1 16.070 Introduction to Computers and Programming March 14 Recitation 6 Spring 2002 Topics: • Quick review of PS4 issues • Fundamental Data types • ASCII / Arithmetic Conversion • Number Systems / Logical Operation • Data Representation • Sampling Common Programming Mistakes At office hours and while grading a number of mistakes have surfaced repeatedly. Here are a few: 1. Pointer variables should be initialized to the constant NULL. 2. The address a pointer points to is incremented by pointer arithmetic according to the size of the variable type that the pointer is declared as. 3. Constants (#define and const) should be all uppercase. 3. Function prototypes specified in homework should be used without modification. Fundamental Data Types The fact is that data types are platform specific. IBM, SGI, Sun and other manufacturers all adhere to their own standards regarding the amount of memory assigned for different data types. ANSI-C does provide a minimum standard, indicating at least how many bytes should be assigned to each standard data type. Variable Type Keyword Bytes Required Range Character char 1 -27 to 27-1 Integer int 2 -215 to 215 -1 Short Integer short 2 -215 to 215 -1 Long Integer long 4 -231 to 231 -1 Unsigned Character unsigned char 1 0 to 28 -1 Unsigned Integer unsigned int 2 0 to 216 -1 Unsigned Short Integer unsigned short 2 0 to 216 -1 Unsigned Long Integer unsigned long 4 0 to 232 -1 Single-Precision Floating-Point* float 4 -1038 to 1038 Double-Precision Floating-Point** double 8 -10308 to 10308 * Approximate precision to 7 digits ** Approximate precision to 19 digits Table 1 ANSI-C minimum memory requirements for standard data types2How do we know how much memory is allotted for each data type on your specific platform? Programming languages have a sizeof(), or equivalent, statement. It is always good style to use sizeof() in C when you rely on the amount of memory used by your program. This statement returns the size of any data type e.g. int a = 0; a=sizeof(int); printf(“integers have size: %d bytes”,a); Output: integers have size: 4 bytes Press any key to continue You will notice that integers on the PC platform, running MS Windows, consume more memory than the minimum ANSI specification of 2 bytes. ASCII Character Table You may have noticed that the above table specifies a variable of type char to have a range from 0 to 255. How can a character correspond to a number? The ASCII (“American Standard Code for Information Interchange”) codes are used to represent characters as one byte integers. The first 128 of them (0 " 127) are the standard ASCII characters, while the next 128 (128 " 255) are the extended ASCII characters (symbols, accented letters, Greek letters, etc...). 0 1 2 3 4 5 6 7 8 9 A B C D E F 0 NUL SOH STX ETX EOT ENQ ACK BEL BS HT LF VT FF CR SO SI 1 DLE DC1 DC2 DC3 DC4 NAK SYN ETB CAN EM SUB ESC FS GS RS US 2 SP ! " # $ % & ' ( ) * + , - . / 3 0 1 2 3 4 5 6 7 8 9 : ; < = > ? 4 @ A B C D E F G H I J K L M N O 5 P Q R S T U V W X Y Z [ \ ] ^ _ 6 ` a b c d e f g h i j k l m n o 7 p q r s t u v w x y z { | } ~ DEL Table 2 Hex-indexed ASCII table Some examples… Character Decimal Hex Octal Binary space 32 20 40 0010 0000 ! 33 21 41 0010 0001 " 34 22 42 0010 0010 # 35 23 43 0010 0011 $ 36 24 44 0010 0100 A 65 41 101 0100 0001 B 66 42 102 0100 00103Control Codes NUL (null) SOH (start of heading) STX (start of text) ETX (end of text) EOT (end of transmission) - Not the same as ETB ENQ (enquiry) ACK (acknowledge) BEL (bell) - Caused teletype machines to ring a bell. Causes a beep in many common terminals and terminal emulation programs. BS (backspace) - Moves the cursor (or print head) move backwards (left) one space. TAB (horizontal tab) - Moves the cursor (or print head) right to the next tab stop. The spacing of tab stops is dependent on the output device, but is often either 8 or 10. LF (NL line feed, new line) - Moves the cursor (or print head) to a new line. On Unix systems, moves to a new line AND all the way to the left. VT (vertical tab) FF (form feed) - Advances paper to the top of the next page (if the output device is a printer). CR (carriage return) - Moves the cursor all the way to the left, but does not advance to the next line. SO (shift out) - Switches output device to alternate character set. SI (shift in) - Switches output device back to default character set. DLE (data link escape) DC1 (device control 1) DC2 (device control 2) DC3 (device control 3) DC4 (device control 4) NAK (negative acknowledge) SYN (synchronous idle) ETB (end of transmission block) - Not the same as EOT CAN (cancel) EM (end of medium) SUB (substitute) ESC (escape) FS (file separator) GS (group separator) RS (record separator) US (unit separator) Type Casting Sometimes we don’t like the standard rules of arithmetic conversion to be applied to us. In such cases we may decide to cast variables or results of arithmetic into specific types. Lets look at an example: double Velocity, Time_Elapsed; Distance = Velocity*Time_Elapsed; We would usually need to define Distance as being of data type double. A compiler like Visual C would even compile the code if Distance was defined as int, but it would issue a warning message. This is not necessarily true for all compilers! Other C compilers, such as Interactive C, which we will use when programming the Handy Boards, are more strict about types. Many compilers don’t even hold with the arithmetic conversion rules that you have learned. The safest bet is to make use of a process called type casting. The correct way to cast a variable of one data type into another would be: /* variable declaration */ int Distance = 0; double Velocity = 10.0; double Time_Elapsed = 100.0; /* Type casting */ Distance = (int)(Velocity*Time_Elapsed);4 Arithmetic Conversions Sometimes equations in your code will contain variables of different types, and all in the same equation. What should the type of the answer to such an equation be? Standard C compiler rules exist that deal with such cases: 1. C temporarily converts the value of the “lower” type to that of the “higher” type, and then performs the operation, producing a value of the “higher” type (see hierarchy table on page 4). 2.