1. C Slides and captured lecture (video and sound) are available at: www.cim.mcgill.ca/~jer/C/

2. Crash Course -- Review Last week, we covered: variable declarations and assignments float x;double d = 5;int *p, i, a[100];char s[21]; y = z = 4 + 5; x += 1; ++x; y = x--; formatted I/O int snum; float gr; char mark; char name[80]; scanf ("%d %f %c %s", &snum, &gr, &mark, name); printf (”%s\’s mark is %f or %c\n”,name, mark, gr); loops while (1) printf ("hi"); for (j = 0; j * j <= up; j++) printf ("%d \n", j * j);

3. Exercise: Caesar's code Input: sequence of capital letters Output: another sequence of capital letters obtained by shifting each letter in the original sequence three places in the alphabet. Note: shifting wraps around. Example: KENNEDY -> NHQQHGB recall the use of this handy string function: int strlen(char *); example: n = strlen ("there"); /* n is assigned 5 */

4. Crash Course -- Part II Today, we will cover: functions types pointers arrays structures using linked lists simple debugging

5. Functions /* Increment; takes an integer argument and * returns the argument plus one. */ int incr (int i) { int j; j = i + 1; return j; } main () { int k, m = 4; k = incr(m); printf ("k = %d, m = %d\n", k, m); } output: k = 5, m = 4

6. More about Functions might have no return type, and no return statement: void printhi () { printf ("hi\n"); } parameters are copied and can be modified int incr (int i) { i++; return i; } default (unspecified) return type is int

7. Variables within Functions But this does not work: void no_incr (int i) { i++; } void main () { int x = 5; no_incr(x); printf ("%d\n", x); } beware that modifications are on internal copies of the parameters. note: void main() since main does not return a value

8. Exercise Write a function that checks whether a sentence has 'title case': Arguments: sentence string Assume the string consists of letters and blanks only. Return true iff each word in the sentence starts with a capital letter and continues with lowercase letters.

9. C Preprocessor Directives Operations before compilation phase: include "header" files that contain pre-supplied functions: #include this line will be substituted by the contents of the file define symbolic constants: #define MY_LARGE_NUMBER 12345 #define MY_NIFTY_STRING "abc"

10. Make your own booleans C has boolean operations on int: == != && || ! However, C has no boolean type. Make it! #define boolean_t int; #define TRUE 1 #define FALSE 0 Example: boolean_t xIsFive, zAsBig, result; xIsFive = (x == 5); zAsBig = (z >= x); if (xIsFive || zAsBig) { result = TRUE; }

11. Naming your own type A better way: use typedef typedef int boolean_t; Syntax: typedef existing-type new-type; Comments: typedef does not create a new type only creates a new label for an existing type makes your code easier to read

12. typedef in action Example: typedef char * string_t; typedef int boolean_t; string_t msg = "No more room"; boolean_t full_class; if (num_students >= MAX_SIZE) full_class= TRUE;

13. i == *pi Pointers C relies heavily on pointers: int i; int *pi; pi = &i; *pi = 3; 3 pi

14. Pointers and Arrays An array name by itself is a pointer to the first element: int a[100]; *a = 5; /* same as a[0] = 5 */ *(a + 3) = 10; /* same as a[3] = 10; */ /* Note: *a + 3 means something else */ Strings are arrays of char: char s[3]; strcpy (s, "hi"); /* same as following line */ s[0] = 'h'; s[1] = 'i'; s[2] = '\0';

15. 4 j i Call by reference void incr (int *i); /* * increment i by 1 */ { *i = *i + 1; } main() { int j; j = 4; incr (&j); printf ("j = %d\n", j); } &j and i are pointers to the same integer: incr can change the integer (side effect), but not the pointer

16. Dynamic Memory Allocation Pointer errors can be nasty void main () { int *p; *p = 5; } Reserve space for the "pointed to" object: #include void main () { int *p; p = (int *)malloc (sizeof (int)); *p = 5; printf ("value at p = %d\n", *p); free (p);/* must do this yourself! */ }

17. Finding Bugs Insert "print" statements: printf ("At point A, %s = %d\n", "x", x); Use a debugger to help you catch bugs in the act: run step-by-step insert breakpoints display variables

18. Programming Errors I #include void main () { int i; scanf ("%d", &i); if (i = 0) puts ("false"); else puts ("true"); } What’s wrong?

19. Programming Errors I #include void main () { int i; scanf ("%d", &i); if (i = 0) puts ("false"); else puts ("true"); } The assignment, i = 0, will return a value of 0, so the program will always output "true" Use: if(0 == i) to be safe

20. Programming Errors II #include void main () { int i; /* echo one number */ scanf ("%d", i); printf ("input = %d\n", i); } What’s wrong? Hint: Segmentation fault / Bus error

21. Programming Errors II #include void main () { int i; /* echo one number */ scanf ("%d", i); printf ("input = %d\n", i); } Must provide the address of an integer if the function is going to assign a value to it.

22. Programming Errors III #include void main () { int *pc; scanf ("%d", pc); printf ("%d\n", pc); } What’s wrong?

23. Programming Errors III #include void main () { int *pc; scanf ("%d", pc); printf ("%d\n", pc); } *pc is an uninitialized pointer so results are unpredictable --> use malloc

24. Programming Errors IV #include void main () { double *p; p = malloc (sizeof (double *)); } What’s wrong?

25. Programming Errors IV #include void main () { double *p; p = malloc (sizeof (double *)); } Insufficient memory allocated for a double (typically, 8 bytes) Use malloc (sizeof (double));

26. Programming Errors V #include void main () { char s[] = "hi"; if (s == "hi") puts ("Strings are equal"); else puts ("Strings are not equal"); } What happens?

27. Programming Errors V #include main () { char s[] = "hi"; if (s == "hi") puts ("Strings are equal"); else puts ("Strings are not equal"); } Use strcmp() to compare strings, otherwise you are comparing addresses.

28. Structures Use struct to create or define a new type: struct str_data { char *string; int length; }; struct str_data s1, s2; Syntax struct structure_name { type1 member1; type2 member2; … }; A member of a structure is referred to by: structure_name.member

29. Structures, cont. Example: struct str_data { char *string; int length; }; struct str_data s1, s2; s1.string = (char *) malloc(80); strcpy (s1.string, "How now brown cow."); s1.length = strlen(s1.string); s2 = s1; /* can copy structures, pass to fcns */

30. Arrays of Structures Example: #define MAX_STUDENT 100 typedef struct { /* define a new type */ char name[80]; int name_len; int student_number; } student_t; /* create list of elements of type student_t */ student_t class[MAX_STUDENT]; for (i = 0; i < MAX_STUDENT; i++) { gets(class[i].name); class[i].name_len = strlen(class[i].name); }