CS 108 Computing FundamentalsNotes for Tuesday, January 27, 2015

GHP #1b and #2• Due today at 8 AM… I am missing about 12 GHP #1b and 12 GHP #2 submissions

• I started grading GHP 1b late night

•I will grade the rest today and tomorrow

• If you have not yet submitted: do so ASAP to minimize the impact of submitting late (I deduct 1.5 points for GHP 1b and 3 points for GHP #2 per day)… if you have not submitted, then you lost 1.5 or 3 points for lateness already… you have until Wednesday afternoon 8 AM to submit before you lose another 1.5 or 3 points for lateness

•I'm available for help… I'm your best resource for CS 108 help

Abstraction• What is abstraction?

• Why is abstraction important?

• In computing, we heavily embrace abstraction

• As programmers, we LOVE abstraction

1. We know what the “thing” does

2. We know how to make “inputs”

3. We get predictable/expected output for a given specific input… an input is transformed into an expected output

4. We do not know HOW an input is transformed into an expected output

Abstraction Means:

• An automobile transmission

• A photo-taking booth at an arcade

• Airline travel

• Anything that acts like a "black-box"

Examples of Abstraction

Abstraction

• Each virtual machine layer is an abstraction to the level above it.

• The machines at each level execute their own particular instructions, calling upon machines at lower levels to perform tasks as required.

• Computer circuits ultimately carry out the work.

Review of Abstraction

1. We know what the “thing” does

2. We know how to make “inputs”

3. We get predictable/expected output for a given specific input… an input is transformed into an expected output

4. We do not know HOW an input is transformed into an expected output

• C and most programming language are all about abstractionThe "form" of C encourages us to break large objective

into smaller tasks and to write small pieces of code to accomplish the smaller tasksIn C we use functions (from provided libraries of

functions or our own libraries) and we also write functions to accomplish smaller tasks

C functions (both from libraries and the functions ones we write) are grouped together to form programs which accomplish a larger, overall objective

The “internals” of C functions are often hidden

Abstraction and the C Programming Language

Let's Jump Into Programming

• We are going to use C in this course as our programming language of choice

The higher-level concepts that we are going to learn are not "C specific"

• C programs are collections of statement and functions (subroutines or sub-programs) woven together to solve a problem or accomplish a task.

• Key point to remember: C (like UNIX) is case sensitive.

You Have Already Learned Some of the Thing's You'll Need to Program

• All of our CS 108 C programs start as text files on Fang (text files that contain C code are call source-code files)

For GHP #2 you created a text file on Fang that contained 3 algorithms… you used the pico text editor on Fang We will use pico on Fang to create our C text files (aka C source-code files)

Our C source code files may have any legal filename, but they must be have a .c filetype

Examples: practice.c ghp3.c stuff.c

Compiling a Source-code File

• Once you have a properly named C source code file, you must use a compiler to transform a C source-code file into an executable file

Our compiler of choice is the gcc compiler

Compiling a Source-code File

• We compile a C source-code file by typing gcc followed by a space followed by the name of the source-code file (we do this at the Fang prompt)

Examples:gcc practice.c gcc ghp3.cgcc stuff.c

Let's Give It a Try

• Let’s logon to Fang using Putty• After we logon we'll create a file named first.c

first.c will contain only one blank space and nothing else Watch what happens when we compile first.c (remember, it

contains only a space):gcc first.c

Let's Give It a Try

• We get an error… something like this: /usr/lib/crt1.o(.text+0x85): In function `_start':

: undefined reference to `main'

• This is to be expected because our source-code file (first.c) didn't contain anything more than a single space… the gcc compiler was expecting to find text in first.c that is part of the C programming language

C source-code files must be composed of text that is part of the C programming language

Let's Give It a Try

• Let's open first.c and add something from the C programming language

Into first.c let's add this line: main ( ) { }

• Now let's save first.c and compile using the gcc compiler: gcc first.c

• What happens?

Let's Give It a Try•Now let's save first.c and compile using the gcc compiler: gcc first.c

• What happened? There were no errors this time… this is a good thing because it means that the source-code file was successfully compiled (transformed into an executable program). How do we run the program?

Type ./a.out at the Fang prompt What happens? Why?

Let's Give It a Try• How do we run the program?

Type ./a.out at the Fang prompt What happens? Why?

