Homework
• Homework
– Continue Reading K&R Chapter 2
– We’ll go over HW2 at end of class today
– Continue working on HW3
• Questions?
1
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’
10100101
n & ‘\x1f’
10100101
& 00011111 turn off all bits
00000101 except bottom 5
• Called "masking" the bits -- only see bits on
in result where 1's found in mask value
2
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 = ~00 . . . 00111111
= 11 . . . 11000000 of required size
Extends itself with 1 bits on left for length of x
3
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':
10100101
| 11000000 turn on MS 2 bits
11100101
4
“Encryption” with Exclusive Or
• Show that x ^ (x ^ y) == y
char y =‘\xa5’ 10100101 (plain text bits)
char x =‘\x69’ 01101001 (encryption key)
x^y
11001100 (cypher text bits)
x ^ (x ^ y)
10100101 (decrypted bits)
Same as original value of y!
5
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 */
6
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 <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!!
7
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
8
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 */
9
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
10
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
11
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!!
12
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)
13
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;
}
14
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;
15
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 */
}
16
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
17
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
18
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)
19
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
String Array
Holding Variable
20
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?
21