Programming Levels Application Languages (Java, C#)

High-Level Languages

Algorithm level

Scripting Interpreted Languages Or Compiled (Perl, Python, VB)

System Programming Languages (C and C++)

Introduction to C

Compilation

Assembly Language (x86, PowerPC, SPARC, MIPS)

Low-Level Languages

Machine Language g g (x86, PowerPC, SPARC, MIPS)

Based on slides © McGraw-Hill Additional material © 2004/2005 Lewis/Martin Modified by Diana Palsetia

Hardware (Application-Specific Integrated Circuits or ASICs)

Assembly

ISA level

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Programming Language Semantics

Programming Language: Syntax

When the computer carries out your instructions (program)

Syntax is the grammar of the language „

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Analogous to rules in English Language

Semantics is the meaning of the program

¾ Missing ss g a pe period od a after e se sentence e ce ¾ Rules using verbs, nouns etc..

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Running or Executing a program

Most languages provide syntax check errors (Interprets/Compilers)

We learn the semantics after we run or execute the program Basically we observe the output

After the executing program, the semantics of the program may or may be correct

¾ Syntax error messages may be helpful ¾ Often, Often they are not ¾ You gain experience with error messages after a while

Semantic errors cause your answers to be wrong „

You may or may not get error messages

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If your program is not doing what you want it to do, though it runs, the error is semantic

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E.g. Error Message – Dividing a number by zero

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Compilation Process

C Programs are Compiled

Entire mechanism is usually called the compiler hello.c

(text file containing C source code)

C Source and Header Files

Preprocessor „ „

Acts upon C preprocessor directives “Source-level” transformations

C Preprocessor

¾ Output is still C

hello.s

Compiler

(text file containing assembler source code)

Source Code Analysis

Compiler „

Symbol Table

Generates object file

Target Code Synthesis

¾ Machine instructions (binaries)

hello (a.out)

Library Object Files

Linker

(binary executable file Containing machine code )

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Combine object files (including libraries) into executable image

We will use the GNU gcc compiler (v3.0 and higher) 5

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Executable Image

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Comments in C

Basic C Elements Variables

Begins with /* and ends with */ „

Linker

„ „

Can span multiple lines

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A data item upon which the programmer performs an operation A named space in memory E.g. z, counter

Operators p „

Begins with // and ends with “end of line” „

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Single-line comment

Expressions

¾ E.g. //This is a comment „ „

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Introduced in C++, later back-ported to C Compiling with ansi standards, gives error

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Why use comments ?

A defined behavior Constitutes a unit of work for high-level language Ends with a semicolon. E.g. z = x * y;

Functions „

Help readers understand programs better

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Operators combined with variables/literals to form expressions E.g. x * y

Statements „

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Predefined actions performed on data items E.g. *, +, /, ||, |, &&

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Named group of statements Provides modularity to program Enforces, DRY principle (Do not Repeat Yourself) 8

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C program structure

Variable Properties Identifier: variable name

int main(){ /* code goes here */ return 0; }

Type: how data is interpreted, and how much space it needs Later we will discuss: Scope: is the region of the program in which the variable is alive and accessible

Every C program must the above format to develop application program(s) „

One of files must contain above structure ¾ main is a special function in C ¾ similar to Java’s main method

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Storage: how C compiler allocates storage and whether or not the variable loses its value when the block that contains it has completed execution

Starting point for every program

All C programs start and finish execution in the main Note: int main(int argc, char **argv) – main function can also take arguments like in Java 9

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Identifier: Variable Names

Identifier Examples Legal

Any combination of letters, numbers, and underscore (_)

i wordsPerSecond same identifier words_p per_second _green aReally_longName_moreThan31chars aReally_longName_moreThan31characters

Case sensitive „

"sum" is different than "Sum"

Cannot begin with a number „

Illegal

Usually, variables beginning with underscore are used only in special library routines

10sdigit ten'sdigit done? double

Only first 31 characters are definitely used „

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Implementations can consider more characters if they like

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reserved keyword

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Types

Additional to Data Type

C has several basic data types

Literal „

int integer (at least 16 bits, commonly 32 bits) long integer (at least 32 bits) 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 „

int is supposed to be "natural" integer size

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So how do I know the size?

