Programming in C Lecture Tiina Niklander

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1 Programming in C Lecture Tiina Niklander Liisa Marttinen & Tiina Niklander C programming

2 Week 4 covers Multidimensional arrays Pointer arrays Command-line arguments Please note that there are less numbered exercises, but each subtask will give you TMC points, so please submit also partially solved exercises Liisa Marttinen & Tiina Niklander C programming

3 More about types: Remember that C does not have fixed type sizes, except char is stored in one byte The type sizes are promised to be short int <= int <= long int (<=long long int) unsigned versions of these float <= double <= long double Special fixed-size types: stdint.h int8_t, int16_t, int32_t, int64_t uint8_t, uint16_t, uint32_t, uint64_t (for portability; standardised in C99) Liisa Marttinen & Tiina Niklander C programming

4 Stack and heap, scope of variable Faculty of Science Department of Computer Science

5 Process structure Stallings: Operating systems Process control block (PCB): Program code Heap for data Stack (grows from larger memory addresses towards smaller) Memory addresses of process s own memory area

6 Stack (pino) (function call stack) Implicit usage (function arguments and local variables) push top pop Advantage: No explicit memory allocation or release. Disadvantage: Elements in the stack can be used only when in the stack (= during function execution) Error case: dangling reference Pointer that points to a released memory location (in stack or in heap)

7 Heap (keko) (dynamic memory alloc.) Explicit usage (malloc etc) Programmer is responsible for memory allocation in C Memory fragmentation is possible free Improper or careless usage might leak memory Memory is a resource (like files) and it must also be managed

8 Variable definitions and scope of names #include <stdio.h> /* global variables here*/ double tulos; int main(int argc, char** argv) { /* Function s variables */ int i; for (i=0; i < 20; i++) { /* Block s variables*/ int j; j = i*4; } return 0; } Global variables Defined outside functions Visible to all functions that defined later in the code Maintains value through the whole program execution Internal variables (block, function) Defined at the beginning of the block Visible only within the block, not outside Maintains value only during single execution of block, not whole program Block anything within { }, also function

9 Refer to a name defined somewhere else > extern int i; static int si; extern int ei; void f() { int fi; static int sif; extern int eif; /* Avoid!*/ } void g(); static void h(); int eif; /* Definition here */ Definition is in - Some other file or library - Same file, but later Avoid usage!! Define variables as local as possible and at least within one file (if necessary, use the header file with macro protection)

10 Function s internal variable persistent and hidden -> static int i; static int si; extern int ei; void f() { int fi; static int sif; extern int eif; /* Avoid! */ } void g(); static void h(); int eif; /* Definition here */ With static definition the variable maintains its value from one execution time to the other. On the other hand, static limits the scope of the function (or variable). It cannot be used by functions in other files. (likejava sprivate)

11 Variable location in the memory / process s address space Extern informs compiler: Do not allocate space now. The allocation happes, where the definition is. Static informs compiler: Allocate space in heap instead of stack. The value has to be maintained, and that can only happen in heap area. Register informs compiler: Reserve a register for this if possible. The variable is used so much that memory access might slow down the execution. No specification = automatic allocation: global variables in heap during compilation, local variables in stack during execution

12 Function call stack The figure shows the stack when main has called function foo and the function has not yet returned Frame contains Function arguments Value of simple type Address of complex type Local variables SP stack pointer FP frame pointer Faculty of Science Department of Computer Science

13 Multidimensional arrays Liisa Marttinen & Tiina Niklander C programming

14 Reminder: Accessing arrays Type definitions: double arr[5]; /*static memory allocation*/ double *arr2; /* dynamic allocation with malloc */ Accessing array elements: *(arr + k) == arr [k] *(arr2+k) == arr2[k] *arr == arr [0] Liisa Marttinen & Tiina Niklander C programming

15 Multidimensional arrays Multidimensional arrays in C are actually single dimensional arrays with elements as arrays int t[3][2] = { {1,2},{11,12}, {21,22}}; for (i=0; i<3; i++) { for (j = 0; j<2; j++) printf ( t[%d][%d] = %d\,i,j, t[i][j]); putchar( \n ); } t[0][0] = 1 t[0][1] = 2 t[1][0] = 11 t[1][1] = 12 t[2][0] = 21 t[2][1] = 22 Liisa Marttinen & Tiina Niklander C programming

16 Two-dimensional array static char days [2][13] ={ {0, 31, 28, 31, 30, 31,30,31, 30, 31,30, 31}, {0, 31, 29, 31, 30, 31,30,31, 30, 31,30, 31} }; count = days[leap][2]; When leap ==0, then count = 28, when leap ==1, then count = 29 Liisa Marttinen & Tiina Niklander C programming

17 Array of pointers Liisa Marttinen & Tiina Niklander C programming

18 Pointer to array of pointers p double **p; Arrays of pointers to individual double elements Dereferencing has to be done twice **p Liisa Marttinen & Tiina Niklander C programming

