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+---
+language: c
+filename: learnc.c
+contributors:
+    - ["Adam Bard", "http://adambard.com/"]
+    - ["Árpád Goretity", "http://twitter.com/H2CO3_iOS"]
+
+---
+
+Ah, C. Still **the** language of modern high-performance computing.
+
+C is the lowest-level language most programmers will ever use, but
+it more than makes up for it with raw speed. Just be aware of its manual
+memory management and C will take you as far as you need to go.
+
+```c
+// Single-line comments start with // - only available in C99 and later.
+
+/*
+Multi-line comments look like this. They work in C89 as well.
+*/
+
+//Special characters:
+'\a' // alert (bell) character
+'\n' // newline character
+'\t' // tab character (left justifies text)
+'\v' // vertical tab
+'\f' // new page (formfeed)
+'\r' // carriage return
+'\b' // backspace character
+'\0' // null character. Usually put at end of strings in C lang. 
+     //   hello\n\0. \0 used by convention to mark end of string. 
+'\\' // backspace
+'\?' // question mark
+'\'' // single quote
+'\"' // double quote
+'\xhh' // hexadecimal number. Example: '\xb' = vertical tab character
+'\ooo' // octal number. Example: '\013' = vertical tab character
+
+//print formatting:
+"%d"    // integer
+"%3d"   // integer with minimum of length 3 digits (right justifies text)
+"%s"    // string
+"%f"    // float
+"%ld"   // long
+"%3.2f" // minimum 3 digits left and 2 digits right decimal float 
+"%7.4s" // (can do with strings too)
+"%c"    // char
+"%p"    // pointer
+"%x"    // hexidecimal
+"%o"    // octal
+"%%"    // prints % 
+
+// Constants: #define <keyword> (no semicolon at end) 
+#define DAYS_IN_YEAR = 365
+
+//enumeration constants are also ways to declare constants. 
+enum days {SUN = 1, MON, TUE, WED, THU, FRI, SAT};
+// MON gets 2 automatically, TUE gets 3, etc. 
+
+// Import headers with #include
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+// (File names between <angle brackets> are headers from the C standard library.)
+// For your own headers, use double quotes instead of angle brackets:
+#include "my_header.h"
+
+// Declare function signatures in advance in a .h file, or at the top of
+// your .c file.
+void function_1();
+void function_2();
+
+// Your program's entry point is a function called
+// main with an integer return type.
+int main() {
+    // print output using printf, for "print formatted"
+    // %d is an integer, \n is a newline
+    printf("%d\n", 0); // => Prints 0
+    // All statements must end with a semicolon
+
+    ///////////////////////////////////////
+    // Types
+    ///////////////////////////////////////
+
+    // ints are usually 4 bytes 
+    int x_int = 0;
+
+    // shorts are usually 2 bytes
+    short x_short = 0;
+
+    // chars are guaranteed to be 1 byte
+    char x_char = 0;
+    char y_char = 'y'; // Char literals are quoted with ''
+
+    // 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; 
+
+    // floats are usually 32-bit floating point numbers
+    float x_float = 0.0;
+
+    // doubles are usually 64-bit floating-point numbers
+    double x_double = 0.0;
+
+    // Integral types may be unsigned.
+    unsigned short ux_short;
+    unsigned int ux_int;
+    unsigned long long ux_long_long;
+
+    // chars inside single quotes are integers in machine's character set. 
+    '0' //==> 48 on the ASCII character set. 
+    'A' //==> 65 on the ASCII character set. 
+
+    // sizeof(T) gives you the size of a variable with type T in bytes
+    // sizeof(obj) yields the size of the expression (variable, literal, etc.).
+    printf("%zu\n", sizeof(int)); // => 4 (on most machines with 4-byte words)
+
+
+    // If the argument of the `sizeof` operator an expression, then its argument
+    // is not evaluated (except VLAs (see below)).
+    // The value it yields in this case is a compile-time constant.
+    int a = 1;
+    size_t size = sizeof(a++); // a++ is not evaluated
+    printf("sizeof(a++) = %zu where a = %d\n", size, a);
+    // prints "sizeof(a++) = 4 where a = 1" (on a 32-bit architecture)
+
+    // 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:
+    char my_array[20] = {0};
+
+    // Indexing an array is like other languages -- or,
+    // rather, other languages are like C
+    my_array[0]; // => 0
+
+    // Arrays are mutable; it's just memory!