Values we write in a conventional form whose value is obvious

¾ 32 bits for most modern processors

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Variable whose values do not change during the execution of the program This done by appending “const” before the type

¾ Use method called sizeof. E.g. sizeof(int), returns answer in bytes

Symbol

Signed vs. unsigned: „ „

Default is 2’s complement signed integers Use “unsigned” keyword for unsigned numbers

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Constants vs. Symbol

Integer

#define RADIUS 15.0

123 /* decimal */ -123 0x123 /* hexadecimal */

symbol

int main(){ constt d double bl PI = 3 3.14159; 14159 double area; constant double circum;

Floating point /* 6.023 x 1023 */ /* 5.0 x 1012 */

literal

area = PI * RADIUS * RADIUS; circum = 2 * PI * RADIUS; return 0;

Character 'c' '\n' /* newline */ '\xA' /* ASCII 10 (0xA) */ CIT 593

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Literals

6.023 6.023e23 5E12

Like constants but is preprocessor directive

} 15

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Symbol is C Preprocessor Directive

Expression Expression

Symbol start with #define „

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Must go before the “main function syntax”

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Example: #define RADIUS 15.0 „ „ „

Before compiling, replace all instances of the string “RADIUS“ in the code with the string “15.0" Also known as a macro Used for values that won't change during execution, but might change if the program is reused reused. (Must recompile.)

Examples: area = PI * RADIUS * RADIUS; counter >= STOP x + sqrt(y) x & z + 3 || 9 - w-- % 6 17

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Any combination of variables, constants, operators, and function calls Every expression has a type type, derived from the types of its components (according to C typing rules)

Statement

Operators

Expresses a complete unit of work „

Three things to know about each operator (1) Function

Executed in sequential order

Simple statement ends with semicolon (note: “;” is nott a commentt in i C) z = x * y; /* assign product to z */ y = y + 1; /* update y */ ; /* null statement */ Compound statement formed with braces „

„

What does it do?

(2) Precedence „ „

In which order are operators combined? Example: "a * b + c * d" is the same as "(a * b) + (c * d)" because multiply (*) has a higher precedence than addition (+)

(3) Associativity „

Syntactically equivalent to a simple statement

„

{ z = x * y; y = y + 1; }

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In which order are operators of the same precedence combined? Example: "a - b - c" is the same as "(a - b) - c" because add/sub associate left-to-right

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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 side ¾ Type conversion might make assigned value different than computed value e.g. int x = 15.6/3 = 5

2. Set value of left-hand side variable to result.

Assignment g associates right g to left. y = x = 3; y gets the value 3, because (x = 3) evaluates to the value 3

y = (x = 3); 21

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Arithmetic Operators Symbol * / % + -

Operation multiply divide modulo addition subtraction

Usage x * y x / y x % y x + y x - y

Arithmetic Expressions

Precedence Assoc 6 l-to-r 6

l-to-r

6

l-to-r

7

l-to-r

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l-to-r

If mixed types, smaller type is "promoted" to larger „ „

„ „

Example: x / 3 if x is int and x=5, result is 1 (not 1.666666...)

Storing mixed type expression values „

* / % have higher precedence than + -

Example

Example: x + 4.3 if x is int, converted to double and result is double

Integer g division -- fraction is dropped pp

„

All associate left to right

int x = 45/7.1; C compiler will do automatic down grade if storage is small. Java compiler will complain.