Nothing happens because our source-code essentially contained no instructions The executable file did, in fact, execute, but it did nothing that we could see (it did interact with the operating system and it did take up a few cycles of the CPU)

Let's Give It a Try• The bare minimum code that's required in every, every, EVERY C source-code text file is this: main ( ) { }

Every source-code C file must have a main function definition

Every C function definition has the function's name followed by a ( ) which encloses the formal parameters/arguments list (this one is empty)

Every C function definition contains instructions that are found between { (signifying the start of the function's instructions) and the } (signifying the end of the function's instructions)… there are no instructions in this source code

Win a Bet With Other C Programmers

• The shortest legal compliable C program is just 8 characters:

main(){}

Open and CloseParenthesis

Open and CloseCurly Brace

Name of the function is main

Some Syntax• The first function in all C programs and the only function required by all C programs: main( )

• main( ) controls the flow of a program… it’s the quarterback

• All functions (including main ( ) ) are composed of statements

• Statements in C end in a ;

• Function statements are grouped together into blocks which are all the statements between { }

More Syntax (1)

• We can make first.c a lot more readable if we rewrite it as follows:

main ( ){

}

•This is known as the Allman style (named for Eric Allman)

http://en.wikipedia.org/wiki/Indent_style#Allman_style

More Syntax (2)

• Normally we expect this in the main ( ) (main function):

main ( ){

C instruction statement ;C instruction statement ;C instruction statement ;

}

• This is known as the Allman style (named for Eric Allman)

http://en.wikipedia.org/wiki/Indent_style#Allman_style

More Syntax (3)• Additionally, we normally return values when our functions complete their work

The main ( ) normally returns an integer value to the operating system when it's complete (think of this as a "signal flag")… the integer value 0 is returned when the program runs successfully… here's how we make this happen :

int main ( ){

return ( 0 ) ;}

return is a special statement in C and it performs a specific task, but it is not a function (even though it looks like a function)… this also works: return 0 ; ( notice no ( ) )

Function's "returned"data type

main( )"returned"value

More Syntax (4)• One more thing: the ( ) holds information that's received from the calling environment for the function to use/work upon/digest… if the function is designed to receive nothing, then we should put the keyword void inside the formal parameters list or formal arguments list ( ) … here's first.c rewritten:

int main ( void ){

return ( 0 ) ;}

• Let's recompile (using gcc) and rerun the resulting executable file ( ./a.out )

More Syntax (5) int main ( void )

{ return ( 0 ) ;

} •The ( ) in a function definition:

Holds information that's received into from the calling environment… the function then uses, manipulates, transforms that received information) If we have nothing received into a function, then we should put the keyword void inside the ( ) of the definition Everything contained inside the ( ) in a function definition is called the function’s formal parameters list or the function’s formal arguments list

More Syntax (6)• Although our source-code compiles and executes, it does nothing… let's add a statement that does something

int main ( void ){

printf ( " C programming is slice of pie. " ) ; return ( 0 ) ;

} • printf( ) is a function in C that is found in a "library" of related functions… libraries hold functions for us to use that have been written by other programmers •In the code above, I am calling the printf( ) and sending it a string constant " C programming is slice of pie. " • Let's recompile (using gcc) and see what happens

More Syntax (7) printf ( " C programming is slice of pie. " ) ;

•The ( ) in a function call: Holds information that is being passed to the function for the function to work on (use, manipulate, transform) If we have nothing to send the function, then we should leave the ( ) empty ( do not use void in a function’s call ) Everything contained inside the ( ) in a function call is known as the function’s actual parameters list or the function’s actual arguments list

More Syntax (8)

• Compiling the source-code below resulted in an error… why?

int main ( void ){

printf ( " C programming is slice of pie. " ) ; return ( 0 ) ;

}

More Syntax (9)• Compiling the source-code below resulted in an error… why?

Because we didn't tell the compiler where to find the function that we called ( printf ( ) )… we need to "include" the specific library necessary We need to include a preprocessor directive ( # include ) that explicitly states the name of the library (< stdio.h >) which we wish to use ( <stdio.h> is where the printf ( ) source-code is located) # include <stdio.h>

int main ( void ){

printf ( " C programming is a slice of pie. " ) ; return ( 0 ) ;

}

Preprocessor command

More Syntax (10)• The output is not as nice as it could be

• Let's add some line-feeds/new-line characters ( \n ) to make the output a little more readable

# include <stdio.h> int main ( void ){

printf ( " \n C programming is a slice of pie. \n " ) ; return ( 0 ) ;

}

• The output was not as nice as it could be, so I added the "line-feeds" escape sequences before and after the text string to improve the appearance

Line-feed"escape sequence"

More Syntax (11)• Comments are very useful/helpful/required in my CS 108 world

/* This is an example program written for class on January 27, 2015 by Chris Urban… it does nothing but display a line of text to the screen. */

