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Diffstat (limited to 'c.html.markdown')
| -rw-r--r-- | c.html.markdown | 129 |
1 files changed, 97 insertions, 32 deletions
diff --git a/c.html.markdown b/c.html.markdown index a57be1dc..7405c7ab 100644 --- a/c.html.markdown +++ b/c.html.markdown @@ -46,31 +46,47 @@ Multi-line comments don't nest /* Be careful */ // comment ends on this line... // Enumeration constants are also ways to declare constants. // All statements must end with a semicolon -enum days {SUN = 1, MON, TUE, WED, THU, FRI, SAT}; +enum days {SUN, MON, TUE, WED, THU, FRI, SAT}; +// SUN gets 0, MON gets 1, TUE gets 2, etc. + +// Enumeration values can also be specified +enum days {SUN = 1, MON, TUE, WED = 99, THU, FRI, SAT}; // MON gets 2 automatically, TUE gets 3, etc. +// WED get 99, THU gets 100, FRI gets 101, 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" +// File names between <angle brackets> tell the compiler to look in your system +// libraries for the headers. +// For your own headers, use double quotes instead of angle brackets, and +// provide the path: +#include "my_header.h" // local file +#include "../my_lib/my_lib_header.h" //relative path // Declare function signatures in advance in a .h file, or at the top of // your .c file. void function_1(); int function_2(void); -// Must declare a 'function prototype' before main() when functions occur after -// your main() function. +// At a minimum, you must declare a 'function prototype' before its use in any function. +// Normally, prototypes are placed at the top of a file before any function definition. int add_two_ints(int x1, int x2); // function prototype // although `int add_two_ints(int, int);` is also valid (no need to name the args), // it is recommended to name arguments in the prototype as well for easier inspection -// Your program's entry point is a function called -// main with an integer return type. +// Function prototypes are not necessary if the function definition comes before +// any other function that calls that function. However, it's standard practice to +// always add the function prototype to a header file (*.h) and then #include that +// file at the top. This prevents any issues where a function might be called +// before the compiler knows of its existence, while also giving the developer a +// clean header file to share with the rest of the project. + +// Your program's entry point is a function called "main". The return type can +// be anything, however most operating systems expect a return type of `int` for +// error code processing. int main(void) { // your program } @@ -96,13 +112,14 @@ int main (int argc, char** argv) // For the sake of the tutorial, variables are declared dynamically under // C99-compliant standards. - // ints are usually 4 bytes + // ints are usually 4 bytes (use the `sizeof` operator to check) int x_int = 0; - // shorts are usually 2 bytes + // shorts are usually 2 bytes (use the `sizeof` operator to check) short x_short = 0; - // chars are guaranteed to be 1 byte + // chars are defined as the smallest addressable unit for a processor. + // This is usually 1 byte, but for some systems it can be more (ex. for TMS320 from TI it's 2 bytes). char x_char = 0; char y_char = 'y'; // Char literals are quoted with '' @@ -224,24 +241,41 @@ 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 are defined by IEEE 754, thus cannot store perfectly + // exact values. For instance, the following does not produce expected results + // because 0.1 might actually be 0.099999999999 insided the computer, and 0.3 + // might be stored as 0.300000000001. + (0.1 + 0.1 + 0.1) != 0.3; // => 1 (true) + // and it is NOT associative due to reasons mentioned above. + 1 + (1e123 - 1e123) != (1 + 1e123) - 1e123; // => 1 (true) + // this notation is scientific notations for numbers: 1e123 = 1*10^123 + + // It is important to note that most all systems have used IEEE 754 to + // represent floating points. Even python, used for scientific computing, + // eventually calls C which uses IEEE 754. It is mentioned this way not to + // indicate that this is a poor implementation, but instead as a warning + // that when doing floating point comparisons, a little bit of error (epsilon) + // needs to be considered. + + // 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. - // (Or _Bool or bool in C99.) + // (C99 introduced the _Bool type provided in stdbool.h) // 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 + 3 > 2; // => 1 + 3 < 2; // => 0 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). @@ -349,25 +383,30 @@ 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." + 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 */ /////////////////////////////////////// @@ -378,13 +417,16 @@ int main (int argc, char** argv) // if you want (with some constraints). int x_hex = 0x01; // You can assign vars with hex literals + // binary is not in the standard, but allowed by some + // compilers (x_bin = 0b0010010110) // 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 + // If you assign a value greater than a types max val, it will rollover + // 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 `unsigned char`, @@ -575,6 +617,24 @@ printf("first: %d\nsecond: %d\n", first, second); // values will be swapped */ +// Return multiple values. +// C does not allow for returning multiple values with the return statement. If +// you would like to return multiple values, then the caller must pass in the +// variables where they would like the returned values to go. These variables must +// be passed in as pointers such that the function can modify them. +int return_multiple( int *array_of_3, int *ret1, int *ret2, int *ret3) +{ + if(array_of_3 == NULL) + return 0; //return error code (false) + + //de-reference the pointer so we modify its value + *ret1 = array_of_3[0]; + *ret2 = array_of_3[1]; + *ret3 = array_of_3[2]; + + return 1; //return error code (true) +} + /* With regards to arrays, they will always be passed to functions as pointers. Even if you statically allocate an array like `arr[10]`, @@ -703,6 +763,10 @@ typedef void (*my_fnp_type)(char *); // my_fnp_type f; +///////////////////////////// +// Printing characters with printf() +///////////////////////////// + //Special characters: /* '\a'; // alert (bell) character @@ -741,11 +805,12 @@ 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 +848,8 @@ 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 +888,7 @@ 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 +898,4 @@ 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) |