Programming Language: Syntax

Syntax is the grammar of the language „ Analogous to rules in English Language ¾Missin g a peri od aft er sent en ce Introduction to C Language ¾Rules using verbs, nouns etc.. Overview, variables, Operators, Statements „ Most languages provide syntax check errors (Interprets/Compilers) ¾Syntax error messages may be helpful ¾Often, they are not ¾You gain experience with error messages after a while Based on slides © McGraw-Hill Additional material © 2004/2005 Lewis/Martin Modified by Diana Palsetia

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Programming Language Semantics C Programs are Compiled When the computer carries out your instructions (program) (text file containing „ Running or Executing a program hello.c C source code) Semantics is the meaning of the program „ We learn the semantics after we run or execute the program „ Basically we observe the output (text file containing After the executing program, the semantics of the program hello.s may or may be correct assembler source code)

Semantic errors cause your answers to be wrong „ You may or may not get error messages hello ¾ E.g. Error Message – Dividing a number by zero (a.out) (binary executable file „ If your program is not doing what you want it to do, though it Containing machine code ) runs, the error is semantic We will use the GNU gcc compiler (v4.0 and higher) CIT 593 3 CIT 593 4

1 Syntax Compiling and Executing C programs Compilation Process

C Entire mechanism is usually called the compiler Source and In the folder that contains the source code do for: Header Files Preprocessor Compiling the program „ Acts upon C preprocessor directives „ “Source-level” transformations C Preprocessor gcc –Wall progname.c ¾ Output is still C

gcc tool for compiling process Compiler Compiler Source Code -Wall to display all warning Analysis „ Generates object (.o) file for each file Symbol Table progname.c is file that contains C code Target Code ¾ Machine instructions (binaries) Synthesis Will produce a file called a.out „ This file is not seen unless special compiler options are used Library Linker Executing Object Files Linker ./a.out „ Combine object files (including libraries) Executable into executable image Image

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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)

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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; Syntax: } Every C program must have the above format to develop Later we will discuss: application program(s) „ One of files must contain above structure Scope: is the region of the program in which the variable is ¾ main is a special function in C alive and accessible ¾ similar to Java’s main method „ Starting point for every program Storage: how C compiler allocates storage and whether or All C programs start and finish execution in the main not the variable loses its value when the section that Note: int main(int argc, char **argv) – main function can contains it has completed execution 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

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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 is done by appending “const” before the type „ So how do I know the size? ¾ Use unary operator called sizeof. E.g. sizeof(int) – returns answer in bytes Symbol Signed vs. unsigned: „ Like constants but is preprocessor directive „ Default is 2’s complement signed integers „ Use “unsigned” keyword for unsigned numbers

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Literals C program showing Basic Syntax Integer #define RADIUS 15.0 123 /* decimal */ symbol -123 int main(){ 0x123 /* hexadecimal */ double area; double circum; DlDeclara tion a var ibliable (declaration statement) Floating point const double PI = 3.14159; 6.023 constant 6.023e23 /* 6.023 x 1023 */ literal 5E12 /* 5.0 x 1012 */ area = PI * RADIUS * RADIUS; Expression Character circum = 2 * PI * RADIUS; 'c' Statement '\n' /* newline */ return 0; Operators '\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 literals, 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 PI * RADIUS * RADIUS; „ Used for values that won't change during execution, but might change if the program is reused(Must x + sqrt(y) //sqrt is function with input y recompile) x % 6 == 0

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Statement Operators Expresses a complete unit of work Three things to know about each operator „ Executed in sequential order (1) Function Simple statement ends with semicolon (note: “;” is not a „ What does it do? comme nt 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 (+) sqrt(y); /* containing a function call*/ (3) Associativity Compound statement formed with braces „ In which order are operators of the same precedence combined? „ Syntactically equivalent to a simple statement „ Example: { z = x * y; y = y + 1; } "a - b - c" is the same as "(a - b) - c" because add/sub associate left-to-right

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5 Assignment Operator Assignment Operator (contd..) Changes the value of a variable All expressions evaluate to a value x = x + 4; „ Even ones with the assignment operator e.g. y = x = 3 For assignment, the result is the value assigned 1. Evaluate right-hand side. „ Usually (but not always) the value of the right-hand 2. Set value of left-hand side variable to result. side ¾Type conversion might make assigned value 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);

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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 * /%/ % + - „ Thus C has less type sa fe ty f eat ures compared t o J ava Example „ 2 + 3 * 4 vs. (2 + 3) * 4 Modulo -- result is remainder „ Example: x % 3 „ 2 * 4 % 5 „ if x is int and x=5, result is 2

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6 Relational Operators AND, OR, NOT

Symbol Operation Usage Precedence Assoc Two states: TRUE=1, FALSE=0 > greater than x > y 9l-to-r >= greater than or equal x >= y 9l-to-r A B A AND B A B A OR B A NOT A < less than x < y 9l-to-r 0 0 0 0 0 0 0 1 0 1 0 0 1 1 1 0 <= less than or equal x <= y 9l-to-r 1 0 0 1 0 1 == equal x == y 10 l-to-r 1 1 1 1 1 1 != not equal x != y 10 l-to-r View n-bit number as a collection of n logical values Result is non-zero (TRUE) or zero (FALSE) „ Operation applied to each bit independently (bitwise operators) „ No boolean data type in C ¾Applications e.g. water marking, cryptography „ All logical work is done via integer data type