+    my_array[1] = 2;
+    printf("%d\n", my_array[1]); // => 2
+
+    // In C99 (and as an optional feature in C11), variable-length arrays (VLAs)
+    // can be declared as well. The size of such an array need not be a compile
+    // time constant:
+    printf("Enter the array size: "); // ask the user for an array size
+    char buf[0x100];
+    fgets(buf, sizeof buf, stdin);
+
+    // strtoul parses a string to an unsigned integer
+    size_t size = strtoul(buf, NULL, 10);
+    int var_length_array[size]; // declare the VLA
+    printf("sizeof array = %zu\n", sizeof var_length_array);
+
+    // A possible outcome of this program may be:
+    // > Enter the array size: 10
+    // > sizeof array = 40
+
+    // 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
+
+    printf("%d\n", a_string[16]); // => 0
+    // i.e., byte #17 is 0 (as are 18, 19, and 20)
+
+    // If we have characters between single quotes, that's a character literal.
+    // It's of type `int`, and *not* `char` (for historical reasons).
+    int cha = 'a'; // fine
+    char chb = 'a'; // fine too (implicit conversion from int to char)
+
+    ///////////////////////////////////////
+    // Operators
+    ///////////////////////////////////////
+
+    int i1 = 1, i2 = 2; // Shorthand for multiple declaration
+    float f1 = 1.0, f2 = 2.0;
+
+    //more shorthands:
+    int a, b, c;
+    a = b = c = 0;
+
+    // Arithmetic is straightforward
+    i1 + i2; // => 3
+    i2 - i1; // => 1
+    i2 * i1; // => 2
+    i1 / i2; // => 0 (0.5, but truncated towards 0)
+
+    f1 / f2; // => 0.5, plus or minus epsilon
+    // Floating-point numbers and calculations are not exact
+
+    // Modulo is there as well
+    11 % 3; // => 2
+
+    // Comparison operators are probably familiar, but
+    // there is no boolean type in c. We use ints instead.
+    // (Or _Bool or bool in C99.)
+    // 0 is false, anything else is true. (The comparison 
+    // operators always yield 0 or 1.)
+    3 == 2; // => 0 (false)
+    3 != 2; // => 1 (true)
+    3 > 2; // => 1
+    3 < 2; // => 0
+    2 <= 2; // => 1
+    2 >= 2; // => 1
+
+    // C is not Python - comparisons don't chain.
+    int a = 1;
+    // WRONG:
+    int between_0_and_2 = 0 < a < 2;
+    // Correct:
+    int between_0_and_2 = 0 < a && a < 2;
+
+    // Logic works on ints
+    !3; // => 0 (Logical not)
+    !0; // => 1
+    1 && 1; // => 1 (Logical and)
+    0 && 1; // => 0
+    0 || 1; // => 1 (Logical or)
+    0 || 0; // => 0
+
+    //Conditional expression ( ? : )
+    int a, b, z;
+    z = (a > b) ? a : b; // "if a > b return a, else return b." 
+
+    //Increment and decrement operators:
+    s[j++]; //returns value of j to s THEN increments value of j. 
+    s[++j]; //increments value of j THEN returns value of j to s. 
+    // same with j-- and --j
+
+    // Bitwise operators!
+    ~0x0F; // => 0xF0 (bitwise negation, "1's complement")
+    0x0F & 0xF0; // => 0x00 (bitwise AND)
+    0x0F | 0xF0; // => 0xFF (bitwise OR)
+    0x04 ^ 0x0F; // => 0x0B (bitwise XOR)
+    0x01 << 1; // => 0x02 (bitwise left shift (by 1))
+    0x02 >> 1; // => 0x01 (bitwise right shift (by 1))
+
+    // Be careful when shifting signed integers - the following are undefined:
+    // - shifting into the sign bit of a signed integer (int a = 1 << 32)
+    // - left-shifting a negative number (int a = -1 << 2)
+    // - shifting by an offset which is >= the width of the type of the LHS:
+    //   int a = 1 << 32; // UB if int is 32 bits wide
+
+    ///////////////////////////////////////
+    // Control Structures
+    ///////////////////////////////////////
+
+    if (0) {
+      printf("I am never run\n");
+    } else if (0) {
+      printf("I am also never run\n");
+    } else {
+      printf("I print\n");
+    }
+
+    // While loops exist
+    int ii = 0;
+    while (ii < 10) { //ANY value not zero is true. 
