CS 108 Computing Fundamentals Notes for Tuesday, January 28, 2014

1. CS 108 Computing FundamentalsNotes for Tuesday, January 28, 2014

2. GHP #1a • I graded all the submissions I received. • I replied to your submissions and included the grade you earned in the reply • I have a submission from everyone in Section 01 • I’m missing 5 submissions from Section 35W (online section) • This means that your grades for GHP #1a are in your Alpine in-box on Fang • Open Alpine and select menu option “I” to get you your in-box

3. GHP #1b and #2 • Due this morning at 8 AM for Section 01 and 11:59 PM this evening for Section 35W. • I will grade them today and tomorrow • Half already graded for Section 01… about 20% submitted and graded for Section 35W • 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 3 points for lateness already… you have until Wednesday morning at 8 AM (for Section 01) or Wednesday at 11:59 PM for Section 35W to submit before you lose another 3 points for lateness

4. Abstraction • What is abstraction? • Why is abstraction important? • In computing, we heavily embrace abstraction • As programmers, we LOVE abstraction

5. We know what the “thing” does We know how to make “inputs” We get predictable/expected output for a given specific input… an input is transformed into an expected output We do notknow HOW an input is transformed into an expected output Abstraction Means:

6. An automobile transmission A photo-taking booth at an arcade Airline travel Anything that acts like a "black-box" Examples of Abstraction

7. 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.

8. Review of Abstraction • We know what the “thing” does • We know how to make “inputs” • We get predictable/expected output for a given specific input… an input is transformed into an expected output • We do notknow HOW an input is transformed into an expected output

9. C is all about abstraction The "form" of C encourages us to break large objective into smaller tasks and to write small pieces of code to accomplish the smaller tasks In 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

10. Let's Jump Into Programming • We are going to use C in this course as our programming language of choice • The 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. • Key point to remember: C (like UNIX) is case sensitive.

11. 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 nano text editor on Fang • We will use nano 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

12. 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

13. 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.c gcc stuff.c

14. Let's Give It a Try • Let’s logon to Fang using SSH • 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

15. 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

16. 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?

17. 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?

18. 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)

19. 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 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) Every source-code C file must have a mainfunction definition Every C function definition and function call has the function name followed by a ( ) (more about function calls soon)

20. Win a Bet With Other C Programmers • The shortest legal compliable C program is just 8 characters: main(){} Open and Close Parenthesis Open and Close Curly Brace Name of the function is main

21. Some Syntax • The first function in all C programs and the onlyfunctionrequired 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 end in a ; • Function statements are grouped together into blocks which are all the statements between { }

22. 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

23. 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

24. 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 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 ( ) ) main( ) "returned" value Function's "returned" data type

25. More Syntax (4) • One more thing: the ( ) holds information that's passed to the function for the function to digest (work on)… if we have nothing to send the function, then we should put the keyword void inside the ( ) … here's first.c rewritten: int main ( void ) { return ( 0 ) ; } • Let's recompile (using gcc) and rerun the resulting executable file ( ./a.out )

26. More Syntax (5) int main ( void ) { return ( 0 ) ; } • The ( ) in a function definition: • Holds information that's received into the function for the function to work on (use, manipulate, transform) • If we have nothing is 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

27. 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

28. 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

29. 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 ) ; }

30. 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

31. 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"

32. More Syntax (11) • Comments are very useful/helpful/required in my CS 108 world /* This is an example program written for class on January 28, 2014 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 */

33. 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

34. A Real C Example of Abstraction • Do we know exactly HOWprintf( ) 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.hlibrary 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)

35. 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 "initialized“ when they are declared • A variable name is the symbolic representation of the memory location that is allocated when a variable is declared

36. 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”

37. 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

38. Data Types • A variable’s data type specifies (de facto): • kind of value that variable can store • set of operations that can be applied to thatvariable • Four main data types int float double char

40. 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' ;

41. printf( ) • This is a standard outputfunction 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 • printf ( " \n I hope you C things my way. \n " ) ; • Try Google’ing this phrase: “C escape sequences”

42. More printf( ) printf ( " The batting average is %f“ , batting_average ) ; • Arguments / actual parameters: everything between the ( ) • Format string or string constant (everything between the " " ) • Placeholder or format specifier or conversion specifier… in this case: %f • Data list ( variable name(s), constant(s), expressions, or function call(s) )… 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

43. scanf( ) • scanf( ) is standard inputfunction 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

44. 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

45. Key Things to Understand and Commit to Memory From Today • abstraction source code executable code • compiler compilation gcc • functions main function main( ).c file type syntax white space • actual parameters actual arguments block • formal parametersformal parameters; • , escape sequences \n • function return data type return variables • intfloatdouble • char format string placeholder • format specifier data list address list • & address of operator = • assignment operator formal parameters ( ) • { }statement pre-processor directive • include <stdio.h> int main (void) • void function definition function call