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Logical Operators Relational and Logical Usage Symbol Operation Usage Precedence Assoc Typically used to construct conditions „ Ultimately conditional will result in TRUE or FALSE ! logical NOT !x 4 r-to-l „ No Boolean type && logical AND x && y 14 l-to-r ¾ Outcome is zero or non-zero (i.e. int) || logical OR x||yx || y 15 l-to-r Example „ (x > y) && (x < z) „ (c == ‘q’) || (c == ‘Q’) Logical Operator is different from bitwise operators Conditions allow change in the control flow „ Can skip certain instructions based on condition „ Treats entire variable (or value) as TRUE (non-zero), or FALSE (zero) if((c == ‘q’) || (c == ‘Q’)){ „ EgE.g. 1 1&&8 && 8 = 1 (True && True = True) //Quit game }

„ If the condition results in TRUE, then statements in the { } will be executed „ { } is known as block. Basically encloses some number of statements to be executed

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7 Bitwise Operators Examples of Bitwise AND, OR, NOT Symbol Operation Usage Precedence Assoc AND ~ bitwise NOT ~x 4 r-to-l 11000101 << left shift x << y 8l-to-r „ Useful for clearing bits >> right shift x >> y 8l-to-r ¾AND with zero = 0 AND 00001111 & bitwise AND x&yx & y 11 l-to-r ¾AND with one = no change 00000101 ^ bitwise XOR x ^ y 12 l-to-r | bitwise OR x | y 13 l-to-r OR 11000101 Bit-wise vs Logical AND „ Useful for setting bits OR 00001111 „ 1 & 8 = 0 (000001 AND 001000 = 000000) ¾OR with zero = no change „ 1 && 8 = 1 (True && True = True) ¾OR with one = 1 11001111

NOT NOT 11000101 „ Unary operation -- one argument 00111010 „ Flips every bit

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Shift Operator: << (Left Shift Operator) …C and the Right Shift Operator (>>) Operate on values -- neither operand is changed Does right shift sign extend or not? „ Answer: Yes and No Left Shift (<<) variable << shift amount Unsigned values: zero extend „ Shifts bits to the left by shift amount „ unsigned int x = ~0; „ Fills spaces equivalent to shift amount on the right side „ Then, (x >> 10) will have 10 leading zeros with zeroes „ x = y << 1 same as x = y + y or 2 * y Signed values: „ “Right shifting a signed quantity will fill with sign bits (“arithmetic shift”) on some machines and with 0-bits (“log ica l s hift”) on others.” - KihKernighan an d Ritchie „ In practice, it does sign extend ¾int x = ~0; /* signed */ ¾Then, (x >> 10) will still be all 1s

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8 Special Operators: ++ and -- Using ++ and -- Changes value of variable by 1 before (or after) its x = 4; value is used in an expression y = x++; Results: x = 5, y = 4 Symbol Operation Usage PrecedenceAssoc (because x is incremen te d a fter ass ignmen t) ++ postincrement x++ 2 r-to-l -- postdecrement x-- 2 r-to-l x = 4; ++ preincrement ++x 3 r-to-l y = ++x; -- predecrement --x 3 r-to-l Results:Results: x = 5, y = 5 (because x is incremented before assignment) Pre: Increment/decrement variable before using its value Post: Increment/decrement variable after using its value

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Special Operators: +=, *=, etc. Input and Output

Arithmetic and bitwise operators can be combined Variety of I/O functions in C Standard Library with assignment operator „ Libraries contain code/programs already written that can be re-used Statement Equivalent assignment „ So we do not have to re-invent the wheel x += y; x = x + y; For I/O, must include i.e.#include x -= y; x = x - y; ¾ Must go above the main function syntax „ #include is a C processor Directive x *= y; x = x * y; ¾ Before compiling, copy contents of stdio.h into source code x /= y; x = x / y; ¾ .h files typically contain descriptions of functions and structs x %= y; x = x % y; All have same printf – Print formatted x &= y; x = x & y; precedence and x|=y;x |= y; x=x|y;x = x | y; „ Performs output to standard output device (monitor) associativity as = „ String contains characters to print and formatting directions for variable x ^= y; x = x ^ y; and associate „ Really useful in debugging x <<= y; x = x << y; right-to-left. x >>= y; x = x >> y; scanf – Scan Formatted „ String contains formatting directions for reading input

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9 Output Examples printf(“Counter value is %d\n", counter); „ print the variable counter as a decimal integer, followed by a This code: linefeed (\n) „ Linefeed will make the cursor go onto next line printf("%d is a prime number.\n", 43); printf("43 plus 59 in decimal is %d.\n", Can print arbitrary expressions, not just variables 4339+59); printf("%d\n", startPoint - counter); printf("43 plus 59 in hex is %x.\n", 43+59); printf("43 plus 59 as a character is Print multiple expressions with a single statement %c.\n", 43+59); printf("%d %d\n", counter, startPoint - counter);

Different formatting options: produces this output: %d decimal integer 43 is a prime number. %x hexadecimal integer 43 plus 59 in decimal is 102. %c ASCII character 43 plus 59 in hex is 66. %f floating-point number 43 plus 59 as a character is f.

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Examples of Input scanf("%d", &startPoint); „ read a decimal integer and assign it to the variable startPoint (& - specifies the address of the operand….more on this when we do pointers)

Same formatting characters are available for user input scanf("%c", &nextChar); „ reads a single character and stores it in nextChar scanf("%f", &radius); „ reads a floating point number and stores it in radius scanf("%d %d", &length, &width); „ reads two decimal integers (separated by whitespace), stores the first one in length and the second in width „ Scanf has some limitations….more on it later

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