Comments, Style, Printing

1
Comments, Style, Printing

• Jan. 15, 2008

2
Hackerz Tip of the Day

• SSH Secure Shell Protocol
• Run commands on remote machines
• From Unix/Linux/Mac
• Open a terminal -gt type
• ssh username_at_hostname
• Then password when prompted
• From Windows
• http//ftp.ssh.com/pub/ssh/SSHSecureShellClient-3
  .2.9.exe
• A graphical SSH client
• Also does file transfer

3
(No Transcript)
4
Quiz!

• Given RAM, hard drive, CPU cache, CPU registers
• Order in terms of access speed (fastest to
  slowest)
• Order in terms of cost per byte (most expensive
  to least expensive)
• Give an example of two input devices and two
  output devices
• What does \n in a printf statement do?
• What symbol is used for division?

5
30-Second Survey

• Finished Lab 1 without any problems?
• How did Lab 1 go?
• Had more trouble working with Cygwin than
  understanding the programming?
• Darn you, Cygwin!
• Have Cygwin/Ubuntu/Mac Developer tools installed
  on personal computer?
• Feels like they understand last week's material
  completely or almost completely?

6
News

• Office hours have been updated
• Monday, 10am-Noon Vivek (I313)?
• Tuesday, 530pm-730pm Eugene (I313)?
• Wednesday, 2pm-4pm Matt (I231)?
• Wednesday, 4pm-6pm Huijun (I313)?
• Thursday, 1pm-3pm John (I313)?
• Friday, 230pm-430pm Zhuofeng (I313)

7
What is a Program?

• Set of computer instructions that define the
  solution to a specific, well-defined problem
• The general name for this is algorithm
• A finite set of steps that solve a well-defined
  problem
• Real world example a cooking recipe

8
Bread Recipe

• Combine 3 cups flour, 1 tsp. salt, ¼ tsp of yeast
  and ¾ cup of water in a large bowl
• Mix thoroughly with a wooden spoon
• Cover with plastic wrap and let rise for 12-18
  hours
• Fold dough over twice and then put into loaf pan
• Let rise 3-4 hours
• Cook at 400 F for 45 minutes

9
Recipe as Algorithm

• Instructions are in sequential order
• Don't do step 2 before doing step 1!
• Instructions are exact
• Exact enough to be easily reproducible by others

10
Recipe as Algorithm

• Recipe solves a well-defined problem
• Lack of yummy bread
• Recipe is finite, it has an end
• We will get bread sometime tomorrow
• Bread has been proven to be not as tasty when it
  takes an infinite number of years to bake it

11
Computer Programs as Algorithms

• Should perform a well-defined task
• Calculations, communications, games, etc.
• Should finish in a finite amount of time
• Should have explicit instructions
• The computer has to be told what to do
• We have to tell it what to printf
• We have to tell it which library printf comes
  from stdio.h
• Instructions are executed in order

12
File system Organization
13
Organizing Directories

• It's important to keep things organized
• We're going to use two directories
• src stores all our source code
• bin stores all our executable applications
• (Why do you think it's called bin?)?

14
Home Directory
i210
src
bin
hello.exe
add.exe
add.c
hello.c
15
Compiling From src to bin

• gcc -Wall -o ../bin/hello.exe hello.c

16
Compiling From src to bin
Relative path to bin folder

• gcc -Wall -o ../bin/hello.c hello.c

17
How are variables stored in memory?
18
Starting Memory (idealized)?
19
Printing Multiple Values

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x 5
• int y 10
• return( 0 )

20
Memory During Execution
21
Memory with Realistic Addresses
22
Memory Over Time

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x 5
• int y 0
• y x 5
• return( 0 )

23
Memory Over Time

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x 5
• int y 0
• y x 5
• return( 0 )

Snapshot 1
Snapshot 2
24
Memory at Snapshot 1
25
Memory at Snapshot 2
26
Starting Memory (Realistic)?

