Merge pull request #34 from theseoafs/master

Making C tutorial better

c.html.markdown

@@ -27,7 +27,7 @@ void function_2();
 
 // Your program's entry point is a function called
 // main with an integer return type.
-int main(){
+int main() {
 
 // print output using printf, for "print formatted"
 // %d is an integer, \n is a newline
@@ -38,36 +38,49 @@ printf("%d\n", 0); // => Prints 0
 // Types
 ///////////////////////////////////////
 
-// Variables must always be declared with a type.
+// You have to declare variables before using them. A variable declaration
+// requires you to specify its type; a variable's type determines its size
+// in bytes.
 
-// 32-bit integer
+// ints are usually 4 bytes 
 int x_int = 0;
 
-// 16-bit integer
+// shorts are usually 2 bytes
 short x_short = 0;
 
-// 8-bit integer, aka 1 byte
+// chars are guaranteed to be 1 byte
 char x_char = 0;
 char y_char = 'y'; // Char literals are quoted with ''
 
-long x_long = 0; // Still 32 bits for historical reasons
-long long x_long_long = 0; // Guaranteed to be at least 64 bits
+// longs are often 4 to 8 bytes; long longs are guaranteed to be at least
+// 64 bits
+long x_long = 0;
+long long x_long_long = 0; 
 
-// 32-bit floating-point decimal
+// floats are usually 32-bit floating point numbers
 float x_float = 0.0;
 
-// 64-bit floating-point decimal
+// doubles are usually 64-bit floating-point numbers
 double x_double = 0.0;
 
-// Integer types may be unsigned
+// Integral types may be unsigned. This means they can't be negative, but
+// the maximum value of an unsigned variable is greater than the maximum
+// value of the same size.
 unsigned char ux_char;
 unsigned short ux_short;
 unsigned int ux_int;
 unsigned long long ux_long_long;
 
+// Other than char, which is always 1 byte, these types vary in size depending
+// on your machine. sizeof(T) gives you the size of a variable with type T in 
+// bytes so you can express the size of these types in a portable way.
+// For example,
+printf("%d\n", sizeof(int)); // => 4 (on machines with 4-byte words)
+
 // Arrays must be initialized with a concrete size.
 char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes
 int my_int_array[20]; // This array occupies 4 * 20 = 80 bytes
+                      // (assuming 4-byte words)
 
 
 // You can initialize an array to 0 thusly:
@@ -81,16 +94,20 @@ my_array[0]; // => 0
 my_array[1] = 2;
 printf("%d\n", my_array[1]); // => 2
 
-// Strings are just lists of chars terminated by a null (0x00) byte.
+// Strings are just arrays of chars terminated by a NUL (0x00) byte,
+// represented in strings as the special character '\0'.
+// (We don't have to include the NUL byte in string literals; the compiler
+//  inserts it at the end of the array for us.)
 char a_string[20] = "This is a string";
+printf("%s\n", a_string); // %s formats a string
 
 /*
 You may have noticed that a_string is only 16 chars long.
-Char #17 is a null byte, 0x00 aka \0. 
+Char #17 is the NUL byte. 
 Chars #18, 19 and 20 have undefined values.
 */
 
-printf("%d\n", a_string[16]);
+printf("%d\n", a_string[16]); => 0
 
 ///////////////////////////////////////
 // Operators
@@ -112,7 +129,8 @@ f1 / f2; // => 0.5, plus or minus epsilon
 
 // Comparison operators are probably familiar, but
 // there is no boolean type in c. We use ints instead.
-// 0 is false, anything else is true
+// 0 is false, anything else is true. (The comparison 
+// operators always return 0 or 1.)
 3 == 2; // => 0 (false)
 3 != 2; // => 1 (true)
 3 > 2; // => 1
@@ -140,33 +158,33 @@ f1 / f2; // => 0.5, plus or minus epsilon
 // Control Structures
 ///////////////////////////////////////
 
-if(0){
+if (0) {
   printf("I am never run\n");
-}else if(0){
+} else if (0) {
   printf("I am also never run\n");
-}else{
+} else {
   printf("I print\n");
 }
 
