Presentation Transcript

1. Integral Data Types in C

2. First lecture response sent out • Lots of good questions • Some questions will be punted to reading when appropriate • 3 statements useful – found a few misunderstandings • Need to focus on statements, not just nouns • Still need pictures • Gedankenexperiment • What happens when you crank a stereo system to 11 (and beyond)?

3. Goals for this Lecture • Help you to learn about: • The binary number system • Why binary? • Converting between decimal and binary • The octal and hexadecimal number systems • Finite representations of binary integers • Unsigned integers • Integer addition • Signed integers • Bitwise operators • And, or, not, and xor • Shift-left and shift-right • The C integral data types • char, short, int, long (signed and unsigned)

4. Why Bits (Binary Digits)? • Computers are built using digital circuits • Inputs and outputs can have only two values • True (high voltage) or false (low voltage) • Represented as 1 and 0 • Can represent many kinds of information • Boolean (true or false) • Numbers (23, 79, …) • Characters (‘a’, ‘z’, …) • Pixels • Sound • Can manipulate in many ways • Read and write • Logical operations • Arithmetic • …

5. Base 10 and Base 2 • Decimal (base 10) • Each digit represents a power of 10 • 4173 = 4 x 103 + 1 x 102 + 7 x 101 + 3 x 100 • Binary (base 2) • Each bit represents a power of 2 • 10110 = 1 x 24 + 0 x 23 + 1 x 22 + 1 x 21 + 0 x 20 = 22 Decimal to binary conversion: Divide repeatedly by 2 and keep remainders 12/2 = 6 R = 0 6/2 = 3 R = 0 3/2 = 1 R = 1 1/2 = 0 R = 1 Result = 1100

6. Writing Bits is Tedious for People • Octal (base 8) • Digits 0, 1, …, 7 • Hexadecimal (base 16) • Digits 0, 1, …, 9, A, B, C, D, E, F Thus the 16-bit binary number 1011 0010 1010 1001 converted to hex is B2A9

7. Representing Colors: RGB • Three primary colors • Red • Green • Blue • Strength • 8-bit number for each color (e.g., two hex digits) • So, 24 bits to specify a color • In HTML, on the course Web page • Red: <font color="#FF0000"><i>Symbol Table Assignment Due</i> • Blue: <font color="#0000FF"><i>Fall Recess</i></font> • Same thing in digital cameras • Each pixel is a mixture of red, green, and blue

8. Finite Representation of Integers • Fixed number of bits in memory • Usually 8, 16, or 32 bits • Unsigned integer • No sign bit • Always positive or 0 • All arithmetic is modulo 2n • Examples of unsigned integers • 00000001  1 • 00001111  15 • 00010000  16 • 00100001  33 • 11111111  255

9. Adding Two Integers: Base 10 • From right to left, we add each pair of digits • We write the sum, and add the carry to the next column 0 1 1 + 0 0 1 Sum Carry 1 9 8 + 2 6 4 Sum Carry 4 0 6 1 2 1 1 0 0 1 0 1

10. Binary Sums and Carries a b Sum a b Carry 0 0 0 0 0 0 0 1 1 0 1 0 1 0 1 1 0 0 1 1 0 1 1 1 AND XOR 69 0100 0101 103 + 0110 0111 172 1010 1100

11. Modulo Arithmetic • Consider only numbers in a range • E.g., five-digit car odometer: 0, 1, …, 99999 • E.g., eight-bit numbers 0, 1, …, 255 • Roll-over when you run out of space • E.g., car odometer goes from 99999 to 0, 1, … • E.g., eight-bit number goes from 255 to 0, 1, … • Adding 2n doesn’t change the answer • For eight-bit number, n=8 and 2n=256 • E.g., (37 + 256) mod 256 is simply 37 • This can help us do subtraction… • Suppose you want to compute a – b • Note that this equals a + (256 -1 - b) + 1

12. One’s and Two’s Complement • One’s complement: flip every bit • E.g., b is 01000101 (i.e., 69 in decimal) • One’s complement is 10111010 • That’s simply 255-69 • Subtracting from 11111111 is easy (no carry needed!) • Two’s complement • Add 1 to the one’s complement • E.g., (255 – 69) + 1  1011 1011 1111 1111 b - 0100 0101 one’s complement 1011 1010

13. Putting it All Together • Computing “a – b” • Same as “a + 256 – b” • Same as “a + (255 – b) + 1” • Same as “a + onesComplement(b) + 1” • Same as “a + twosComplement(b)” • Example: 172 – 69 • The original number 69: 0100 0101 • One’s complement of 69: 1011 1010 • Two’s complement of 69: 1011 1011 • Add to the number 172: 1010 1100 • The sum comes to: 0110 0111 • Equals: 103 in decimal 1010 1100 + 1011 1011 1 0110 0111

14. Signed Integers • Sign-magnitude representation • Use one bit to store the sign • Zero for positive number • One for negative number • Examples • E.g., 0010 1100  44 • E.g., 1010 1100  -44 • Hard to do arithmetic this way, so it is rarely used • Complement representation • One’s complement • Flip every bit • E.g., 1101 0011  -44 • Two’s complement • Flip every bit, then add 1 • E.g., 1101 0100  -44