19 Allocating space to array and the referenced elements Allocation for the array double **block; #define SIZE 3 if((block=calloc(size, sizeof(double*)))==null) error; for(i = 0; i < SIZE; i++) if((block[i]=calloc(1, sizeof(double)))==null) error; *(*block) = 2.1; block[0][0] = 2.1; Allocation for one element at a time Liisa Marttinen & Tiina Niklander C programming

20 Referencing the element and freeing them OR: bptr = block; for { **btr=2.1+i; bptr++; } Setting the values to the referenced elements for(i = 0; i < SIZE; i++) block[i][0] = i; Freeing the memory: referenced elements and pointer array Elements first for(i = 0; i < SIZE; i++) free(block[i]); free(block);..and then array block = NULL; Liisa Marttinen & Tiina Niklander C programming

21 kk joulu tammi helmi maalis syys loka marras huhti touko kesä elo invalid heinä Pointer arrays: array of strings String arrays are always pointer arrays, since each string is accessed via a pointer. char *monthfinnish(int k) { static char *kk[] = { invalid, tammi, helmi, maalis, huhti, touko, kesä, heinä, elo, syys, loka, marras, joulu }; return ( (k<1 k > 12)? kk[0] : kk[k] ); } Liisa Marttinen & Tiina Niklander C programming

22 Command-line arguments Liisa Marttinen & Tiina Niklander C programming

23 Command line arguments int main (int argc, char **argv); int main (int argc, char *argv[]); programname arg1 arg2 argc count of strings argv pointer to string array (contains argument values) echo Hello, world argc = 3 argv argv[0] argv[1] argv[2] echo\0 Hello,\0 world\0 echo Hello, world argc = 2 work in some systems! argv argv[0] argv[1] echo\0 Hello, world\0 Liisa Marttinen & Tiina Niklander C programming

24 Using command line arguments find Virtanen Ville regfile argc: 4 argv: argv[0] argv[1] argv[2] argv[3] find\0 Virtanen\0 Ville\0 regfile\0 argv[0] or argv contains 1. arg, that is the program name ( find ), argv[1] or argv+1 contains 2. argument ( Virtanen ) argv[2] or argv+2 contains 3. argument ( Ville ) argv[3] or argv+3 contains 4. argument ( regfile ) argv[0][0] or (*argv)[0] or **argv is the first character of the first arg ( f ) argv[2][4] or (*(argv+2))[4)] or *(*(argv+2)+4) is fifth char of third arg. Liisa Marttinen & Tiina Niklander C programming

25 Checking the number of arguments /* Check that count is correct*/ int main(int argc, char **argv) {. switch(argc) { case 4: /* all information on command line*/ case 3: /*OK! Use the preset file name*/ default: fprintf(stderr, Incorrect usage: %s.. \n", argv[0]); /*Would be better to inform correct usage*/ return EXIT_FAILURE; } Liisa Marttinen & Tiina Niklander C programming

26 Example program What does this program do? #include <stdio.h> /* Explaining comment removed */ int main(int argc, char** argv) { int i; for (i=0; i < argc; i++) printf( %s%s, argv[i], (i <argc-1)? : ); printf( \n ); return 0; } Liisa Marttinen & Tiina Niklander C programming

27 Example program: a.out print command line #include <stdio.h> /* Echo the command line with params */ int main(int argc, char** argv) { int i; for (i=0; i < argc; i++) printf( %s%s, argv[i], (i <argc-1)? : ); printf( \n ); return 0; } NOTICE: - Parameters - Array indexing Liisa Marttinen & Tiina Niklander argv: 0 a.out\0 print\0 command\0 line\0 Modification: How would you avoid printing the name of the program? C programming

28 Using command line arguments: Changing program behaviour with options name #define DEFAULT 10 #define MAX 80 /*Prints the n lines of the file to stdout*/ int display(const char *fname, int n, int Max); - = option; int main(int argc, char **argv) { int lines = DEFAULT; -n can be missing switch(argc) { case 3: /* process the number of lines option*/ if(argv[1][0]!= '-' sscanf(argv[1] + 1, "%d", &lines)!=1 lines <= 0) return EXIT_FAILURE; argv++; /* no break! Continues to case 2!!*/ case 2: if(display(argv[1], lines, MAX) == 0) return EXIT_FAILURE; break; default: return EXIT_FAILURE; } return EXIT_SUCCESS; } argv argv[0] argv[1] argv[2] show\0-20\0 testi\0 argv Number of lines show -n fname argv[0] argv[1] File name show\0 testi\0 Liisa Marttinen & Tiina Niklander C programming

29 Command line parameters: typical usage -options static int process_parameters(int argc, char *argv[]) { int i, string_found =0; for(i=1; i<argc; i++){ /* process command switches. Note side effects! */ if (argv[i][0] == '-') { switch (argv[i][1]) { options: -c, -i, and -b case 'c': count_lines = TRUE; break; case 'i': ignore_case = TRUE; break; case 'b : line_beginning = TRUE; break; default: printf("unknown option %s - ignored \n", argv[i]); break; }} else { Search string cannot start with character - if (!string_found) { copy(string, argv[i], STRINGSIZE); string_found =1; } else{ printf("only one search string! \n"); return FALSE; } }} if (!string_found) { Functions always return value! printf("the search string must be given!\n"); return FALSE; } return TRUE; Liisa Marttinen } & Tiina Niklander C programming

