About the Crash Course

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 <stdio.h> • 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 <stdio.h> is compulsory; more about it later

9. Formatted Input • Example: • #include <stdio.h> • 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 scanfmodifies 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 <stdio.h> • 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 <stdio.h> • 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 • 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 <string.h> • 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.