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Homework

• 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’ 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

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 = ~00 . . . 00111111

= 11 . . . 11000000 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': 10100101

| 11000000 turn on MS 2 bits

11100101

5

“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!

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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 */

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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!!

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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

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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 */

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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

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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

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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!!

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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)

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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;

}

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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;

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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 */

}

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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

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trim

• Pseudo code for the trim programwhile 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

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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)

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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 VariableString Array

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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?