30 Advanced aspects of C operators Faculty of Science Department of Computer Science

31 Assignment combinations t /= n -= m *= k += 7; Assignment-combination operators have low precedence are associated from right to left So in the above code fragment k = k+7 is evaluated first m = m*k is next n = n-m then and t = t /n is the last one Faculty of Science Department of Computer Science

32 Lazy evaluation When evaluating a condition operation evaluate the first (left-most) operator If condition result already known, second operator is not evaluated : first is true, second not evaluated && : first is false, second not evaluated When skip, always skip the second operand! Müldner calls this feature short-circuit evaluation rule Faculty of Science Department of Computer Science

33 Lazy evaluation y = a < 10 a >= 2 * b && b!= 1; Evaluation order of this example: First operator: a < 10 and second a >= 2 * b && b!= 1 If a = 7 -> skip the second and y = 1 If a = 17 and b = 20 start evaluating the first part of second operator result: skip second part b!=1 and y = 0 NOTE: The evaluation order is from left to right (and it does not follow the precedence) Faculty of Science Department of Computer Science

34 Lazy evaluation: guarding if ((x!= 0) && (total / x < minimum)) if ((ptr!= NULL) && (ptr->next == NULL)) Faculty of Science Department of Computer Science

35 Lazy evaluation y = a < 0 a > b && b > c b > 10; Which parts will be evaluated/skipped when (and what is the result) a = -3, b =17, c = 6 a = 17, b= 17, c = 17 a = 15, b = 11, c = 9 a = 12, b = 9, c = 6 Faculty of Science Department of Computer Science

36 What if you have: y = a < 0 a > b && b > c b++ > 10; How would you make sure that b gets incremented correctly! This could cause errors, if you assume that b is always incremented!! b would be incremented only when, its value is between a and c and a is positive. Recommendation: Avoid increment and decrement with complex condition operations Faculty of Science Department of Computer Science

37 Some aspects of numerical calculations (Not C specific, but more general) Faculty of Science Department of Computer Science

38 Choosing the proper data type Integer or real? - in most cases obvious Float vs. double can be combined freely in expressions degree of precision required for the data dictates the use of double all math.h functions use double - easier to use double float uses less space (half of the space of double) Integer calculations are faster that real calculations, float calculations faster than double calculations Faculty of Science Department of Computer Science

39 Value range For integers: signed, unsigned, short and long. Max short signed: Max long signed: Max long unsigned: For float the minimum value range is 1.175E E For double: 2.225E E Faculty of Science Department of Computer Science

40 Representation error float and double are approximate representations, but with differing precision. Consider the code below: float w = 4.4; double x = 4.4; printf(" Is x == (double) w? %i \n", (x == (double)w) ); printf(" Is (float)x == w? %i \n", ((float)x == w) ); The output may be unexpected, if you forgot that the two numbers are represented with limited, and different, precision and that the == operator tests for exact bit-by-bit equality: Is x == (double)w? 0 Is (float)x == w? 1 Faculty of Science Department of Computer Science

41 Representation error Explanation: When a more-precise value is cast to the less-precise type, the extra bits are truncated and the numbers are exactly equal. When a shorter value is cast to the longer type, it is lengthened by adding zero bits at the end of the mantissa, not by recomputing the additional bit values. In general, these zeros are not equal to the meaningful bits in the double value. Faculty of Science Department of Computer Science

42 Representation error float w; double x, y = 11.0, z = 9.0; x = z * (y / z); w = y-x; The result is as expected, w = 0, x = However, change the starting values y = 15.0 and z = 11.0, and the result is w = e 15, x = Why? small precision and truncation error in calculations Faculty of Science Department of Computer Science

43 Comparing floating point numbers bit-by-bit comparison does not work. The precision might be different in two calculated values Compare difference of two numbers to a preset epsilon value. double epsilon = 1.0e-3; double number, target; if (fabs( number -target) < epsilon) /* then we consider that number == target */ else /* we consider the values different; */ Faculty of Science Department of Computer Science

44 Overflow / underflow Integer overflow value wraps from large positive to large negative (or vise versa) could be potential security hole Floating point overflow special bit pattern, called infinity, used in overflow HUGE_VAL (in math.h) is this infinity value Floating point underflow magnitude of the number falls below the smallest number in the representable range 0 exponent and a nonzero mantissa, denormalized Faculty of Science Department of Computer Science

45 Order of magnitude and other problems If you add a small float number to a large one, and their exponents differ by more than 107, the addition likely will have no effect. The answer will be the same large number that you started with. A special value called NaN, which stands for not a number, can be generated through operations such as 0/0. This is another special bit pattern that does not correspond to a real value. The IEEE standard defines that any further operation attempted using a NaN or Infinity as an operand will return the same value. Faculty of Science Department of Computer Science