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x
• printf( x is d\n, x )
• return( 0 )

27
Starting Memory (Realistic)?

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x
• printf( x is d\n, x )
• return( 0 )

No initial value given to the variable, x
28
Starting Memory (Realistic)?

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x
• printf( x is d\n, x )
• return( 0 )

Can we assume this value is 0? No.
29
Starting Memory (Realistic)?
Garbage Values
Garbage Values
30
Printing Multiple Values

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x 5
• int y 10
• printf( "x is d, y is d\n", x, y )
• return( 0 )

31
Review Assignment Operator

• Use to assign new values to variables

x y 10
32
The Assignment Operator

• Use to assign new values to variables

x y 10
x is getting a new value
33
The Assignment Operator

• Use to assign new values to variables

x y 10
x is getting a new value
Right-hand side is evaluated first, by looking up
y's value and multiplying by 10
34
The Assignment Operator

• Right-hand side can even have the old value of
  the variable

x x 8
35
The Assignment Operator

• Right-hand side can even have the old value of
  the variable

x x 8
Look up old value of x and add 8.
36
The Assignment Operator

• Right-hand side can even have the old value of
  the variable

x x 8
Look up old value of x and add 8.
Assign the result back into x's memory slot
37
Making your Programs Human Understandable
38
Programming code can be cryptic
include ltstdio.hgt main(t,_,a)char
areturn!0ltt?tlt3?main(-79,-13,amain(-87,1-_, ma
in(-86,0,a1)a))1,tlt_?main(t1,_,a)3,main(-94,-
27t,a)t2?_lt13? main(2,_1,"s d
d\n")916tlt0?tlt-72?main(_,t, "_at_n','/w/wc
dnr/,r/de,/,/w,/wqn,/l,,/nn,/
n,/\ qn,/k,/'r 'd'3,wK
w'K'e'dq'l \ q'd'K!/kq'reKKw'reKK
nl'/qn'))w'))nl'/n'drw' i
\ )nl!/nn' rw'r ncnl'/l,'K rw'
iKnl'/wqn'wk nw' \ iwkKKnl!/w'lw'
i nl'/q'ldr'nlwb!/de'c
\ nl'-rw'/,'nc,',nw'/kd'e'rdq
w! nr'/ ') rl'n' ') \ '(!!/")? tlt-50
?_a?putchar(31a)main(-65,_,a1)main((a'/
')t,_,a1)? 0ltt?main(2,2,"s")a'/'main(0
,main(-61,a, "!ekdc i_at_bK'(q)-wnr3l,\nuw
loca-Om .vpbks,fxntdCeghiry"),a1)
39
Clarity Helper 1 Comments

• Comments help programmers figure out what
  different instructions are doing
• They're basically human-readable notes that don't
  affect the program's execution
• Comments are completely ignored by the compiler
  when building an executable

40
Single Line Comment

• A comment that ends at the end of the line
• Denoted in the code by the use of //
• Everything that follows // is part of the comment
  and doesn't do anything when the program is
  executed

41
Single Line Comment Example

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x 5
• int y 10
• // Print out the values of x and y
• printf( "x is d, y is d\n", x, y )
• return( 0 )

42
Multi-line Comment

• Sometimes we need to write comments that cover
  several lines
• Option 1
• Put single line comment markers, //, at the
  beginning of each line of the multi-line comment
• What if we have several paragraphs of description
  in a very complicated program?