Modulo -- result is remainder „

„2

+ 3 * 4 vs. (2 + 3) * 4 „2* 4% 5 CIT 593

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Example: x % 3 if x is int and x=5, result is 2

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Relational Operators Symbol Operation > greater than >= greater than or equal < less than y 9 x >= y 9 x < y 9 x & ^ |

Ultimately conditional will result in TRUE or FALSE Note that see has no Boolean type ¾ Outcome is zero or non-zero (i.e. int)

Example „ (x > y) && (x < z) „ (c == ‘q’) || (c == ‘Q’)

Conditions allow change in the control flow „

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Usage ~x x > y x & y x ^ y x | y

Precedence 4 8 8 11 12 13

Assoc r-to-l l-to-r l-to-r l-to-r l-to-r l-to-r

Bit-wise vs Logical

Can skip certain instructions based on condition if((c == ‘q’) || (c == ‘Q’)){ //Quit game }

„

Operation bitwise NOT left shift right shift bitwise AND bitwise XOR bitwise OR

„

1 & 8 = 0 (000001 AND 001000 = 000000) 1 && 8 = 1 (True && True = True)

Shift operations

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 27

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Operate on values -- neither operand is changed x = y >)

Special Operators: ++ and -Changes value of variable by 1 before (or after) its value is used in an expression

Does right shift sign extend or not? „

Answer: Yes and No

Unsigned g values: zero extend „ „

Symbol Operation ++ postincrement -- postdecrement ++ preincrement -- predecrement

unsigned int x = ~0; Then, (x >> 10) will have 10 leading zeros

Signed values: „

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“Right shifting a signed quantity will fill with sign bits (“arithmetic shift”) on some machines and with ith 0 0-bits bit (“l (“logical i l shift”) hift”) on others.” th ” - Kernighan K i h and d Ritchie In practice, it does sign extend

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2

r-to-l

2

r-to-l

3

r-to-l

3

rr-to-l to l

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Using ++ and --

Special Operators: +=, *=, etc. Arithmetic and bitwise operators can be combined with assignment operator Statement Equivalent assignment

x = 4; y = x++; Results: x = 5, y = 4 (b (because x iis iincremented t d after ft assignment) i t)

x x x x x x x x x x

x = 4; y = ++x; Results: x = 5, y = 5 (because x is incremented before assignment)

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PrecedenceAssoc

Pre: Increment/decrement variable before using its value Post: Increment/decrement variable after using its value

¾ int x = ~0; /* signed */ ¾ Then, (x >> 10) will still be all 1s CIT 593

Usage x++ x-++x --x x

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+= y; -= y; *= y; /= y; %= y; &= y; |= y; ^= y; = y;

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x x x x x x x x x x

= = = = = = = = = =

x x x x x x x x x x

+ y; - y; * y; / y; % y; & y; | y; ^ y; > y;

All have same precedence and associativity as = and associate right-to-left.

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Input and Output

Output printf(“Counter value is %d\n", counter);

Variety of I/O functions in C Standard Library „ „

Libraries are contain code/programs already written that can be re-used So we do not have to re-invent the wheel

This call says to print the variable counter as a decimal integer, followed by a linefeed (\n) Linefeed will make the cursor go onto next line

„ „

For I/O, must include stdio.h i.e.#include

Can print arbitrary expressions, not just variables

¾ Must go below the main function syntax „

#include is a C processor Directive

printf("%d\n", startPoint - counter);

¾ Before compiling, copy contents of stdio.h into source code ¾ .h files typically contain descriptions of functions and structs

Print multiple expressions with a single statement printf("%d %d\n", counter, startPoint - counter);

printf – Print formatted „ „

Performs output to standard output device String contains characters to print and formatting directions for variable

Different formatting options: %d decimal integer %x hexadecimal integer %c ASCII character %f floating-point number

scanf – Scan Formatted „

String contains formatting directions for reading input

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Examples

Examples of Input scanf("%d", &startPoint);

This code:

„

printf("%d is a printf("43 plus 43+59); 3 9 printf("43 plus 43+59); printf("43 plus %c.\n", 43+59);

prime number.\n", 43); 59 in decimal is %d.\n", 59 in hex is %x.\n",

This call says to 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);

59 as a character is

„

reads a single character and stores it in nextChar

scanf("%f", &radius); „

produces this output: 43 43 43 43 CIT 593

is a plus plus plus

prime 59 in 59 in 59 as

reads a floating point number and stores it in radius

scanf("%d %d", &length, &width);

number. decimal is 102. hex is 66. a character is f.

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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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