15. Overflow: Running Out of Room • Adding two large integers together • Sum might be too large to store in the number of bits available • What happens? • Unsigned integers • All arithmetic is “modulo” arithmetic • Sum would just wrap around • Signed integers • Can get nonsense values • Example with 16-bit integers • Sum: 10000+20000+30000 • Result: -5536

16. Bitwise AND (&) Mod on the cheap! E.g., h = 53 & 15; Bitwise OR (|) | & 0 0 1 1 0 0 0 0 0 1 1 1 0 1 1 1 Bitwise Operators: AND and OR 53 0 0 1 1 0 1 0 1 & 15 0 0 0 0 1 1 1 1 5 0 0 0 0 0 1 0 1

17. Bitwise Operators: Not and XOR • One’s complement (~) • Turns 0 to 1, and 1 to 0 • E.g., set last three bits to 0 • x = x & ~7; • XOR (^) • 0 if both bits are the same • 1 if the two bits are different ^ 0 1 0 0 1 1 1 0

18. Bitwise Operators: Shift Left/Right • Shift left (<<): Multiply by powers of 2 • Shift some # of bits to the left, filling the blanks with 0 • Shift right (>>): Divide by powers of 2 • Shift some # of bits to the right • For unsigned integer, fill in blanks with 0 • What about signed negative integers? Varies across machines… • Can vary from one machine to another! 53 0 0 1 1 0 1 0 0 53<<2 1 1 0 1 0 0 0 0 53 0 0 1 1 0 1 0 0 53>>2 0 0 0 0 1 1 0 1

19. Data Types • Programming languages combine: • Bits into bytes • Bytes into larger entities • Combinations of bytes have types; why? • Facilitates abstraction • Enables compiler to do type checking • C has 11 primitive data types • 8 integral data types (described in this lecture) • 3 floating-point data types (described in next lecture)

20. Integral types: * On hats; C90 standard does not specify size C Integral Data Types

21. C Integral Data Types (cont.) • Why char vs. short vs. int vs. long? • Small sizes conserve memory • Large sizes provide more range • Why signed vs. unsigned? • Signed types allow negatives • Unsigned types allow greater positives • (Dubious value: Java omits unsigned types) • When to use unsigned? • When you really need that extra bit • When you’ll do lots of bit shifting • When you’ll never do (a < 0) test

22. The int Data Type • Description: A positive or negative integer • Same as signed int • Size: System dependent • 16 <= bits in short <= bits in int <= bits in long • Usually 16 bits (alias 2 bytes) or 32 bits (alias 4 bytes) • The “natural word size” of the computer

23. Two’s complement High-order bit indicates sign Leading zero means octal Leading zero-x means hexadecimal The int Data Type (cont.) • Example constants (assuming 4 bytes)

24. Same range as int, but shifted on number line Note “U” suffix The unsigned int Data Type • Description: A positive integer • Size: System dependent • Same as int • Example constants (assuming 4 bytes)

25. Note “L” suffix The long Data Type • Description: A positive or negative integer • Same as signed long • Size: System dependent • 16 <= bits in short <= bits in int <= bits in long • Usually 32 bits, alias 4 bytes • Example constants (assuming 4 bytes)

26. Note “UL” suffix The unsigned long Data Type • Description: A positive integer • Size: System dependent • Same as long • Example constants (assuming 4 bytes)

27. No way to express constant of type short, so must use cast The short Data Type • Description: A positive or negative integer • Same as signed short • Size: System dependent • 16 <= bits in short <= bits in int <= bits in long • Usually 16 bits, alias 2 bytes • Example constants (assuming 2 bytes)

28. No way to express constant of type unsigned short, so must use cast The unsigned short Data Type • Description: A positive integer • Size: System dependent • Same as short • Example constants (assuming 4 bytes)

29. No way to express constant of type signed char, so must use cast The signed char Data Type • Description: A (small) positive or negative integer • Size: 1 byte • Example constants

30. No way to express constant of type unsigned char, so must use cast The unsigned char Data Type • Description: A (small) positive integer • Size: 1 byte • Example constants

31. The char Data Type • On some systems, char means signed char • On other systems, char means unsigned char • Obstacle to portability int a[256]; char c; c = (char)255; … … a[c] … /* char is unsigned => a[255] => OK */ /* char is signed => a[-1] => out of bounds */

32. The char Data Type (cont.) • On your system, is char signed or unsigned? • Output on hats #include <stdio.h> int main(void) { char c = (char)0x80; if (c > 0) printf("unsigned"); else printf("signed"); return 0; } signed

33. The char Data Type (cont.) • Q: • Why is type char called “char” (meaning “character”)? • A: • Type char can be used for limited range arithmetic • As indicated previously • However, type char is used more often to store a charactercode • As shown next lecture

34. Summary • Computer represents everything in binary • Integers, floating-point numbers, characters, addresses, … • Pixels, sounds, colors, etc. • Binary arithmetic through logic operations • Sum (XOR) and Carry (AND) • Two’s complement for subtraction • Binary operations in C • AND, OR, NOT, and XOR • Shift left and shift right • Useful for efficient and concise code, though sometimes cryptic • C integral data types • char, short, int, long (signed and unsigned)