43
Multi-line Comments using Single Line syntax

• int main( int argc, char argv )?
• int x 5
• // It is now vital to show the user
• // the value of x.
• // In order to do this we will use
• // the printf function to display the
• // value on the monitor of the user.
• printf( "x is d\n", x )
• return( 0 )

44
Option 2 Real Multi-line Comments

• Use the multi-line comment syntax
• / is the starting marker
• / is the ending marker
• Everything in between these markers is part of
  the comment

45
Multi-line Comments using Single Line syntax

• int main( int argc, char argv )?
• int x 5
• / It is now vital to show the user
• the value of x.
• In order to do this we will use
• the printf function to display the
• value on the monitor of the user. /
• printf( "x is d\n", x )
• return( 0 )

46
When to use Comments

• Use comments to describe complicated parts of a
  program
• Makes it easier for others you're working with to
  understand your code
• It is very easy to forget what your old code was
  supposed to be doing
• Help out your future self!
• Use comments to take out parts of your code, so
  they're not executed
• Example with printf line

47
Header Comments

• Each program you write should start with a
  multi-line comment that includes essential
  information about the program
• Your name
• Date
• Which assignment/lab/project it is for
• Any unusual requirements to compile or run your
  program
• A brief overview of what the program does

48

• /
• Matt Whitehead
• Jan. 15, 2008
• Lecture Example 6
• This program shows how to define variables and
  perform simple mathematical operations on them.
• /
• include ltstdio.hgt
• int main( int argc, char argv )?
• ...

49
Clarity Helper 2 Appropriate Variable Names

• Variable names can be nearly anything
• xouw15erjfjjj88 is a legal variable name
  (unfortunately)?
• Variable names should be short and descriptive
• The name should make it clear what the value
  being stored represents
• daysUntilSummer
• booksCheckedOut
• hatsOwned

50
Case of Variable Names

• Variable names should be in Camel Case
• First letter is lowercase
• Subsequent letters that start words should be
  uppercase
• daysUntilSummer
• booksCheckedOut
• hatsOwned

51
Clarity Helper 3 Program Layout

• Keep your program nicely organized in terms of
  spacing and indentation
• These help improve the readability of your code
  and make it far easier to work with

52
Manage Vertical Spacing Adding Blank Lines

• Add blank lines to separate logical blocks of
  code
• Add a blank line above the return statement
• Add a blank line above the main function header
• Add a blank line after your variable declarations

53
BAD Vertical Space Management

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x 5 int y 0
• y x 5
• return( 0 )

54
GOOD Vertical Space Management

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x 5
• int y 0
• y x 5
• return( 0 )

55
Manage Horizontal Spacing Adding Spaces/Tabs

• Code Indentation
• Indent code inside the main function
• Indentation should be 4 spaces in
• Indentation helps us see which code is inside the
  main function (or other block of code, as we'll
  see later on)?
• Spacing around operators
• Leave a space around mathematical operators
• Leave space inside parentheses

56
BAD Horizontal Space Management

• include ltstdio.hgt
• int main(int argc,charargv)?
• int x 5
• int y0
• y x 5
• return(0)

57
GOOD Horizontal Space Management

• include ltstdio.hgt
• int main( int argc, char argv )?
• int x 5
• int y 0
• y x 5
• return( 0 )

58
New Datatypes
59
Floating Point Values

• So far we've just used integers
• Obviously, sometimes we need the fractional part
  of numbers to perform accurate calculations
• Just as simple to declare and use as integers

60
Declaring and Using Floats

• include ltstdio.hgt
• int main( int argc, char argv )?
• float x 5.55
• printf( x is f\n, x )
• return( 0 )

61
Declaring and Using Floats
Use the data type float for numbers with decimal
values

• include ltstdio.hgt
• int main( int argc, char argv )?
• float x 5.55
• printf( x is f\n, x )
• return( 0 )

62
Declaring and Using Floats
Now you can use a decimal point in your
initializations
Use the data type float for numbers with decimal
values

• include ltstdio.hgt
• int main( int argc, char argv )?
• float x 5.55
• printf( x is f\n, x )
• return( 0 )

63
Declaring and Using Floats
Now you can use a decimal point in your
initializations
Use the data type float for numbers with decimal
values

• include ltstdio.hgt
• int main( int argc, char argv )?
• float x 5.55
• printf( x is f\n, x )
• return( 0 )

Printing a floating point value is similar, but
uses f instead of d
64
Float Examples