1. C

2. About the Crash Course Cover sufficient C for simple programs: variables and statements control functions arrays and strings pointers Slides and captured lecture (video and sound) are available at: www.cim.mcgill.ca/~jer/C/

3. About the Crash Course Next installments part II: Wednesday, November 24, 16:30-18:00 part III: Thursday, November 25, 18:00-19:30 Reference Kernighan & Ritchie: "The C programming language", 2nd edition. Prentice-Hall, 1988.

4. Why C? C is not always a good choice... Safety-critical systems Component-based programming / RAD...... but in many applications you do want C Legacy systems Speed, memory requirements Low-level programming (close to the machine)...

5. Your First C Program "The only way to learn a new programming language is by writing programs in it" [K&R] /* A simple program that prints something */ #include main () { printf ("Hello, world!\n"); }

6. Declarations Example: float x; double d = 5; int *p, i, a[100]; char s[21]; Syntax: type variable_name,... [= value]; Rules: declarations must precede executable statements int type may be modified by: long, short, unsigned

7. Changing Variable Values Example: int x, y, z; x = 2; x = x + 1; Getting Fancy y = z = 4 + 5; x += 1; ++x; x++; y = x--; Note: assignment statements return value, which may or may not be ignored; same goes for increment statements

8. Formatted Output Example: int i = 10; float f = 2.5; char s [] = "hi"; printf ("Jack\'s integer is %d\n", i); printf ("Jill\0x27s float is %f\n", f); printf ("My string s = %s\n", s); Syntax: printf (string_with_formatting, var1, var2,...); Formats: %d integer, %f float, %c character, %s string, … Note "escape sequences": \n newline, \' quote, \0x27, etc. #include is compulsory; more about it later

9. Formatted Input Example: #include int i; float f; scanf ("%d %f\n", &i, &f); /* inputs an integer and a float */ Syntax: scanf (string_with_formatting, &var1, &var2,...); Note: The ampersand ( & ) is necessary because scanf modifies the values stored in the respective variables; by comparison, printf only uses the values, without modifying them. More about this later

10. I/O Example What does this print? #include main () { int n; float x; char mark; scanf ("%d %f %c", &n, &x, &mark); printf ("Of %d %s,\n%f got %c\’s\n", n, "students", x, mark); } Type in the following input: 86 85.999 A

11. I/O Example #include main () { int n; float x; char mark; scanf ("%d %f %c", &n, &x, &mark); printf ("Of %d %s,\n%f got %c\’s\n", n, "students", x, mark); } Input : 86 85.999 A Output : Of 86 students, 85.999001 got A's

12. Conditional Statements Example: if (age < 0) { printf ("warning: negative age\n"); age = -age; } Syntax: if (condition) statement if (condition) statement else statement Rules: the condition is an int ! (no booleans) parentheses required around conditional expression use curly braces to make a compound statement

13. More Conditionals Example: if (x < 0) printf ("x is less than 0\n"); else if (x == 0) printf ("x is equal to 0\n"); else printf ("x is greater than 0\n"); What’s wrong with this? if (x < 0) if (y < z) printf ("y is less than z\n"); else printf ("x not less than 0\n");

14. While Loops Example: /* print "hi" forever */ while (1) printf ("hi"); Syntax: while (condition) statement Rules (again): the condition is an int ! (no booleans) parentheses required around conditional expression use curly braces to make a compound statement

15. For Loops Example: /* print "hi" three times */ int i; /* i continues to exist when loop ends */ for (i = 0; i < 3 ; i++) printf ("hi"); Syntax: for (statement1; condition; statement2) statement3; Equivalent to: statement1; while (condition) { statement3; statement2; }

16. Loop Example /* print squares up to 100 */ main ( ) { int j, up = 100; for (j = 0; j * j <= up; j++) printf ("%d \n", j * j); } Note: can’t do: for (int j = 0;... waste of one multiplication per iteration can you make it more efficient?

17. Example (cont’d) /* print squares up to 100 */ void main ( ) { int j, up = 100, sq; for (j = 0; (sq = j * j) <= up; j++) printf ("%d \n", sq); } Note: recall equivalence to a while loop: condition is evaluated before the loop body

18. Arrays int years[45]; float temperatures [11]; void main () { years[0] = 2000; temperatures[11] = -45.67; } Rules: indices start at zero maximum valid index is the size of the array minus 1 but C lets you go beyond the declared boundaries temperatures[11] is illegal

19. Characters char a, b, c1, c2; a = '0'; b = '\037'; c1 = 'K'; c2 = c1 + 1; Assigns values: 48, 31, 75, 76 The sequences '0',...,'9', 'a',...,'z', 'A',...,'Z' contain characters numbered consecutively Casting printf ("%c %d\n", c1, (int) c1); Outputs: K 75

20. Strings Strings are '\0' -terminated arrays of char : char s[3] = "hi"; /* invisible '\0' */ char t[3] = {'h', 'i', '\0'}; String operations #include strlen ("there"); /* returns 5 */ strcpy (s, t); /* copy t to s */ strcmp (s, t) /* alphabetical comparison */

21. Exercise: Caesar's code A simple code used by Caesar in the Gallic wars. 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

22. 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

23. 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

24. 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

25. 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.