1. CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall alphonce@cse.buffalo.edu 1

2. Announcements If this is your first class, pick up a syllabus. Recitations begin next week Make your name signs Cell phones off! 2

3. Today’s lessons Information encoding/decoding Examples –digital photos –mp3s –morse code –binary numbers Bit strings 3

4. Images 4 Each pixel encodes the amount of RED, GREEN and BLUE (RGB). This is an additive color scheme. Printing uses CYAN, MAGENTA, YELLOW and BLACK (CMYK). This is a subtractive color scheme.

5. Morse Code Dots, dashes and spaces used to represent letters/digits http://www.planetofnoise.com/midi /morse2mid.phphttp://www.planetofnoise.com/midi /morse2mid.php Two features: –variable length encodings –not a prefix code 5

6. 6 Spaces of different lengths is needed to decode unambiguously. Without spaces, how many ways can six dots in a row be decoded?

7. 7 5 five cinq

8. Counting Decimal (base 10) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 etc. Binary (base 2) 0 1 10 11 100 101 110 111 1000 1001 1010 1011 1100 1101 etc. 8

9. Bit string A ‘0’ or ‘1’ is a binary digit, or a bit. A sequence of bits is called a bit string. For example: –1101 is a bit string 9

10. Number systems Decimal (base 10) Each position is weighted by a power of 10. E.g. 734 = –7*100 + 3*10 + 4*1 –7*10 2 + 3*10 1 + 4*10 0 E.g. 1101 = –1*1000 + 1*100 + 0*10 + 1*1 –1*10 3 + 1*10 2 + 0*10 1 + 1*10 0 Binary (base 2) Each position is weighted by a power of 2. E.g. 111 = –1*4 + 1*2 + 1*1 = “seven” –1*2 2 + 1*2 1 + 1*2 0 E.g. 1101 = –1*8 + 1*4 + 0*2 + 1*1 = “thirteen” –1*2 3 + 1*2 2 + 0*2 1 + 1*2 0 10

11. Interpretation QUESTION: –What does the bit string 1101 represent? 11

12. Interpretation QUESTION: –What does the bit string 1101 represent? ANSWER: –Whatever we want it to represent! 12

13. Representations Binary (non-negative numbers) Two’s complement (integers) IEEE 754 (approx. floating point numbers) ASCII / EBCDIC / Unicode (characters) etc. 13

14. Questions? 14