+        printf("%d, ", ii++); // ii++ increments ii AFTER using it's current value.
+    } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
+
+    printf("\n");
+
+    int kk = 0;
+    do {
+        printf("%d, ", kk);
+    } while (++kk < 10); // ++kk increments kk BEFORE using it's current 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++) {
+        printf("%d, ", jj);
+    } // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
+
+    printf("\n");
+
+    // *****NOTES*****:
+    // Loops MUST always have a body. If no body is needed, do:
+    for (i = 0; i <= 5; i++) {
+        ; // use semicolon to act as the body (null statement)
+    }
+
+    // branching with multiple choices: switch()
+    switch (some_integral_expression) {
+    case 0: // labels need to be integral *constant* epxressions
+        do_stuff();
+        break; // if you don't break, control flow falls over labels
+    case 1:
+        do_something_else();
+        break;
+    default:
+        // if `some_integral_expression` didn't match any of the labels
+        fputs("error!\n", stderr);
+        exit(-1);
+        break;
+    }
+    
+
+    ///////////////////////////////////////
+    // Typecasting
+    ///////////////////////////////////////
+
+    // Every value in C has a type, but you can cast one value into another type
+    // if you want (with some constraints).
+
+    int x_hex = 0x01; // You can assign vars with hex literals
+
+    // Casting between types will attempt to preserve their numeric values
+    printf("%d\n", x_hex); // => Prints 1
+    printf("%d\n", (short) x_hex); // => Prints 1
+    printf("%d\n", (char) x_hex); // => Prints 1
+
+    // Types will overflow without warning
+    printf("%d\n", (unsigned char) 257); // => 1 (Max char = 255 if char is 8 bits long)
+
+    // For determining the max value of a `char`, a `signed char` and an `unisigned char`,
+    // respectively, use the CHAR_MAX, SCHAR_MAX and UCHAR_MAX macros from <limits.h>
+
+    // 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
+    ///////////////////////////////////////
+
+    // A pointer is a variable declared to store a memory address. Its declaration will
+    // also tell you the type of data it points to. You can retrieve the memory address 
+    // of your variables, then mess with them.
+
+    int x = 0;
+    printf("%p\n", (void *)&x); // Use & to retrieve the address of a variable
+    // (%p formats an object pointer of type void *)
+    // => Prints some address in memory;
+
+
+    // Pointers start with * in their declaration
+    int *px, not_a_pointer; // px is a pointer to an int
+    px = &x; // Stores the address of x in px
+    printf("%p\n", (void *)px); // => Prints some address in memory
+    printf("%zu, %zu\n", sizeof(px), sizeof(not_a_pointer));
+    // => Prints "8, 4" on a typical 64-bit system
+
+    // To retreive the value at the address a pointer is pointing to,
+    // put * in front to de-reference it.
+    // Note: yes, it may be confusing that '*' is used for _both_ declaring a
+    // pointer and dereferencing it.
+    printf("%d\n", *px); // => Prints 0, the value of x
+
+    // You can also change the value the pointer is pointing to.
+    // We'll have to wrap the de-reference in parenthesis because
+    // ++ has a higher precedence than *.
+    (*px)++; // Increment the value px is pointing to by 1
+    printf("%d\n", *px); // => Prints 1
+    printf("%d\n", x); // => Prints 1
+
+    // Arrays are a good way to allocate a contiguous block of memory
+    int x_array[20]; //declares array of size 20 (cannot change size)
+    int 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;
+    // x_ptr now points to the first element in the array (the integer 20). 
+    // This works because arrays often decay into pointers to their first element.
+    // For example, when an array is passed to a function or is assigned to a pointer,
+    // it decays into (implicitly converted to) a pointer.
+    // Exceptions: when the array is the argument of the `&` (address-od) operator:
+    int arr[10];
+    int (*ptr_to_arr)[10] = &arr; // &arr is NOT of type `int *`!
+                                  // It's of type "pointer to array" (of ten `int`s).
