Merge pull request #4097 from Naereen/patch-7

[c/en] Just a few explanations, a couple of tiny changes

c.html.markdown

@@ -224,10 +224,18 @@ int main (int argc, char** argv)
   (float)i1 / i2; // => 0.5f
   i1 / (double)i2; // => 0.5 // Same with double
   f1 / f2; // => 0.5, plus or minus epsilon
-  // Floating-point numbers and calculations are not exact
   
-  // Modulo is there as well
-  11 % 3; // => 2
+  // Floating-point numbers and calculations are not exact
+  // for instance it is not giving mathematically correct results
+  (0.1 + 0.1 + 0.1) != 0.3; // => 1 (true)
+  // and it is NOT associative
+  1 + (1e123 - 1e123) != (1 + 1e123) - 1e123; // => 1 (true)
+  // this notation is scientific notations for numbers: 1e123 = 1*10^123
+
+  // Modulo is there as well, but be careful if arguments are negative
+  11 % 3;    // => 2 as 11 = 2 + 3*x (x=3)
+  (-11) % 3; // => -2, as one would expect
+  11 % (-3); // => 2 and not -2, and it's quite counter intuitive
 
   // Comparison operators are probably familiar, but
   // there is no Boolean type in C. We use ints instead.
@@ -241,7 +249,7 @@ int main (int argc, char** argv)
   2 <= 2; // => 1
   2 >= 2; // => 1
 
-  // C is not Python - comparisons don't chain.
+  // C is not Python - comparisons do NOT chain.
   // Warning: The line below will compile, but it means `(0 < a) < 2`.
   // This expression is always true, because (0 < a) could be either 1 or 0.
   // In this case it's 1, because (0 < 1).
@@ -349,26 +357,31 @@ int main (int argc, char** argv)
     break;
   }
   /*
-  using "goto" in C
+    Using "goto" in C
   */
   typedef enum { false, true } bool;
   // for C don't have bool as data type before C99 :(
   bool disaster = false;
   int i, j;
-  for(i=0;i<100;++i)
-  for(j=0;j<100;++j)
+  for(i=0; i<100; ++i)
+  for(j=0; j<100; ++j)
   {
     if((i + j) >= 150)
         disaster = true;
     if(disaster)
-        goto error;
+        goto error;  // exit both for loops
   }
-  error :
+  error: // this is a label that you can "jump" to with "goto error;"
   printf("Error occurred at i = %d & j = %d.\n", i, j);
   /*
     https://ideone.com/GuPhd6
     this will print out "Error occurred at i = 51 & j = 99."
   */
+  /*
+    it is generally considered bad practice to do so, except if
+    you really know what you are doing. See 
+    https://en.wikipedia.org/wiki/Spaghetti_code#Meaning
+  */
 
   ///////////////////////////////////////
   // Typecasting
@@ -741,11 +754,12 @@ typedef void (*my_fnp_type)(char *);
 // Order of Evaluation
 ///////////////////////////////////////
 
+// From top to bottom, top has higher precedence
 //---------------------------------------------------//
 //        Operators                  | Associativity //
 //---------------------------------------------------//
 // () [] -> .                        | left to right //
-// ! ~ ++ -- + = *(type)sizeof       | right to left //
+// ! ~ ++ -- + = *(type) sizeof      | right to left //
 // * / %                             | left to right //
 // + -                               | left to right //
 // << >>                             | left to right //
@@ -783,8 +797,8 @@ as the C file.
 /* included into files that include this header.                       */
 #include <string.h>
 
-/* Like c source files macros can be defined in headers and used in files */
-/* that include this header file.                                         */
+/* Like for c source files, macros can be defined in headers */
+/* and used in files that include this header file.          */
 #define EXAMPLE_NAME "Dennis Ritchie"
 
 /* Function macros can also be defined.  */
@@ -823,7 +837,7 @@ Best to find yourself a copy of [K&R, aka "The C Programming Language"](https://
 It is *the* book about C, written by Dennis Ritchie, the creator of C, and Brian Kernighan. Be careful, though - it's ancient and it contains some
 inaccuracies (well, ideas that are not considered good anymore) or now-changed practices.
 
-Another good resource is [Learn C The Hard Way](http://learncodethehardway.org/c/).
+Another good resource is [Learn C The Hard Way](http://learncodethehardway.org/c/) (not free).
 
 If you have a question, read the [compl.lang.c Frequently Asked Questions](http://c-faq.com).
 
@@ -833,4 +847,4 @@ Readable code is better than clever code and fast code. For a good, sane coding
 
 Other than that, Google is your friend.
 
-[1] [Why isn't sizeof for a struct equal to the sum of sizeof of each member?](http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member)
+[1] [Why isn't sizeof for a struct equal to the sum of sizeof of each member?](https://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member)