G52CFJ CC for Java Programmers Demo for Lectures 5 and 6

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G52CFJ C/C for Java Programmers Demo for
Lectures 5 and 6

• Debugging
• Stack vs Heap
• Zombies

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Contents

• Simple debugging using printf()
• Creating your own arrays of strings
• From user input
• How NOT to create arrays of struct pointers
• Zombies sample

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Debugging

• A few slides for reference

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Compiler and linker errors

• Will differ between compilers
• For gcc you may want to read
• http//www.network-theory.co.uk/docs/
• gccintro/index.html
• For compiler error messages
• http//www.network-theory.co.uk/docs/
• gccintro/gccintro_94.html
• For linker error messages
• http//www.network-theory.co.uk/docs/
• gccintro/gccintro_95.html
• For runtime errors
• http//www.network-theory.co.uk/docs/
• gccintro/gccintro_96.html

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Common compilation problems

• Not matching and
• This is why I line them up on separate lines
• Not matching if / ifdef / ifndef / else and
  endif
• Using something before it has been declared
• E.g. a type or function
• Did you remember to include the header file?
• Typo, or missing a
• Usually gives a parse error/syntax error?
• Trying to initialise from something which is not
  a constant (or a non-constant array size)

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Common code errors

• These wont necessarily crash
• May give a warning, often do not give a
  compilation error!
• vs (assignment vs equivalence)
• Missing breaks from switch statements
• vs , vs (bitwise vs logical operators)
• Not matching the printf parameter types with the
  letters in the control string (e.g. s, c)
• Single vs double quotes a vs a
• Should get a warning probably about pointer vs
  integer
• Write out of bounds
• Over the end or beginning of an array
• E.g. array size too small, or bad index value
• Dereference a bad pointer (e.g. memory freed)

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Tips

• Unit testing
• Test functions independently
• Test groups of functions together
• Rather than whole program together
• Look for errors that you know you commonly make
• e.g. I always double-check switches (because I
  know I forget breaks
• A handy tip is to always comment when a break is
  deliberately missing
• Check if statements, ensure there is a double
  equals not a single equals, unless deliberate
• If deliberate then add a comment!

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Catching runtime crashes
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Debugging runtime problems

• Find out where the problem is
• Be able to reproduce the error
• So that you can test the fix
• Find out what the problem is
• Fix the problem
• Test the fix
• Look for similar problems elsewhere
• If you made the mistake once it could be
  elsewhere too

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printf() debugging

• Scatter calls to printf() throughout your code
• You could define a debug function
• printf() in debug, vanishes in release
• See zombies code
• printf() a checkpoint string
• Like the program announcing I got here
• So you know where it crashed
• printf() to see the value of a variable
• To see why it crashed
• This is a simple solution
• But, you have to recompile and re-execute each
  time you add more printf()s

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printf() as a progress point

• Example of checkpoint code
• char c h
• printf( Checkpoint 3\n )
• printf( s, c )
• printf( Checkpoint 4\n )
• Prints Checkpoint 3 but then crashes.
• You know the error is between the two checkpoint
  printfs

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printf() to show variable value

• Bugs are usually for one of two reasons
• The code was wrong
• The data provided to the code was wrong
• In the previous case, the code was wrong
  printf( s, c )
• You can also use printf debugging to identify
  when data is wrong
• Once you find out what went wrong, print the
  values of variables prior to executing the
  statement which went wrong

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Storing strings and structs
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Accepting a string from the user

• / Macro for string length /
• define MAX_STRING_LENGTH 50
• / A simple function to get a string from the
  user. /
• char getString()
• char StringMAX_STRING_LENGTH1 /Allow for 0
  at end/
• int i -1
• do
• / Check that we do not overrun the string
  length /
• if ( i gt MAX_STRING_LENGTH )
• break / Stop the do-while loop /
• Stringi getchar()
• while ( (Stringi ! '\n') (Stringi !
  EOF) )
• Stringi '\0' / Ensure zero-termination of
  string /
• return String / Return the string that was
  loaded /

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Asking for and storing strings

• / Main function - asks for 3 strings, then shows
  them. /
• int main( int argc, char argv )
• / Array of chars, i.e. array of pointers /
• char myinputstringsNUMBER_STRINGS
• int i
• / Ask for some strings and display them /
• for ( i 0 i lt NUMBER_STRINGS i )
• printf( "Please specify string d ", i )
• myinputstringsi getString()
• printf( "\nString d was 's'\n", i,
  myinputstringsi )
• printf( "\nThank you, all strings loaded. Press
  ENTER\n" )
• while ( getchar() ! '\n' ) / Skip chars until
  ENTER /
• for ( i 0 i lt NUMBER_STRINGS i )

