C Language Summary (Continued)

Transcription

1 Chris J Michael cmicha1@lsu.edu 11 September 2008 C Language Summary (Continued) Heavily Influenced by the GNU C Reference Manual:

2 Q/A -Somebody brought up a nice point after class: What is the value of 'x' after evaluation of the code segment... int x = 3; x = x++; -The behavior of x=x++; is undefined! Different compilers may evaluate this expression in different ways. 2

3 Some Operators in Detail Increment: ++x Increments before evaluation x++ Increments after evaluation int x = 3; struct particle particle_array[10]; struct particle *p = &particle_array; p points to particle_array[0] x++; /* x is now 4 */ p++; /* p is now &particle_array[1] */ /* More specifically, p is &particle_array + sizeof(particle) */ Logical Negation:!x Evaluates to 0 if x!= 0; when x is 0, evaluates to 1. 3

4 Some Operators in Detail Size of: sizeof(x) Returns size of x in bytes size_t x = sizeof(int); size_t y = sizeof(struct particle); size_t is a type defined in <stddef.h>. It is usually an "unsigned int". Type casting: (int)x (float)x (type)x float a = (float) x / y; /* If x = 3 and y = 2, a = 1.5 */ If this result weren't cast to float, then "/" would perform integer division and a would equal 1. 4

5 Order of Evaluation -This is very important!!! Try to understand the sense behind the decisions that were made. -It is key to understand all the rules as a whole. 5

6 Statements -Labels rarely_necessary: -If if( a < 0 ) a = -a; usually used in combination with "goto" if( a < 0 ) a = -a; negative_count++; else if( a == 0 ) zero_count++; else positive_count++; 6

7 Statements -Switch test variable switch( number ) case 0: printf("number is 0"); break; case 1: printf("number is 1"); break; default: printf("number is neither 0 or 1"); break; condition (literal integer) /* The following is a functionally equivalent "if" statement */ if( number == 0 ) printf("number is 0"); else if( number == 1 ) printf("number is 1"); else printf("number is neither 0 or 1"); Make sure to break each condition or control will follow through to the next condition If none of the given conditions are met, then the "default" statements are executed. 7

8 -While (checks first) int i = 0; Statements while( i < 10 ) printf("i is %d", i++); Prints i from 0 to 9 -Do (checks last) int i = 0; do printf("i is %d", i++); while( i < 10 ); Note the difference if "i" was initialized to 10! "While" would not print anything while "do" would. 8

9 Statements -For int i; Initialize Test Step for( i = 0; i < 10; i++ ) printf("i is %d", i); -- int i = 0; Null statement Prints i from 0 to 9 for( ; i < 10; i++ ) printf("i is %d", i); -- int i, j; for( i = 0, j = 10; i < 10, j > 0; i++, j-- ) printf("i is %d, j is %d", i); 9

10 Statements -Blocks int i = 10; printf("i is %d and will be gone after this block", i); -Break int i = 0; do printf("i is %d", i++); if( i == 6 ) break; while( i < 10 ); "Breaks" out of loops and switch statements 10

11 Statements -Continue int i; Terminates current loop iteration for( i = 0; i < 10; i++ ) if( i == 1 ) continue; printf("i is %d", i); -Return void sample_1()... return; int two() return 2; 11

12 Statements -Typedef struct particle int x, y, z; float v_x, v_y, v_z; ; Terminates current loop iteration typedef struct particle particle_t;... /* Now instead of declaring as "struct particle p", we can do as follows */ particle_t p; /* You can also use typedefs when defining... */ typedef struct particle int x, y, z; float v_x, v_y, v_z; particle_t;... particle_t p; 12

13 Functions -Declaration Return type Parameter List void print_message(char *message); Name int difference(int a, int b); -Definition and call void print_message(char *message) puts(message); int difference(int a, int b) return a b;... print_message("almost done."); int d = difference(10, 2); 13

14 Functions -Parameters are copied in to the function locally. Their value is not passed back upon return. void bad(int a) a++;... int a_number = 5; bad(a_number); "a" is incremented locally only. Since variables local to functions are clobbered after the function returns, this function "bad" really doesn't change the program state. does nothing to "a_number" void good(int *a) (*a)++;... int a_number = 5; good(&a_number); Here, we copy-in the memory location of an integer, and increment the value found at that location. This function, unlike the last, has side-effects. after this call, "a_number" will be 6 14

15 -C supports recursion void factorial(int n) if( n < 1 ) return n; else return n * factorial(n-1); Functions -Main function with parameters int main(int argc, char *argv[]) int counter; the amount of command line arguments, including program name for( counter = 0; counter < argc; counter++ ) printf("%s\n", argv[counter]); an array of strings providing the argument list return 0; 15

16 Looking Back at Arrays -Arrays are always passed to functions as memory addresses (pointers). void print_and_zero_array(int *a, int amount) int i; for( i = 0; i < amount; i++ ) printf("%d ", a[i]); a[i] = 0; array initialization Note that 2-D arrays are passed as double pointers, 3-D are triple, etc.... int my_array[5] = 5,4,3,2,1; print_and_zero_array(my_array, 5); 16

17 Program Snippets -A set of small programs have been made available to you. $ mkdir test_programs $ cd test_programs $ mv <wherever you put it>/os4103_ _c.tar.bz2. $ bunzip2 os4103_ _c.tar.bz2; tar -xvf os4103_ _c.tar 00-hello_world.c 01-function_basics.c 02-dynamic_allocation.c 03-functions_as_pointers.c 04-linked_list_basics.c $ make gcc -pedantic -ansi -Wall 00-hello_world.c -o 00-hello_world.out gcc -pedantic -ansi -Wall 01-function_basics.c -o 01-function_basics.out gcc -pedantic -ansi -Wall 02-dynamic_allocation.c -o 02- dynamic_allocation.out gcc -pedantic -ansi -Wall 03-functions_as_pointers.c -o 03- functions_as_pointers.out gcc -pedantic -ansi -Wall 04-linked_list_basics.c -o 04- linked_list_basics.out $./00-hello_world.out Hello, world. 17

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