Programming Levels Algorithm level Application Scripting Interpreted Languages Languages Or Compiled High-Level (Java, C#) (Perl, Python, VB) Languages System Programming Languages Introduction to C (C and C++) Compilation Assembly Language (x86, PowerPC, SPARC, MIPS) Low-Level Assembly Languages Machine Langgguage (x86, PowerPC, SPARC, MIPS) Based on slides © McGraw-Hill ISA level Additional material © 2004/2005 Lewis/Martin Hardware Modified by Diana Palsetia (Application-Specific Integrated Circuits or ASICs) CIT 593 2 Programming Language: Syntax Programming Language Semantics Syntax is the grammar of the language When the computer carries out your instructions (program) Running or Executing a program Analogous to rules in English Language ¾Missin g a peri od a fte r se nt en ce Semantics is the meaning of the program ¾Rules using verbs, nouns etc.. We learn the semantics after we run or execute the program Basically we observe the output Most languages provide syntax check errors After the executing program, the semantics of the program (Interprets/Compilers) may or may be correct ¾Syntax error messages may be helpful ¾Often, they are not Semantic errors cause your answers to be wrong ¾You gain experience with error messages after a while You may or may not get error messages ¾ E.g. Error Message – Dividing a number by zero If your program is not doing what you want it to do, though it runs, the error is semantic CIT 593 3 CIT 593 4 1 C Programs are Compiled Compilation Process C Entire mechanism is usually called the compiler Source and Header Files (text file containing hello.c Preprocessor C source code) Acts upon C preprocessor directives “Source-level” transformations C Preprocessor ¾ Output is still C Compiler (text file containing Source Code hello.s assembler source code) Analysis Compiler Symbol Table Generates object file Target Code Synthesis ¾ Machine instructions (binaries) Library hello Linker (a.out) (binary executable file Linker Object Files Containing machine code ) Combine object files (including libraries) into executable image Executable We will use the GNU gcc compiler (v3.0 and higher) Image CIT 593 5 CIT 593 6 Comments in C Basic C Elements Variables Begins with /* and ends with */ A data item upon which the programmer performs an operation A named space in memory Can span multiple lines E.g. z, counter Operators Predefined actions performed on data items Begins with // and ends with “end of line” E.g. *, +, /, ||, |, && Single-line comment Expressions ¾E.g. //This is a comment Operators combined with variables/literals to form expressions E.g. x * y Introduced in C++, later back-ported to C Statements Compiling with ansi standards, gives error A defined behavior Constitutes a unit of work for high-level language Ends with a semicolon. E.g. z = x * y; Why use comments ? Functions Help readers understand programs better Named group of statements Provides modularity to program Enforces, DRY principle (Do not Repeat Yourself) CIT 593 7 CIT 593 8 2 C program structure Variable Properties int main(){ Identifier: variable name /* code goes here Type: how data is interpreted, and how much space it */ needs return 0; } Every C program must the above format to develop Later we will discuss: application program(s) Scope: is the region of the program in which the variable is One of files must contain above structure alive and accessible ¾ main is a special function in C ¾ similar to Java’s main method Storage: how C compiler allocates storage and whether or Starting point for every program not the variable loses its value when the block that All C programs start and finish execution in the main contains it has completed execution Note: int main(int argc, char **argv) – main function can also take arguments like in Java CIT 593 9 CIT 593 10 Identifier: Variable Names Identifier Examples Any combination of letters, numbers, and Legal underscore (_) i wordsPerSecond same identifier words_per_ second Case sensitive _green "sum" is different than "Sum" aReally_longName_moreThan31chars aReally_longName_moreThan31characters Cannot begin with a number Usually, variables beginning with underscore Illegal are used only in special library routines 10sdigit ten'sdigit Only first 31 characters are definitely used done? reserved keyword Implementations can consider more characters if they like double CIT 593 11 CIT 593 12 3 Types Additional to Data Type C has several basic data types Literal int integer (at least 16 bits, commonly 32 bits) Values we write in a conventional form whose value is long integer (at least 32 bits) obvious float floating point (at least 32 bits) double floating point (commonly 64 bits) char character (at least 8 bits) Constant Exact size can vary, depending on processor Variable whose values do not change during the int is supposed to be "natural" integer size execution of the program ¾ 32 bits for most modern processors This done by appending “const” before the type So how do I know the size? ¾ Use method called sizeof. E.g. sizeof(int), returns answer in bytes Signed vs. unsigned: Symbol Default is 2’s complement signed integers Like constants but is preprocessor directive Use “unsigned” keyword for unsigned numbers CIT 593 13 CIT 593 14 Literals Constants vs. Symbol Integer #define RADIUS 15.0 123 /* decimal */ -123 symbol 0x123 /* hexadecimal */ int main(){ const dou ble PI = 3. 