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Took a run at C
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---
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language: c
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author: Adam Bard
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author_url: http://adambard.com/
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---
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Ah, C. Still the language of modern high-performance computing.
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C is the lowest-level language most programmers will ever use, but
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it more than makes up for it with raw speed. Just be aware of its manual
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memory management and C will take you as far as you need to go.
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```c
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// Single-line comments start with //
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/*
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Multi-line comments look like this.
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*/
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// Import headers with #include
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#include <stdlib.h>
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#include <stdio.h>
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// Declare function signatures in advance in a .h file, or at the top of
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// your .c file.
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void function_1();
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void function_2();
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// Your program's entry point is a function called
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// main with an integer return type.
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int main(){
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// print output using printf, for "print formatted"
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// %d is an integer, \n is a newline
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printf("%d\n", 0); // => Prints 0
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// All statements must end with a semicolon
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///////////////////////////////////////
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// Types
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///////////////////////////////////////
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// Variables must always be declared with a type.
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// 32-bit integer
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int x_int = 0;
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// 16-bit integer
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short x_short = 0;
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// 8-bit integer, aka 1 byte
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char x_char = 0;
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char y_char = 'y'; // Char literals are quoted with ''
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long x_long = 0; // Still 32 bytes for historical reasons
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long long x_long_long = 0; // Guaranteed to be at least 64 bytes
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// 32-bit floating-point decimal
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float x_float = 0.0;
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// 64-bit floating-point decimal
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double x_double = 0.0;
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// Integer types may be unsigned
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unsigned char ux_char;
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unsigned short ux_short;
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unsigned int ux_int;
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unsigned long long ux_long_long;
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// Arrays must be initialized with a concrete size.
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char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes
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int my_int_array[20]; // This array occupies 4 * 20 = 80 bytes
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// You can initialize an array to 0 thusly:
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char my_array[20] = {0};
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// Indexing an array is like other languages -- or,
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// rather, other languages are like C
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my_array[0]; // => 0
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// Arrays are mutable; it's just memory!
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my_array[1] = 2;
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printf("%d\n", my_array[1]); // => 2
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// Strings are just lists of chars terminated by a null (0x00) byte.
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char a_string[20] = "This is a string";
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/*
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You may have noticed that a_string is only 16 chars long.
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Char #17 is a null byte, 0x00 aka \0.
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Chars #18, 19 and 20 have undefined values.
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*/
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printf("%d\n", a_string[16]);
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///////////////////////////////////////
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// Operators
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///////////////////////////////////////
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int i1 = 1, i2 = 2; // Shorthand for multiple declaration
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float f1 = 1.0, f2 = 2.0;
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// Arithmetic is straightforward
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i1 + i2; // => 3
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i2 - i1; // => 1
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i2 * i1; // => 2
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i1 / i2; // => 0 (0.5, but truncated towards 0)
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f1 / f2; // => 0.5, plus or minus epsilon
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// Modulo is there as well
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11 % 3; // => 2
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// Comparison operators are probably familiar, but
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// there is no boolean type in c. We use ints instead.
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// 0 is false, anything else is true
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3 == 2; // => 0 (false)
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3 != 2; // => 1 (true)
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3 > 2; // => 1
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3 < 2; // => 0
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2 <= 2; // => 1
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2 >= 2; // => 1
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// Logic works on ints
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!3; // => 0 (Logical not)
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!0; // => 1
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1 && 1; // => 1 (Logical and)
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0 && 1; // => 0
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0 || 1; // => 1 (Logical or)
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0 || 0; // => 0
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// Bitwise operators!
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~0x0F; // => 0xF0 (bitwise negation)
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0x0F & 0xF0; // => 0x00 (bitwise AND)
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0x0F | 0xF0; // => 0xFF (bitwise OR)
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0x04 | 0x0F; // => 0x0A (bitwise XOR)
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0x01 << 1; // => 0x02 (bitwise left shift (by 1))
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0x02 >> 1; // => 0x01 (bitwise right shift (by 1))
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///////////////////////////////////////
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// Control Structures
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///////////////////////////////////////
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if(0){
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printf("I am never run\n");
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}else if(0){
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printf("I am also never run\n");
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}else{
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printf("I print\n");
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}
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// While loops exist
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int ii = 0;
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while(ii < 10){
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printf("%d, ", ii++); // ii++ increments ii in-place, after using its value.
