1. Homework Homework Questions? Continue Reading K&R Chapter 2
We’ll go over HW2
HW3 is posted
Questions?

2. Review HW2 Solution Let’s go over solutions to HW2
Learn to write pseudo code effectively
Learn to think about the problem logically
Learn to write C code from pseudo code
These are key for being a good programmer

3. 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

4. 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)

5. trim C code for the trim program
while ((len = getline(line, MAXLINE)) > 0) {
stop = 0;
for (i=len-1; i>=0 && !stop; --i) {
if (line[i] != ' ' && line[i] != '\t' && line[i] != '\n') {
stop = 1; /* stop on any non-blank,tab,eol */
line[i+1] = '\n'; /* add an eol */
line[i+2] = '\0'; /* and a zero to term the string */
printf("%s",line); }

6. 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
You can define another array and copy the characters on to it
Or you can define temporary location to store. So you need less space
String Array
Holding Variable 2 3 1

7. 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?

8. reverse C code for the reverse function int len, i; char c;
len = strlen(line) - 1; /* array index = len - 1 */
for (i=0; i<=len/2; ++i) { /* go halfway through the line */
c = line[i]; /* save a character from line */
line[i] = line[len-i]; /* overwrite that character */
line[len-i] = c; /* save reversed character */ }

9. 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
Sometimes when you do C programming you’ll deal with hardware.
You want to see which bit of the register is set or not
Turning off the bits that are 0 in the mask

10. 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
If you are interested in upper bits of X

11. 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

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

13. String String: “I am a string.” Also have string functions:
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 <string.h>
With these definitions, can use: len = strlen(msg);
where msg is string in a string array
NOTE: Write your own string functions for homework!!
‘0’ -> ascii code of 0
“0” -> the string of 2 locations: ascii value of 0 and the terminating character (\0)

14. 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
Variables inside enum, doesn’t care what you called them. It assigns 0 to first, 1 to second, 2 to third, …
So use enum instead of multiple #define statements

15. Enumeration Symbolic Constants
We can assign values to some name in any order. All unassigned names get value as value of previous name plus one
enum day {sunday = 1, monday, tuesday = 5, wednesday, thursday = 10, friday, saturday};
Enum variables are automatically assigned values, but Macros not 2 6 11 12

16. 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};
Debugger might print value 2 as FEB
Add a junk word before your variables to start from 1 or type JAN=1,

17. Code Example of the enum type
int main() { enum month {ERR, JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC}; enum month this_month; this_month = FEB; … }

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

19. Operators Arithmetic Operators: Examples: + Add - Subtract * Multiply
/ Divide
% Modulo (Remainder after division )
e.g. 5%7 = %7 = %7 = 3
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
Use modulo in hw3!

20. Operators Logical Operators: Example:
&& logical and
|| logical or
! Not
Example: …
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 no inner parentheses needed?
See precedence table, P 53. Good questions for exams!!
Bit wise operators in past
Precedence table!

21. Precedence and Associativity of Operators
sign
dereference
address
casting
Associativity
( ) [ ] -> Left to right
! ~ * & (type) sizeof right to left
* / % left to right
left to right
<< >> left to right
< <= > >= left to right
= = != left to right
& left to right
^ left to right
| left to right
&& left to right
|| left to right
?: right to left
= += -= *= /= %= &= ^= |= <<= >>= right to left
, left to right
Precedence

22. Precedence and Associativity
Operators in the same row have the same precedence
Operators are in order of decreasing precedence
Associativity
Determine the order if the operators have the same precedence
e.g
x = y +=z -= 4 means
x = y +=(z -= 4)
x =(y +=(z -= 4))

23. Evaluate a Logical Expression
Evaluation of a logical expression is tricky
Right-hand part of the expression is evaluated conditionally on the value of the left-hand part
Depending on the expression, the right-hand part may not be evaluated

24. Relations / Comparisons
We call a comparison between two arithmetic expressions a "relation" 
ae1 <= ae2 (Comparisons: <, <=, = =, !=, >=, > )
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)