summaryrefslogtreecommitdiffhomepage
path: root/c.html.markdown
diff options
context:
space:
mode:
Diffstat (limited to 'c.html.markdown')
-rw-r--r--c.html.markdown132
1 files changed, 84 insertions, 48 deletions
diff --git a/c.html.markdown b/c.html.markdown
index 15bfa05e..69bf099e 100644
--- a/c.html.markdown
+++ b/c.html.markdown
@@ -2,6 +2,7 @@
language: c
author: Adam Bard
author_url: http://adambard.com/
+filename: learnc.c
---
Ah, C. Still the language of modern high-performance computing.
@@ -12,6 +13,7 @@ memory management and C will take you as far as you need to go.
```c
// Single-line comments start with //
+
/*
Multi-line comments look like this.
*/
@@ -19,6 +21,7 @@ Multi-line comments look like this.
// Import headers with #include
#include <stdlib.h>
#include <stdio.h>
+#include <string.h>
// Declare function signatures in advance in a .h file, or at the top of
// your .c file.
@@ -27,7 +30,7 @@ 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 +41,49 @@ 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 bytes for historical reasons
-long long x_long_long = 0; // Guaranteed to be at least 64 bytes
+// 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("%lu\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 +97,20 @@ 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 +132,8 @@ 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 +161,33 @@ 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 +197,8 @@ 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
@@ -188,32 +209,53 @@ 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
///////////////////////////////////////
-// You can retrieve the memory address of your variables,
-// then mess with them.
+// 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", &x); // Use & to retrieve the address of a variable
// (%p formats a pointer)
// => Prints some address in memory;
+// Pointer types end with * in their declaration
+int* px; // px is a pointer to an int
+px = &x; // Stores the address of x in px
+printf("%p\n", px); // => Prints some address in memory
+
+// To retreive the value at the address a pointer is pointing to,
+// put * in front to de-reference it.
+printf("%d\n", *px); // => Prints 0, the value of x, which is what px is pointing to the address of
+
+// 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
+
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
-// Pointer types end with *
+// Declare a pointer of type int and initialize it to point to x_array
int* x_ptr = x_array;
-// This works because arrays are pointers to their 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.
-// Put a * in front to de-reference a pointer and retrieve the value,
-// of the same type as the pointer, that the pointer is pointing at.
+// Arrays are pointers to their first element
printf("%d\n", *(x_ptr)); // => Prints 20
printf("%d\n", x_array[0]); // => Prints 20
@@ -221,33 +263,27 @@ 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
@@ -260,12 +296,12 @@ function_1();
// <return type> <function name>(<args>)
int add_two_ints(int x1, int x2){
- return x1 + x2; // Use return a return a value
+ return x1 + x2; // Use return to return a value
}
/*
-Pointers are passed-by-reference (duh), so functions
-can mutate their values.
+Functions are pass-by-value, but you can make your own references
+with pointers so functions can mutate their values.
Example: in-place string reversal
*/