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} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
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printf("\n");
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int kk = 0;
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do{
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printf("%d, ", kk);
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}while(++kk < 10); // ++kk increments kk in-place, before using its value
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// => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
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printf("\n");
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// For loops too
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int jj;
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for(jj=0; jj < 10; jj++){
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printf("%d, ", jj);
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} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
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printf("\n");
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///////////////////////////////////////
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// Typecasting
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///////////////////////////////////////
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// Everything in C is stored somewhere in memory. You can change
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// the type of a variable to choose how to read its data
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int x_hex = 0x01; // You can assign vars with hex literals
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// Casting between types will attempt to preserve their numeric values
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printf("%d\n", x_hex); // => Prints 1
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printf("%d\n", (short) x_hex); // => Prints 1
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printf("%d\n", (char) x_hex); // => Prints 1
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// Types will overflow without warning
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printf("%d\n", (char) 257); // => 1 (Max char = 255)
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printf("%d\n", (short) 65537); // => 1 (Max short = 65535)
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///////////////////////////////////////
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// Pointers
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///////////////////////////////////////
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// You can retrieve the memory address of your variables,
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// then mess with them.
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int x = 0;
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printf("%p\n", &x); // Use & to retrive the address of a variable
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// (%p formats a pointer)
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// => Prints some address in memory;
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int x_array[20]; // Arrays are a good way to allocate a contiguous block of memory
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int xx;
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for(xx=0; xx<20; xx++){
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x_array[xx] = 20 - xx;
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} // Initialize x_array to 20, 19, 18,... 2, 1
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// Pointer types end with *
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int* x_ptr = x_array;
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// This works because arrays are pointers to their first element.
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// Put a * in front to de-reference a pointer and retrieve the value,
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// of the same type as the pointer, that the pointer is pointing at.
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printf("%d\n", *(x_ptr)); // => Prints 20
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printf("%d\n", x_array[0]); // => Prints 20
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// Pointers are incremented and decremented based on their type
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printf("%d\n", *(x_ptr + 1)); // => Prints 19
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printf("%d\n", x_array[1]); // => Prints 19
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// Array indexes are such a thin wrapper around pointer
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// arithmatic that the following works:
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printf("%d\n", 0[x_array]); // => Prints 20;
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printf("%d\n", 2[x_array]); // => Prints 18;
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// The above is equivalent to:
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printf("%d\n", *(0 + x_ptr));
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printf("%d\n", *(2 + x_ptr));
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// You can give a pointer a block of memory to use with malloc
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int* my_ptr = (int*) malloc(sizeof(int) * 20);
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for(xx=0; xx<20; xx++){
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*(my_ptr + xx) = 20 - xx;
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} // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
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// Dereferencing memory that you haven't allocated gives
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// unpredictable results
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printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what?
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// When you're done with a malloc'd block, you need to free it
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free(my_ptr);
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// Strings can be char arrays, but are usually represented as char
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// pointers:
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char* my_str = "This is my very own string";
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printf("%d\n", *my_str); // 84 (The ascii value of 'T')
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function_1();
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} // end main function
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///////////////////////////////////////
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// Functions
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///////////////////////////////////////
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// Function declaration syntax:
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// <return type> <function name>(<args>)
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int add_two_ints(int x1, int x2){
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return x1 + x2; // Use return a return a value
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}
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/*
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Pointers are passed-by-reference (duh), so functions
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can mutate their values.
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Example: in-place string reversal
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*/
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// A void function returns no value
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void str_reverse(char* str_in){
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char tmp;
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int ii=0, len = strlen(str_in); // Strlen is part of the c standard library
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for(ii=0; ii<len/2; ii++){
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tmp = str_in[ii];
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str_in[ii] = str_in[len - ii - 1]; // ii-th char from end
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str_in[len - ii - 1] = tmp;
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}
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}
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/*
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char c[] = "This is a test.";
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str_reverse(c);
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printf("%s\n", c); // => ".tset a si sihT"
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*/
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///////////////////////////////////////
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// User-defined types and structs
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///////////////////////////////////////
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// Typedefs can be used to create type aliases
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typedef int my_type;
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my_type my_type_var = 0;
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// Structs are just collections of data
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struct rectangle {
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int width;
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int height;
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};
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void function_1(){
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struct rectangle my_rec;
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// Access struct members with .
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my_rec.width = 10;
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my_rec.height = 20;
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// You can declare pointers to structs
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struct rectangle* my_rec_ptr = &my_rec;
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// Use dereferencing to set struct pointer members...
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(*my_rec_ptr).width = 30;
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// ... or use the -> shorthand
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my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10;
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}
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// You can apply a typedef to a struct for convenience
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typedef struct rectangle rect;
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int area(rect r){
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return r.width * r.height;
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}
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```
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## Further Reading
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Best to find yourself a copy of [K&R, aka "The C Programming Language"](https://en.wikipedia.org/wiki/The_C_Programming_Language)
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Other than that, Google is your friend.
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