COS 140: Foundations of Computer Science

Transcription

1 COS 140: Foundations of Computer Science Variables and Primitive Data Types Fall 2017 Introduction 3 What is a variable? Variable attributes Binding Variable names 6 Variable names Name length Special names Assignment 11 Data types 13 Primitive data types 14 Numeric types Booleans Characters Arrays Records Pointers

2 Homework Reading: Chapter 16 Homework: Exercises at end of chapter Homework due 10/23 Copyright c UMaine Computer Science Department 2 / 29 Introduction 3 / 29 What is a variable? In this lecture: imperative/oo languages A variable is an area of memory that: can have a value can have its value changed by the program Different sizes: byte, word, multiple words Copyright c UMaine Computer Science Department 3 / 29 2

3 Variable attributes Name what do we call it? Address where does it live in memory? Type what kind of thing can it hold? Value contents of the variable Scope who can see this variable? Lifetime duration of program, or shorter period? Copyright c UMaine Computer Science Department 4 / 29 Binding Associates an attriburte value with an attribute Not just value attribute Two types: Static binding: occurs before run-time, not changed afterward Allows compiler to check for errors, but......need information about the binding at compile time Dynamic binding: takes place/is changed at run-time Copyright c UMaine Computer Science Department 5 / 29 3

4 Variable names 6 / 29 Variable names Name: identifier associated variable Same name different variables in different contexts Different names same variable Unnamed variables: access via pointer Copyright c UMaine Computer Science Department 6 / 29 Variable names Strings (symbols) in a symbol table Symbol tables created at compile/interpretation time Associate names with entities (e.g., variables) Copyright c UMaine Computer Science Department 7 / 29 4

5 Variable names Which characters to allow? Have to recognize names during lexical analysis no spaces Sometimes special initial character e.g., a letter For enhanced readability, may allow connectors (e.g., -, ), case-sensitivity Copyright c UMaine Computer Science Department 8 / 29 Length of variable names Names should be meaningful longer names But symbol table should be reasonable size, easy to maintain shorter names BASIC: one letter + optional number Can t complain about the size! Readable for mathematical applications, but not much else Limited number of characters e.g., FORTRAN (6 or 31), C (63 significant) Trades-off readability and wasted space Table maintenance easy, but whatever the size, someone always wants more Unlimited number of characters e.g., Ada, Lisp Complete flexibility, but......potentially great deal of space, hard to maintain Copyright c UMaine Computer Science Department 9 / 29 5

6 Special names Some names not allowed or limited Reserved words can only be used in language-defined context(s) if in C, e.g. Predefined words defined in the language, but can be changed (e.g., Ada, C [libraries]) Keywords special meaning in some contexts, but can be used as a name (e.g., for in Lisp, if in FORTRAN) Copyright c UMaine Computer Science Department 10 / 29 Assignment 11 / 29 Giving variables values Assignment construct binds a value to a variable Often, a construct such as: x = a+b x s value is bound to result of a+b Not algebra: can have: x = x+1 Copyright c UMaine Computer Science Department 11 / 29 6

7 Assignment construct examples C, Java, Python... x = x + 1; Pascal x := x + 1 Lisp (setf x (1+ x)) R x <<- x + 1 or x + 1 -> x Smalltalk x x + 1 Forth x 1 + x! COBOL ADD X 1 GIVING X TCL set x x + 1 Copyright c UMaine Computer Science Department 12 / 29 Data types 13 / 29 Data types All data has a type: integer, floating point, etc. Type species size, interpretation, operations for data All variables have a type attribute Depending on language, variables type static or dynamic Determining variable type: Declared somewhere in program (static, explicit) Inferred by compiler/interpreter (static, implicit; dynamic) Copyright c UMaine Computer Science Department 13 / 29 7

8 Primitive data types 14 / 29 What are primitive data types? Primitive data types: defined by language Not (usually) defined in terms of other types Building blocks of new types Unstructured/atomic data types (scalars): Integers Floating point numbers Booleans Characters Pointers Structured data types: Strings Arrays Records Copyright c UMaine Computer Science Department 14 / 29 Integers Integer type represents subset of the integers Z (duh) Most common data type You ve seen: sign-magnitude, two s complement Most languages: several sizes (byte, 2 byte, 4 byte...) Can t represent all possible integers in computer instead i...j, where i 0 j For two s complement, with n bits, can represent 2 n n 1 1 E.g., 8 bits can represent (= ) to (= ). Some languages: unsigned integers (C) Copyright c UMaine Computer Science Department 15 / 29 8

9 Floating point numbers Represent subset of real numbers R Usually two sizes Two parts: fraction and exponent E.g.: , exponent Fractional part = = Exponent = 5, so multiply fraction by 2 5 So number is = Can t represent all numbers accurately e.g., π, 1 3 Copyright c UMaine Computer Science Department 16 / 29 Booleans True/false values Good for flags (switches), conditions to be checked Sometimes languages provide as primitive data type If not: 0 is false, everything else true (C) 0,, [], etc., is false, everything else is true (Python) Representation: smallest efficiently-addressable unit (often byte) Copyright c UMaine Computer Science Department 17 / 29 9

