1 Homework –Continue Reading K&R Chapter 2 –We’ll go over HW2 at end of class today –Continue working on HW3 Questions?

2 Forcing Groups of Bits Off Given char n, how to turn off all bits except the least significant 5 bits: n = n & ‘\x1f’ n = ‘\xa5’ n & ‘\x1f’ & turn off all bits except bottom 5 Called "masking" the bits -- only see bits on in result where 1's found in mask value

3 Forcing Groups of Bits Off x = x & ~077 (octal for a change) Sets least significant 6 bits in x to 0 Even if you don't know size of x (e.g. size of int) ~077 = ~ = of required size Extends itself with 1 bits on left for length of x

4 Forcing Groups of Bits On Given n, how to turn on the MS two bits (if already on, leave on). n = n | ‘\xc0’ n = '\xa5' n | '\xc0': | turn on MS 2 bits

5 “Encryption” with Exclusive Or Show that x ^ (x ^ y) == y char y =‘\xa5’ (plain text bits) char x =‘\x69’ (encryption key) x ^ y (cypher text bits) x ^ (x ^ y) (decrypted bits) Same as original value of y!

6 Exchanging Data with Exclusive Or You can use exclusive or to exchange the values of two variables without using a temporary location for a “three way move” int i = 0x12, j = 0x34; i = i ^ j; /* i = 0x26 */ j = i ^ j; /* j = 0x12 */ i = i ^ j; /* i = 0x34 */

7 String Constants String constant: "I am a string.“ –An array (a pointer to a string) of char values somewhere ending with NUL = '\0', the char with zero value. –"0" is not same as '0'. The value "0" can't be used in an expression - only in arguments to functions like printf(). Also have string functions: See pg. 241, Appendix B and B3, pg. 249 #include With these definitions, can use: len = strlen(msg); where msg is string in a string array NOTE: Write your own string functions for homework!!

8 Enumeration Symbolic Constants enum boolean {FALSE, TRUE}; Enumerated names assigned values starting from 0 FALSE = 0 TRUE = 1 Now can declare a variable of type enum boolean: enum boolean x; x = FALSE; Just a shorthand for creating symbolic constants instead of with #define statements

9 Enumeration Symbolic Constants If you define months as enum type enum months {ERR, JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC}; enum months birthMonth = MAY; printf("%d\n", birthMonth); /* prints 5 */

10 const "const" declaration is like "final" in Java –warns compiler variable value shouldn't change –Commonly used for function arguments const char msg[ ] = "Warning:..."; int copy(char to[ ], const char from[ ]); If logic of copy function attempts to modify “from” string, compiler will give a warning Exact form of warning and actual behavior of the code is installation defined

11 Operators Arithmetic Operators: +Add -Subtract *Multiply /Divide % Remainder after division (modulo) Examples: int x, y; x / y truncates (no fractional part) x % y is the remainder when x is divided by y. Always true that: x == y*(x/y) + x%y

12 Operators Logical Operators: &&logical and ||logical or ! Not Examples: int year; if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) printf( "%d is a leap year\n", year); else printf( "%d is not a leap year\n", year); Why are inner parentheses actually not needed? See precedence table, P 53. Good to mark for exams!!

13 Relations / Comparisons We call a comparison between two arithmetic expressions a "relation" ae1 =, > ) A relation is evaluated as true or false (1 or 0) based on values of given arithmetic expressions if ( i < lim-1 == j < k) –What's it mean? –See precedence table P 53 Instead of c != EOF, could write !(c == EOF)

14 Type Conversion /* atoi: convert character string of digits to int (base 10) */ int atoi(char s[ ]) /* name based on “ascii to integer” */ { int i, n; n = 0; for (i=0; s[i] >= '0' && s[i] <= '9'; ++i) /* "is a digit" */ n = 10*n + (s[i] - '0'); /* s[i]-'0' is char add to 10*n is int */ return n; }

15 Type Conversion /* itoa: convert int n to characters in base 10 in array s */ void itoa (int n, char s[ ]) { int i, sign; if ((sign = n) < 0)/* record sign */ n = -n;/* make n positive */ i = 0;

16 Type Conversion /* generate digits in reverse order */ do {/* new loop type */ s[i++] = n % 10 + '0'; /* generate next digit */ /* what conversion takes place here? */ } while(( n /= 10) > 0); /* delete digit from end of int */ if (sign < 0) s[i++] = '-'; s[i] = '\0'; reverse (s); /* reverse digit string */ }

17 Review HW2 Solution Let’s go over solutions to HW2 –Learn to break down the problem logically –Learn to write pseudo code effectively –Learn to write C code from pseudo code These are key for being a good programmer

18 trim Pseudo code for the trim program while there is still a line to process for each character starting at the end of the line find the first non blank character or the beginning of the line if a non-blank character is found add an EOL and a terminating zero and print out the string

19 trim While writing the pseudo code, think about: –A good strategy (e.g. scan line from the end backwards rather than trying to go forwards) –Stopping each scan (e.g. not blank, not tab, etc) –What’s needed to make a shorter line printable (e.g. add a new EOL and terminating zero) –What indicates that it is time to stop looping on input lines (e.g. return from getline > 0 is false)

20 reverse Pseudo code for the reverse function find length of the string to reverse for each character from the beginning of the string to half way copy the character here to a holding variable copy the character the same distance from end to here copy the holding variable to the same distance from end Holding Variable String Array

21 reverse While writing the pseudo code, think about: –The basic steps needed (e.g., three way move) –The loop conditions (e.g., go halfway through) –The beginning/end as possible special cases What about a string that has an odd length? What about a string that has an even length? –Does it matter if we move the middle character in an odd length string out and back in or not?