summaryrefslogtreecommitdiffhomepage
path: root/c.html.markdown
blob: a8df2e1b5f02551a30f99ab463fb995f0207a1b9 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
---
language: c
author: Adam Bard
author_url: http://adambard.com/
---

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 //
/*
Multi-line comments look like this.
*/

// Import headers with #include
#include <stdlib.h>
#include <stdio.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
///////////////////////////////////////

// Variables must always be declared with a type.

// 32-bit integer
int x_int = 0;

// 16-bit integer
short x_short = 0;

// 8-bit integer, aka 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

// 32-bit floating-point decimal
float x_float = 0.0;

// 64-bit floating-point decimal
double x_double = 0.0;

// Integer types may be unsigned
unsigned char ux_char;
unsigned short ux_short;
unsigned int ux_int;
unsigned long long ux_long_long;

// 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


// 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

// Strings are just lists of chars terminated by a null (0x00) byte.
char a_string[20] = "This is a string";

/*
You may have noticed that a_string is only 16 chars long.
Char #17 is a null byte, 0x00 aka \0. 
Chars #18, 19 and 20 have undefined values.
*/

printf("%d\n", a_string[16]);

///////////////////////////////////////
// Operators
///////////////////////////////////////

int i1 = 1, i2 = 2; // Shorthand for multiple declaration
float f1 = 1.0, f2 = 2.0;

// 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

// 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.
// 0 is false, anything else is true
3 == 2; // => 0 (false)
3 != 2; // => 1 (true)
3 > 2; // => 1
3 < 2; // => 0
2 <= 2; // => 1
2 >= 2; // => 1

// 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

// Bitwise operators!
~0x0F; // => 0xF0 (bitwise negation)
0x0F & 0xF0; // => 0x00 (bitwise AND)
0x0F | 0xF0; // => 0xFF (bitwise OR)
0x04 | 0x0F; // => 0x0A (bitwise XOR)
0x01 << 1; // => 0x02 (bitwise left shift (by 1))
0x02 >> 1; // => 0x01 (bitwise right shift (by 1))

///////////////////////////////////////
// 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){
    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{
    printf("%d, ", kk);
}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++){
    printf("%d, ", jj);
} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "

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

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", (char) 257); // => 1 (Max char = 255)
printf("%d\n", (short) 65537); // => 1 (Max short = 65535)

///////////////////////////////////////
// Pointers
///////////////////////////////////////

// You can retrieve the memory address of your variables,
// then mess with them.

int x = 0;
printf("%p\n", &x); // Use & to retrive the address of a variable
// (%p formats a pointer)
// => Prints some address in memory;

int x_array[20]; // Arrays are a good way to allocate a contiguous block of memory
int 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 *
int* x_ptr = x_array;
// This works because arrays are 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.
printf("%d\n", *(x_ptr)); // => Prints 20
printf("%d\n", x_array[0]); // => Prints 20

// Pointers are incremented and decremented based on their type
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
// arithmatic 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
int* my_ptr = (int*) malloc(sizeof(int) * 20);
for(xx=0; 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
printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what?

// When you're done with a malloc'd block, you need to free it
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')

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 a return a value
}

/*
Pointers are passed-by-reference (duh), so functions
can mutate their values.

Example: in-place string reversal
*/

// A void function returns no value
void str_reverse(char* str_in){
    char tmp;
    int ii=0, 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;
    }
}

/*
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
struct rectangle {
    int width;
    int height;
};


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 use the -> shorthand
    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;
}

```

## 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)

Other than that, Google is your friend.