, 97, and 7. Similarly we can work from right to left: 3797, 379, 37, and 3.
+ *
+ * Find the sum of the only eleven primes that are both truncatable from left to right and right to left.
+ *
+ * NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.*/
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <math.h>
@@ -14,6 +21,7 @@ int main(int argc, char **argv)
 
    clock_gettime(CLOCK_MONOTONIC, &start);
 
+   /* Check every number until 11 truncatable primes are found.*/
    while(i < 11)
    {
       if(is_tr_prime(n))
@@ -40,11 +48,15 @@ int is_tr_prime(int n)
 {
    int i, tmp;
 
+   /* One-digit numbers and non-prime numbers are
+    * not truncatable primes.*/
    if(n < 11 || !is_prime(n))
    {
       return 0;
    }
 
+   /* Remove one digit at a time from the right and check
+    * if the resulting number is prime. Return 0 if it isn't.*/
    tmp = n / 10;
 
    while(tmp > 0)
@@ -56,6 +68,9 @@ int is_tr_prime(int n)
       tmp /= 10;
    }
 
+   /* Starting from the last digit, check if it's prime, then
+    * add back one digit at a time on the left and check if it
+    * is prime. Return 0 when it isn't.*/
    i = 10;
    tmp = n % i;
 
@@ -69,5 +84,6 @@ int is_tr_prime(int n)
       tmp = n % i;
    }
 
+   /* If it gets here, the number is truncatable prime.*/
    return 1;
 }

C/p038.c

@@ -1,3 +1,16 @@
+/* Take the number 192 and multiply it by each of 1, 2, and 3:
+ *
+ * 192 × 1 = 192
+ * 192 × 2 = 384
+ * 192 × 3 = 576
+ *
+ * By concatenating each product we get the 1 to 9 pandigital, 192384576. We will call 192384576 the concatenated product of 192 and (1,2,3)
+ *
+ * The same can be achieved by starting with 9 and multiplying by 1, 2, 3, 4, and 5, giving the pandigital, 918273645, which is the concatenated
+ * product of 9 and (1,2,3,4,5).
+
+What is the largest 1 to 9 pandigital 9-digit number that can be formed as the concatenated product of an integer with (1,2, ... , n) where n > 1?*/
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
@@ -12,6 +25,12 @@ int main(int argc, char **argv)
 
    clock_gettime(CLOCK_MONOTONIC, &start);
 
+   /* A brute force approach is used, starting with 1 and multiplying
+    * the number by 1, 2 etc., concatenating the results, checking if 
+    * it's 1 to 9 pandigital, and going to the next number when the
+    * concatenated result is greater than the greatest 9 digit pandigital
+    * value. The limit is set to 10000, since 1*10000=10000, 2*10000=20000 and
+    * concatenating this two numbers a 10-digit number is obtained.*/
    for(i = 1; i < 10000; i++)
    {
       n = 0;
@@ -60,4 +79,3 @@ int main(int argc, char **argv)
 
    return 0;
 }
-

C/p039.c

@@ -1,3 +1,9 @@
+/* If p is the perimeter of a right angle triangle with integral length sides, {a,b,c}, there are exactly three solutions for p = 120.
+ *
+ * {20,48,52}, {24,45,51}, {30,40,50}
+ *
+ * For which value of p ≤ 1000, is the number of solutions maximised?*/
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
@@ -11,6 +17,7 @@ int main(int argc, char **argv)
 
    clock_gettime(CLOCK_MONOTONIC, &start);
 
+   /* Start with p=12 (the smallest pythagorean triplet is (3,4,5) and 3+4+5=12.*/
    for(p = 12; p <= 1000; p++)
    {
       count = 0;
@@ -18,6 +25,7 @@ int main(int argc, char **argv)
       b = 0;
       c = 0;
 
+      /* Generate pythagorean triplets.*/
       for(m = 2; m * m < p; m++)
       {
          for(n = 1; n < m; n++)
@@ -26,6 +34,9 @@ int main(int argc, char **argv)
             b = 2 * m * n;
             c = m * m + n * n;
 
+            /* Increase counter if a+b+c=p and the triplet is new,
+             * then save the value of c to avoid counting the same 
+             * triplet more than once.*/
             if(a + b + c == p && !savedc[c])
             {
                savedc[c] = 1;
@@ -37,12 +48,15 @@ int main(int argc, char **argv)
             tmpb = b;
             tmpc = c;
 
+            /* Check all the triplets obtained multiplying a, b and c
+             * for integer numbers, until the perimeters exceeds p.*/
             while(tmpa + tmpb + tmpc < p)
             {
                tmpa = a * i;
                tmpb = b * i;
                tmpc = c * i;
 
+               /* Increase counter if the new a, b and c give a perimeter=p.*/
                if(tmpa + tmpb + tmpc == p && !savedc[tmpc])
                {
                   savedc[tmpc] = 1;
@@ -54,6 +68,8 @@ int main(int argc, char **argv)
          }
       }
 
+      /* If the current value is greater than the maximum,
+       * save the new maximum and the value of p.*/
       if(count > max)
       {
          max = count;

C/p040.c

@@ -1,3 +1,13 @@
+/* An irrational decimal fraction is created by concatenating the positive integers:
+ *
+ * 0.123456789101112131415161718192021...
+ *
+ * It can be seen that the 12th digit of the fractional part is 1.
+ *
+ * If d_n represents the nth digit of the fractional part, find the value of the following expression.
+ *
+ * d_1 × d_10 × d_100 × d_1000 × d_10000 × d_100000 × d_1000000*/
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
@@ -14,6 +24,9 @@ int main(int argc, char **argv)
    i = 1;
    value = 1;
 
+   /* Loop on all numbers and put the digits in the right place
+    * in an array. Use modulo and division to get the digits
+    * for numbers with more than one digit.*/
    while(i <= 1000000)
    {
       if(value < 10)
@@ -64,6 +77,7 @@ int main(int argc, char **argv)
       value++;
    }
 
+   /* Calculate the product.*/
    n = digits[0] * digits[9] * digits[99] * digits[999] * digits[9999] * digits[99999] * digits[999999];
 
    clock_gettime(CLOCK_MONOTONIC, &end);