# include <stdio.h> // My file first.c int main ( void ){ printf (" \n I hope you C things my way. \n\n\n " ) ; return ( 0 ) ;}

/* End of program */

More Syntax (12) • main( ) can be read as

the function named main the line of code in the function named main that is literally main( )

• Functions are of two different types: Standard-library (predefined) functions

Source-code is found in code libraries Programmer-created functions (PCFs)

Programmers create the source-code themselves

• main( ) is followed by { which signifies the beginning of the main( ) function block of statements

• eventually } is used to signify the end of the main( ) block of statements

A Real C Example of Abstraction

• Do we know exactly HOW printf( ) works?

printf (" \n I hope you C things my way. \n\n\n " ) ;

•All the details are abstracted away… those details reside in the stdio.h library file… that’s why we have to “link” to that library (the compiler needs the contents of the library file for printf( ) so it can obtain and use printf( ) source-code with our source-code during compilation)

Variables• Variables are basic data objects

• Variables are "declared" or created before they are used

• Every variable has characteristics or properties– Name (also called an identifier)– Data type (this determines encoding/decoding) – Value– An address in main memory

• Variables may be/should be "initialized“ when they are declared

• A variable name is the symbolic representation of the memory location that is allocated when a variable is declared

Variables

• A variable name is the symbolic representation of the memory location that is allocated when a variable is declared

• Let’s do some chalk talk and discuss the concept of a “symbol table”

More on Variables• First character of a variable name/identifier must be a letter

• Only 31 spaces are allowed in any identifier

• Can’t use reserved words (like main or int)

• Suggestions: short, sweet, meaningful, lowercase, use underscores

• ;

• We often use the assignment operator ( = ) to assign a value to a variable

Data Types• A variable’s data type specifies (de facto):

the kind of value that variable can store

the range of values that a variable can store on the specific machine (computer) we are using

the set of operations that can be applied to that variable

• Four main data types

int

float

double

char

Declaration of Variable Statements(The following is a “snippet” of C code)

# include <stdio.h>

int main(void)

{

int first_val = 0 , second_val = 0 , sum = 0 ;

float batting_ average = 0.0 ;

char selection = 'z' ;

printf( )

• This is a standard output function found in stdio.h library

• Display info to the monitor

• The arguments list (aka "actual parameters list") is enclosed in parentheses… notice " " which signifies a string constant… this string constant may contain text, punctuation, and escape-sequence or escape characters, and format specifiers/placeholders/conversion specifiers printf ( " \n I hope you C things my way. \n " ) ;

•Try Google’ing this phrase: “C escape sequences”

More printf( )

printf ( " The batting average is %f " , batting_average ) ;

• Arguments / actual arguments / actual parameters: everything between the ( ) of a function's call

Format string or string constant (everything between the " " )

Placeholder or format specifier or conversion specifier… in this case: %f … must match the data type of the corresponding data list item

Data list ( variable name(s), constant(s), expressions, or function call(s)… each of which resolves to a single value )… in this case a variable name: batting_average

Data list identifiers must correspond one-to-one with the format specifiers in the format string/string constant

scanf( ) • scanf( ) is standard input function found in stdio.h

• Copies the data from an input device into the variable specified as an argument

scanf( " %f ", &batting_average ) ;

• Arguments / argument list (everything between the ( ) )

Format string/string constant

Format specifier or placeholder or conversion specifier

Address list ( variable name(s) ) including &

& is the “address of” operator and translates to

“the address of variable____ ”

"Address of "Operator

Google is Your Friend• Google : c escape characters …the top response is:

http://msdn.microsoft.com/en-us/library/h21280bw.aspx

• Google: c format specifiers …top response is:

http://www.codingunit.com/printf-format-specifiers-format-conversions-and-formatted-output

•Google: c keywords …top response is (C has 32 keywords):

http://tigcc.ticalc.org/doc/keywords.html

•Google: c operators …4th response is: http://www.lix.polytechnique.fr/~liberti/public/computing/prog/c/C/CONCEPT/expressions.html

Key Things to Understand and Commit to Memory From Today

abstraction source code executable codecompiler compilation gccfunctions main function main( ).c file type syntax white spaceactual parameters actual arguments blockformal parameters formal parameters ;, escape sequences \nfunction return data type return variablesint float doublechar format string placeholderformat specifier data list address list& address of operator =assignment operator formal parameters ( ){ } statement pre-processor directiveinclude <stdio.h> int main (void)void function definition function call