+    // or when the array is a string literal used for initializing a char array:
+    char arr[] = "foobarbazquirk";
+    // or when it's the argument of the `sizeof` or `alignof` operator:
+    int arr[10];
+    int *ptr = arr; // equivalent with int *ptr = &arr[0];
+    printf("%zu %zu\n", sizeof arr, sizeof ptr); // probably prints "40, 4" or "40, 8"
+
+
+    // Pointers are incremented and decremented based on their type
+    // (this is called pointer arithmetic)
+    printf("%d\n", *(x_ptr + 1)); // => Prints 19
+    printf("%d\n", x_array[1]); // => Prints 19
+
+    // You can also dynamically allocate contiguous blocks of memory with the
+    // standard library function malloc, which takes one argument of type size_t
+    // representing the number of bytes to allocate (usually from the heap, although this
+    // may not be true on e. g. embedded systems - the C standard says nothing about it).
+    int *my_ptr = malloc(sizeof(*my_ptr) * 20);
+    for (xx = 0; xx < 20; xx++) {
+        *(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx
+    } // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
+
+    // Dereferencing memory that you haven't allocated gives
+    // "unpredictable results" - the program is said to invoke "undefined behavior"
+    printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what? It may even crash.
+
+    // 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
+    // (this is called a "memory leak"):
+    free(my_ptr);
+
+    // Strings are arrays of char, but they are usually represented as a
+    // pointer-to-char (which is a pointer to the first element of the array).
+    // It's good practice to use `const char *' when referring to a string literal,
+    // since string literals shall not be modified (i. e. "foo"[0] = 'a' is ILLEGAL.)
+    const char *my_str = "This is my very own string literal";
+    printf("%c\n", *my_str); // => 'T'
+
+    // This is not the case if the string is an array
+    // (potentially initialized with a string literal)
+    // that resides in writable memory, as in:
+    char foo[] = "foo";
+    foo[0] = 'a'; // this is legal, foo now contains "aoo"
+
+    function_1();
+} // end main function
+
+///////////////////////////////////////
+// Functions
+///////////////////////////////////////
+
+// Function declaration syntax:
+// <return type> <function name>(<args>)
+
+int add_two_ints(int x1, int x2)
+{
+    return x1 + x2; // Use return to return a value
+}
+
+<<<<<<< HEAD
+// Must declare a 'function prototype' before main() when creating functions
+// in file. 
+=======
+// Must declare a 'funtion prototype' when creating functions before main()
+>>>>>>> f28d33fb187bc834e6e2956117039f9abe3b6d9b
+void getInt(char c); // function prototype
+int main() {         // main function
+    return 0;
+}
+void getInt(char w) { //parameter name does not need to match function prototype
+    ;
+}
+
+//if function takes no parameters, do: 
+int getInt(void); for function prototype
+//  and for the function declaration: 
+int getInt(void) {}
+//  (this is to keep compatibility with older versions of C). 
+
+/*
+Functions are call by value. So when a function is called, the arguments passed
+to the function are copies of original arguments (except arrays). Anything you  
+do to your arguments do not change the value of the actual argument where the
+function was called. 
+
+You can use pointers if you need to edit the original argument values. 
+
+Example: in-place string reversal
+*/
+
+// A void function returns no value
+void str_reverse(char *str_in)
+{
+    char tmp;
+    int ii = 0;
+    size_t len = strlen(str_in); // `strlen()` is part of the c standard library
+    for (ii = 0; ii < len / 2; ii++) {
+        tmp = str_in[ii];
+        str_in[ii] = str_in[len - ii - 1]; // ii-th char from end
+        str_in[len - ii - 1] = tmp;
+    }
+}
+
+/////////////////////////////////////
+// Built in functions:
+/////////////////////////////////////
+// from stdio.h:
+// getchar()
+int c = getchar(); //reads character from input. 
+// If input = hi, 'h' is returned then next call, 'i' returned. 
+while ((c = getchar()) != EOF) { // EOF constant "end of file". 
+                                 //   Linux: CTRL+D, Windows: CTRL+X
+    // must have () around getchar() as != is run before =. 
+    putchar(c); //prints character (without newline at end)
+    char c = getchar(); 
+}
+
+//if referring to external variables outside function, must use extern keyword.
