Programming

Elementary Concepts

Summary} Computer

} Notion of an algorithm

} Programming language} Lexicon, syntax and semantics} Grammars

} Flowchart

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Why C ?} http://crashworks.org/if_programming_languages_were_vehicles/

} http://spectrum.ieee.org/computing/software/the-2015-top-ten-programming-languages

} http://www.slant.co/topics/25/~programming-language-to-learn-first

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Processor} Functional unit of a computer

} Accesses memory locations} Place where you store data} Scalar data

} Sequentially performs very simple instructions} Reads value, value stores} Sums, subtracts, etc..} Compare} Conditional jumps

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Machine 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Low-level Language (Assembly)} .cstring} LC0:} .ascii "%d \0"} .text} .globl _main} _main:} LFB3: pushq %rbp} LCFI0: movq %rsp, %rbp} LCFI1: subq $16, %rsp} LCFI2: movl $0, -4(%rbp)} jmp L2} L3: movl -4(%rbp), %esi} leaq LC0(%rip), %rdi} movl $0, %eax} call _printf

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Algorithm} Informal definition:

} Sequence of instructions to perform a task

} Examples:} Recipes} Kits assembly instructions} Instructions for washing hands

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Algorithm} Formal definition:

} Finite sequence of well-defined and unambiguous instructions } Instructions with precise execution in finite time and finite

effort

} Examples:} Directions to somebody’s house} A recipe for cooking a cake} The steps to compute the cosine of 90°} Instructions to sort a list of integers

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Programming Language} Well-defined notation

} Allows to describe a sequence of instructions that the computer can execute

} Machine language} Elementary instructions} Dependent of the processor

} High-level languages} Complex instructions} Independent of the processor} Example

} If (a <0) print ("negative")

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Programming Language} Lexical level: how characters are combined to produce

language elements } i and f produces if

} Syntactical level: how language elements are combined to produce language expressions} If (42 == answer) } Exit(1)

} Semantic level: how language expressions are combined in order to form a meaning} Exit from the program if the answer is 42

} Two programs written in different languages could do the same thing (semantics) but the symbols used to write the program may be different (syntax).

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Syntax and Grammar} Grammar

} Formal description of the syntax rules of a language} Precise rules to construct a program

} Comply with the rules of grammar} Compile errors

} Compiler checks the syntax of your program} But doesn’t find all semantic errors !

} Grammar description:} BNF (Backus Naur-Form)} Syntactic diagrams

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Example: Control Flow} if (value <10) instructions} Phrase consisting of:

} Word “if”} Condition value <10 surrounded by parentheses} Instructions

} Terminal symbols} if (,) and <

} Non-terminal symbols} Condition and instructions} Defined elsewhere in the grammar

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How do You Write a Program?} Decide what steps are needed to complete the task} Write the steps in pseudocode (written in English) or

as a flowchart (graphic symbols)} Translate into the programming language} Try out the program and debug it (fix it if necessary)

What is Pseudocode?} List of steps written in English} Like the instructions for a recipe} Must be in the right sequence

} Imagine saying “bake the cake” and then “mix it up”

Sample Pseudocode} Task: add two numbers} Pseudocode:

} Start } Get two numbers} Add them} Print the answer} End

Another Sample: Calculating Age} Pseudocode:

} Start } Get year born} Calculate age} Print age} If age > 50 print OLD} End

Flowcharts Elements

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Oval

Parallelogram

Rectangle

Diamond

Hybrid

Name Symbol Use in Flowchart

Denotes the beginning or end of the program

Denotes an input operation

Denotes an output operation

Denotes a decision (or branch) to be made. The program should continue along one of two routes. (e.g. IF/THEN/ELSE)

Denotes a process to be carried oute.g. addition, subtraction, division etc.

Flow line Denotes the direction of logic flow in the program

Flowchart Example} The pseudocode would look like this as a flowchart

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Start

Get 2 numbers

Add them

Print answer

End

Symbol Explained !

} START/END} Used at the beginning and end of each

flowchart

} INPUT/OUTPUT} Shows when information/data comes

into a program or is printed out

} PROCESS} Used to show calculations, storing of

data in variables, and other processing steps that may occur

Symbols Explained !

} DECISION} Program must decide whether some statement (usually a

comparison) is true or false.} YES and NO (or T/F) branches are usually shown.

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Y

N

X>7?

To Review} Marques de Sá

} Capítulo 1

} Damas} Introdução

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