14159; Floating point double area; constant literal 6.023 double circum; 6.023e23 /* 6.023 x 1023 */ 5E12 /* 5.0 x 1012 */ area = PI * RADIUS * RADIUS; Character circum = 2 * PI * RADIUS; 'c' return 0; '\n' /* newline */ } '\xA' /* ASCII 10 (0xA) */ CIT 593 15 CIT 593 16 4 Symbol is C Preprocessor Directive Expression Symbol start with #define Expression Must go before the “main function syntax” Any combination of variables, constants, operators, and function calls Every expression has a type, derived from the types Example: #define RADIUS 15.0 of its components (according to C typing rules) Before compiling, replace all instances of the string “RADIUS“ in the code with the string “15.0" Examples: Also known as a macro area = PI * RADIUS * RADIUS; Used for values that won't change during execution, but might change if the program is reused. (Must counter >= STOP recompile.) x + sqrt(y) x & z + 3 || 9 - w-- % 6 CIT 593 17 CIT 593 18 Statement Operators Expresses a complete unit of work Three things to know about each operator Executed in sequential order (1) Function What does it do? Simple statement ends with semicolon (note: “;” is nottiC)t a comment in C) (2) Precedence z = x * y; /* assign product to z */ In which order are operators combined? Example: y = y + 1; /* update y */ "a * b + c * d" is the same as "(a * b) + (c * d)" ; /* null statement */ because multiply (*) has a higher precedence than addition (+) Compound statement formed with braces (3) Associativity Syntactically equivalent to a simple statement In which order are operators of the same precedence combined? Example: { z = x * y; y = y + 1; } "a - b - c" is the same as "(a - b) - c" because add/sub associate left-to-right CIT 593 19 CIT 593 20 5 Assignment Operator Assignment Operator (contd..) Changes the value of a variable All expressions evaluate to a value Even ones with the assignment operator e.g. y = x = 3 x = x + 4; For assignment, the result is the value assigned Usually (but not always) the value of the right-hand 1. Evaluate right-hand side. side ¾Type conversion might make assigned value 2. Set value of left-hand side variable to result. different than computed value e.g. int x = 15.6/3 = 5 Assigggnment associates right to left. y = x = 3; y gets the value 3, because (x = 3) evaluates to the value 3 y = (x = 3); CIT 593 21 CIT 593 22 Arithmetic Operators Arithmetic Expressions If mixed types, smaller type is "promoted" to larger Symbol Operation Usage Precedence Assoc Example: x + 4.3 * multiply x * y 6l-to-r if x is int, converted to double and result is double divide 6l-to-r / x / y Integer division -- fraction is dropped % modulo x % y 6l-to-r Example: x / 3 + addition x + y 7l-to-r if x is int and x=5, result is 1 (not 1.666666...) - subtraction x - y 7l-to-r Storing mixed type expression values int x = 45/7.1; All associate left to right C compiler will do automatic down grade if storage is small. Java compiler will complain. * /%/ % have higher precedence than + - Example Modulo -- result is remainder 2 + 3 * 4 vs. (2 + 3) * 4 Example: x % 3 if x is int and x=5, result is 2 2 * 4 % 5 CIT 593 23 CIT 593 24 6 Relational Operators Logical Operators Symbol Operation Usage Precedence Assoc Symbol Operation Usage Precedence Assoc > greater than x > y 9l-to-r ! logical NOT !x 4 r-to-l logical AND 14 l-to-r >= greater than or equal x >= y 9l-to-r && x && y || logical OR x||yx || y 15 l-to-r < less than x < y 9l-to-r <= less than or equal x <= y 9l-to-r Logical Operator is different from bitwise operators == equal x == y 10 l-to-r Treats entire variable (or value) as TRUE (non-zero), or != not equal x != y 10 l-to-r FALSE (zero) EgE.g. 1&&81 && 8 = 1 (True && True = True) Result is non-zero (TRUE) or zero (FALSE) CIT 593 25 CIT 593 26 Relational and Logical Usage Bitwise Operators Typically used to construct conditions Symbol Operation Usage Precedence Assoc Ultimately conditional will result in TRUE or FALSE ~ bitwise NOT ~x 4 r-to-l Note that see has no Boolean type << left shift x << y 8l-to-r ¾ Outcome is zero or non-zero (i.e.