+int i = 0;
+void testFunc() {
+    extern int i; //i here is now using external variable i
+}
+
+/*
+char c[] = "This is a test.";
+str_reverse(c);
+printf("%s\n", c); // => ".tset a si sihT"
+*/
+
+///////////////////////////////////////
+// User-defined types and structs
+///////////////////////////////////////
+
+// Typedefs can be used to create type aliases
+typedef int my_type;
+my_type my_type_var = 0;
+
+// Structs are just collections of data, the members are allocated sequentially,
+// in the order they are written:
+struct rectangle {
+    int width;
+    int height;
+};
+
+// It's not generally true that
+// sizeof(struct rectangle) == sizeof(int) + sizeof(int)
+// due to potential padding between the structure members (this is for alignment
+// reasons). [1]
+
+void function_1()
+{
+    struct rectangle my_rec;
+
+    // Access struct members with .
+    my_rec.width = 10;
+    my_rec.height = 20;
+
+    // You can declare pointers to structs
+    struct rectangle *my_rec_ptr = &my_rec;
+
+    // Use dereferencing to set struct pointer members...
+    (*my_rec_ptr).width = 30;
+
+    // ... or even better: prefer the -> shorthand for the sake of readability
+    my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10;
+}
+
+// You can apply a typedef to a struct for convenience
+typedef struct rectangle rect;
+
+int area(rect r)
+{
+    return r.width * r.height;
+}
+
+// if you have large structs, you can pass them "by pointer" to avoid copying
+// the whole struct:
+int area(const rect *r)
+{
+    return r->width * r->height;
+}
+
+///////////////////////////////////////
+// Function pointers 
+///////////////////////////////////////
+/*
+At runtime, functions are located at known memory addresses. Function pointers are
+much like any other pointer (they just store a memory address), but can be used 
+to invoke functions directly, and to pass handlers (or callback functions) around.
+However, definition syntax may be initially confusing.
+
+Example: use str_reverse from a pointer
+*/
+void str_reverse_through_pointer(char *str_in) {
+    // Define a function pointer variable, named f. 
+    void (*f)(char *); // Signature should exactly match the target function.
+    f = &str_reverse; // Assign the address for the actual function (determined at runtime)
+    // f = str_reverse; would work as well - functions decay into pointers, similar to arrays
+    (*f)(str_in); // Just calling the function through the pointer
+    // f(str_in); // That's an alternative but equally valid syntax for calling it.
+}
+
+/*
+As long as function signatures match, you can assign any function to the same pointer.
+Function pointers are usually typedef'd for simplicity and readability, as follows:
+*/
+
+typedef void (*my_fnp_type)(char *);
+
+// Then used when declaring the actual pointer variable:
+// ...
+// my_fnp_type f; 
+
+
+///////////////////////////////////////
+// Order of Evaluation
+///////////////////////////////////////
+
+//---------------------------------------------------//
+//        Operators                  | Associativity //
+//---------------------------------------------------//
+// () [] -> .                        | left to right //
+// ! ~ ++ -- + = *(type)sizeof       | right to left //
+// * / %                             | left to right //
+// + -                               | left to right //
+// << >>                             | left to right //
+// < <= > >=                         | left to right //
+// == !=                             | left to right //
+// &                                 | left to right //
+// ^                                 | left to right //
+// |                                 | left to right //
+// &&                                | left to right //
+// ||                                | left to right //
+// ?:                                | right to left //
+// = += -= *= /= %= &= ^= |= <<= >>= | right to left //
+// ,                                 | left to right //
+//---------------------------------------------------//
+
+```
+
+## Further Reading
+
+Best to find yourself a copy of [K&R, aka "The C Programming Language"](https://en.wikipedia.org/wiki/The_C_Programming_Language)
+It is *the* book about C, written by the creators of C. 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://c.learncodethehardway.org/book/).
+
+If you have a question, read the [compl.lang.c Frequently Asked Questions](http://c-faq.com).
+
+It's very important to use proper spacing, indentation and to be consistent with your coding style in general.
+Readable code is better than clever code and fast code. For a good, sane coding style to adopt, see the
+[Linux kernel coding stlye](https://www.kernel.org/doc/Documentation/CodingStyle).
+
+Other than that, Google is your friend.
+
+[1] http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member