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Improvement static local

• char getString()
• static char StringMAX_STRING_LENGTH1
• int i -1
• do
• / Check that we do not overrun the string
  length /
• if ( i gt MAX_STRING_LENGTH )
• break / Stop the do-while loop /
• Stringi getchar()
• while ( (Stringi ! '\n') (Stringi !
  EOF) )
• / Ensure zero-termination of string /
• Stringi '\0'
• / Return the string that was loaded /
• return String

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Improvement use malloc()

• char getString()
• char String malloc(MAX_STRING_LENGTH)
• int i -1
• do
• if ( i gt MAX_STRING_LENGTH )
• break / Stop the do-while loop /
• Stringi getchar()
• while ( (Stringi ! '\n') (Stringi !
  EOF) )
• / Ensure zero-termination of string /
• Stringi '\0'
• return String
• / Since memory was malloc'd it must be freed by
  caller /
• for ( i 0 i lt NUMBER_STRINGS i )
• free( myinputstringsi )

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Or get caller to allocate the memory

• char getString( char String, int iBufferLength)
• int i -1
• do
• if ( i gt (iBufferLength-1) )
• break
• Stringi getchar()
• while((Stringi ! '\n') (Stringi !
  EOF))
• Stringi '\0'
• return String
• myinputstringsi malloc( MAX_STRING_LENGTH 1
  )
• getString(myinputstringsi, MAX_STRING_LENGTH
  1)
• And also free() it again, like before

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Creating structs a problem!

• MyStruct structsNUMBER_STRUCTS
• int i
• srand(1000) / Ensure same results every time
  executed./
• for ( i 0 i lt NUMBER_STRUCTS i )
• MyStruct a_new_struct / Create a new struct /
• / Fill the members of the struct /
• a_new_struct.v1 rand()
• a_new_struct.v2 rand()
• / Store the struct in the array /
• structsi a_new_struct
• for ( i 0 i lt NUMBER_STRUCTS i )
• printf( "\nStruct d has random values d and
  d\n\n",
• i, structsi-gtv1, structsi-gtv2 )

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Games, puzzles,

• (And other programs which have user interaction)

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Interactive program structure
Initialisation Allocate any resources Create any
windows Request memory allocations
Main loop / game loopDisplay state Request
Input Accept/handle Input Update state
De-initialisation Close any windows Release any
resources Free any memory allocations
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Zombies

• How good/bad is the code?
• How would you improve it?

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Key elements of Zombies

• defines for constants
• Optional display for zombie i
• define ZOMBIE 'Z'
• or define ZOMBIE ('a'i)
• Removal of printf when not in debug using a macro
• define debug2(a,b,c) printf(a,b,c)
• or define debug2(a,b,c)
• Use of srand() and rand()
• Another malloc() and free() example

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Questions to consider

• You should try commenting out parts of the code
  to see what they do, or changing the defines at
  the top
• What happens if the size is changed?
• Can the number of zombies and pits be changed?
• What happens if someone enters too much input?
• e.g. multiple numbers/letters
• Why do the zombies and player need to be
  undrawn?
• What memory needs to be freed?
• How do the zombies home in on the player
• How does the code control their movement

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Zombies heading for player

• X and Y coordinates kept separately
• Home in
• if ( zombie_x gt player_x ) zombie_x--
• if ( zombie_x lt player_x ) zombie_x
• if ( zombie_y gt player_y ) zombie_y--
• if ( zombie_y lt player_y ) zombie_y

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Moving zombies/player

• When moving a zombie/player
• Undraw it at the old position
• Need to put the empty map position back again
• Otherwise you get a trail of Zs or Ps on the
  map
• Try it yourself
• Move it
• Update the drawing positions of zombies/player
• Draw it at the new position
• Use the new drawing positions to draw

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Good or bad design?

• Does a function do multiple different things?
• Yes? If so then split it up
• Zombies is an example of a function which is far
  too big
• Where should I store my data?
• Currently all data is in local variables
• If data is used in multiple functions then it
  will need to be passed around, or made global
  (more options in C)
• Can I group data
• For zombies, could produce a struct for a
  position (x,y) and previous contents of map
  location, grouping data
• Then have one struct per player/zombie

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Exercise

• Consider the zombies example code
• You should now be able to work through it
• Make sure that you understand it
• Criticise it
• Is it a good design?
• Is it easy to understand?
• Is the structure nice?
• How could you improve it?
• Do this now it will help with the coursework!

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Next lecture (Friday 3pm)(LT2)

• Now (at 11am)
• Lab session (to try things yourself)
• in A25 (Computer Science Lab)
• With another lab session on
• Tuesday at 1pm