10 Characters Character represented as a pattern of 1s and 0s Representations: ASCII 7 bits code; 8-bit variants of this (e.g., ISO ) are the most common codes used E.g.: a = 33, b = 34,...; space = 32 Unicode 16 bits (or longer); accommodates other alphabets and symbols E.g.: - codepoints 47196, 51060; ; EBCDIC 8 bits; old IBM code Copyright c UMaine Computer Science Department 18 / 29 Strings Sequence of characters: textual data Special operators (e.g., substring, concatenation), special relational operators, sometimes special assignment Lengths: Fixed length: always same size Limited-dynamic can change, but there s a maximum (e.g., C) Length + sequence (often array) of characters Null-terminated sequence of characters Dynamic can change without limit (e.g., Lisp) more flexible, but overhead for allocation/deallocation Copyright c UMaine Computer Science Department 19 / 29 10

11 Arrays Sometimes need to represent groups of related data items (e.g., integers, characters, etc.) An array is a data type that stores homogeneous data in contiguous memory Can be single-dimensional (vectors) or multi-dimensional Use index to find element Value of index doesn t affect time taken to find element random access storage Copyright c UMaine Computer Science Department 20 / 29 Design issues for Arrays Syntax for array indices e.g., Temperatures(20) or Temperatures[20] Subscripts: what is the lower bound? bounds checking or not? must subscripts be integers? Number of dimensions allowed Usually no real limits C: limits to one dimension, but allows array elements to be themselves arrays Copyright c UMaine Computer Science Department 21 / 29 11

12 Array descriptors What information is needed by the programming language/program about an array? Base address where is the first element Element type Index type doesn t have to be integer for some languages Index lower, upper bounds Number of locations in array Copyright c UMaine Computer Science Department 22 / 29 Finding address of array element Element location (or address, A) depends on base address (B), index (I), index lower bound (L), and element size (S) A = B +(I L) S Given an array Taxes whose start location is 1024, element type is a long integer (8 bytes), and whose indices start at 0, find location of element Taxes[15] Array "Taxes" Taxes[0] Taxes[1] Taxes[15] A = 1024+(15 0) 8 = 1144 Copyright c UMaine Computer Science Department 23 / 29 12

13 Another example Suppose array Foo has elements of a type that requires 6 bytes to store, that it begins at location 4096, and that its indices begin at 10. What is the address of Foo[201]? Answer: A = B +(I L) S = 5242 = 4096+(201 10) 6 Copyright c UMaine Computer Science Department 24 / 29 Optimizing address computation Can write expression so static parts can be calculated at compile time and stored in a constant: A = B +(I L) S = B +(I S) (L S) = (I S)+(B L S) constant part Copyright c UMaine Computer Science Department 25 / 29 13

14 Multidimensional arrays Can have more than 1 dimension E.g., array is 2D, 100 entries, 10 rows, 10 columns Indices: [row,column] or [row][column] How to store dimensions? Row-major order: Rows are kept together, elements stored one row after another T: array 1..10, of integers row 1 row 2 row Column-major order: keeps columns together Row-major probably more common Copyright c UMaine Computer Science Department 26 / 29 Address calculation Array T is an 2D array of temperatures taken over a 1-meter grid, 100 m on a side Assume row-major order, float data type (assume 4 bytes), base address for T = 2048, indices each start at 0 (so 0..99) What is the address of T[20,15]? First: what is start of row 20? A [20,0] = B +(I L) S = 2048+(20 0) (4 100) = 10,048 Next, find offset of element in row 20 same kind of calculation: A [20,15] = 10,048+(I L) S = 10,048+(15 0) 4 = 10,108 Copyright c UMaine Computer Science Department 27 / 29 14

15 Heterogenous types: Records, structs Records contain heterogeneous related data E.g., data about employee: name (string), address (string), salary (fixed-point integer), height (float),... Most languages support this type: struct in C, defstruct in Lisp, record in Pascal, etc. Design issues: How are fields selected? How is field checked? Can we assign one structure to another? How to implement e.g., how to find element from selector? (field must have type, offset) Copyright c UMaine Computer Science Department 28 / 29 Pointers These point to an object: contain its address Used for user-created dynamic variables (from heap) Used for indirect addressing (e.g., C) abstraction of assembly language s indirect addressing mode Can assign to and through them: int a = 3; // integer int* p; // pointer to integer p = &a; // p = a s address *p = 4; // a now = 4 Often used (in C, e.g.) to access array elements int a[100]; // 100-element array of ints int* p = a; // p = addr of a s start (no &) for (int i=0;i<100;i++) { *p = 0; p++; } Copyright c UMaine Computer Science Department 29 / 29 15