08. C programming language at a glance

Expressions

• all of it

• @= where "@" is operation

• post and per ++ and --

• a=b=c (i. e. a=(b=c))

• a = b, c (i. e. a=b and c i calculated)

  • what does: a = n, c = d

Types

See previous lecture

+:

• void — not a type but

  • void function(…) indicates no return value is expected

  • void *pointer indicates «universal» pointer (address) type that can contain any pointer without a cast, but has no address arithmetic nor can be dereferenced

• static modifier

  • on globals — not goes to global symbol talbe
  • on locals — statically placed on memory instead of stack (⇒ keeped between calls):

       1 #include <stdio.h>
       2 
       3 void fstat(void) {
       4         static int count = 1;
       5         printf("#%d\n", count++);
       6 }
       7 
       8 int main(int argc, char *argv[]) {
       9         int i;
      10         for(i=0; i<5; i++)
      11                 fstat();
      12         return 0;
      13 }
    

• …

Functions

See previous lecture

+:

• we can always skip return value (esp. int)

• arguments are passed only by value

  • Arrays, functions and other names are in fact pointers

  • If you want to change a variable, pass pointer to this variable

  • Thus «pointer to pointer» and other such stuff:

       1 #include <stdio.h>
       2 
       3 
       4 int A[10] = {1,2,3,4,5,6,7,8,9,0};
       5 
       6 void findcell(int **p, int *arr, int len, int value) {
       7         int i;
       8         *p = NULL;
       9         for(i=0; i<len; i++)
      10                 if(arr[i] == value)
      11                         *p = &arr[i]; /* = arr+i */
      12 }
      13 
      14 
      15 int main(int argc, char *argv[]) {
      16   int *p;
      17   findcell(&p, A, 10, 7);
      18   printf("%d==%d\n", *p, A[6]);
      19   *p = 100500;
      20   printf("%d==%d\n", *p, A[6]);
      21   return 0;
      22 }
    

• …

Control flow

• operator grouping by {}

• if()

• while() {} and do {} while();

• for(;;)

  • not a «cycle with counter»

  • 4-point canonical cycle scheme

• switch()

  • may be or may be not a sequence of if-s (e. g. arithmetic goto)

  • Duff's_device

H/W

• EJudge: HelloWorld 'Hello world'

  Write a simple program that prints "Hello, World!"

  Input:
  Output:

  Hello, World!

• EJudge: DummyCalc 'Dummy calculator'

  Write a program that reads expressions by "%d%c%d" pattern (first and third arguments are integers and second is operation) and prints the result of operation. Operations are '+', '-', '*', '/' and '^' (integer power). If operation is unknown, program should print (to stdout, not to stderr) an "Unknown operation" message. If input doesn't correspond the pattern (e. g. "END" is given instead of expression), the program stops. Zero division shall be equal to 0 and negative power shall be equal to 1 (NB: $$-1^-1=1$$ also!)

  Input:

  3+3
  100*7
  12^3
  42/0
  100500!500100
  2^12
  100/3
  -1^-1
  25-50
  .

  Output:

  6
  700
  1728
  0
  Unknown operation
  4096
  33
  1
  -25

• EJudge: ShuffleArray 'Shuffle an array'

  Write a program, that reads a cardinal N — an array size, then N integers. Reorder the array in such a way that the elements of it's first half take corresponded even positions, and the elements of it's second half reordered backwards and take odd positions. If N is odd, first half is bigger than second.

  Input:

  11
  23 89 34 35 79 26 78 72 37 86 82

  Output:

  23 82 89 86 34 37 35 72 79 78 26