 // While loops exist
 int ii = 0;
-while(ii < 10){
+while (ii < 10) {
     printf("%d, ", ii++); // ii++ increments ii in-place, after using its value.
 } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
 
 printf("\n");
 
 int kk = 0;
-do{
+do {
     printf("%d, ", kk);
-}while(++kk < 10); // ++kk increments kk in-place, before using its value
+} while (++kk < 10); // ++kk increments kk in-place, before using its value
 // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
 
 printf("\n");
 
 // For loops too
 int jj;
-for(jj=0; jj < 10; jj++){
+for (jj=0; jj < 10; jj++) {
     printf("%d, ", jj);
 } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
 
@@ -176,8 +194,8 @@ printf("\n");
 // Typecasting
 ///////////////////////////////////////
 
-// Everything in C is stored somewhere in memory. You can change
-// the type of a variable to choose how to read its data
+// Every value in C has a type, but you can cast one value into another type
+// if you want.
 
 int x_hex = 0x01; // You can assign vars with hex literals
 
@@ -188,7 +206,11 @@ printf("%d\n", (char) x_hex); // => Prints 1
 
 // Types will overflow without warning
 printf("%d\n", (char) 257); // => 1 (Max char = 255)
-printf("%d\n", (short) 65537); // => 1 (Max short = 65535)
+
+// Integral types can be cast to floating-point types, and vice-versa.
+printf("%f\n", (float)100); // %f formats a float
+printf("%lf\n", (double)100); // %lf formats a double
+printf("%d\n", (char)100.0);
 
 ///////////////////////////////////////
 // Pointers
@@ -221,13 +243,14 @@ printf("%d\n", x); // => Prints 1
 
 int x_array[20]; // Arrays are a good way to allocate a contiguous block of memory
 int xx;
-for(xx=0; xx<20; xx++){
+for (xx=0; xx<20; xx++) {
     x_array[xx] = 20 - xx;
 } // Initialize x_array to 20, 19, 18,... 2, 1
 
 // Declare a pointer of type int and initialize it to point to x_array
 int* x_ptr = x_array;
-// This works because an array name is bound to the address of its first element
+// x_ptr now points to the first element in the array (the integer 20). 
+// This works because arrays are actually just pointers to their first element.
 
 // Arrays are pointers to their first element
 printf("%d\n", *(x_ptr)); // => Prints 20
@@ -237,33 +260,27 @@ printf("%d\n", x_array[0]); // => Prints 20
 printf("%d\n", *(x_ptr + 1)); // => Prints 19
 printf("%d\n", x_array[1]); // => Prints 19
 
-// Array indexes are such a thin wrapper around pointer
-// arithmetic that the following works:
-printf("%d\n", 0[x_array]); // => Prints 20;
-printf("%d\n", 2[x_array]); // => Prints 18;
-
-// The above is equivalent to:
-printf("%d\n", *(0 + x_ptr));
-printf("%d\n", *(2 + x_ptr));
-
-// You can give a pointer a block of memory to use with malloc
+// You can also dynamically allocate contiguous blocks of memory with the
+// standard library function malloc, which takes one integer argument 
+// representing the number of bytes to allocate from the heap.
 int* my_ptr = (int*) malloc(sizeof(int) * 20);
-for(xx=0; xx<20; xx++){
-    *(my_ptr + xx) = 20 - xx;
+for (xx=0; xx<20; xx++) {
+    *(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx would also work here
 } // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
 
 // Dereferencing memory that you haven't allocated gives
 // unpredictable results
 printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what?
 
-// When you're done with a malloc'd block, you need to free it
+// When you're done with a malloc'd block of memory, you need to free it, 
+// or else no one else can use it until your program terminates
 free(my_ptr);
 
 // Strings can be char arrays, but are usually represented as char
 // pointers:
 char* my_str = "This is my very own string";
 
-printf("%d\n", *my_str); // 84 (The ascii value of 'T')
+printf("%c\n", *my_str); // => 'T'
 
 function_1();
 } // end main function