Administration

1
Administration

• Upcoming deadlines
• Milestone 1 code due Monday Feb. 2
• Graphics proposal document due Friday, Feb. 13
• Milestone 2 on web due Monday, Feb. 23

2
Good Coding Style

• Continued

3
5. Many Short Functions

• Short functions
• Easier to re-use
• Fix bugs in one place, not many
• Make code easier to read more abstraction
• How long should functions be?
• Should have many 5 to 10 line functions
• Should very rarely have a function gt 100 lines

4
Thoughts on This Code?
include ltmath.hgt void myFunc (float a, float
b, int nvals) float absAvg1 0
for (int i 0 i lt nvals i) absAvg1
abs(ai) absAvg1 / nvals float absAvg2
0 for (int i 0 i lt nvals i)
absAvg2 abs(bi) absAvg2 / nvals
...

• There is no honour in copy
• and paste coding!

• Refactor!

5
Better Version?
include ltmath.hgt float compAbsAvg (float
array, int nvals) float absAvg 0 for
(int i 0 i lt nvals i) absAvg abs
(arrayi) return (absAvg / nvals) void
myFunc (float a, float b, int nvals)
float absAvg1 compAbsAvg (a, nvals) float
absAvg2 compAbsAvg (b, nvals) ...

• 1. Not much shorter, but easier to read
• 2. Less chance of more code copying ? future code
  will be shorter

6
6. Dont Do Too Much in a Statement / Line
include ltstringgt include StreetsDatabaseAPI.h
string name getIntersectionName(getStreetSegmen
tEnds( getIntersectionStreetSegmen
t(id,0)).to) // Hard to read! unsigned
firstSeg getIntersectionStreetSegment
(myInterId,0) unsigned destInterId
getStreetSegmentEnds(firstSeg).to string
destInterName getIntersectionName
(destInterId) // Show your work ? divide into
several steps on several lines // Use good
variable names to show what intermediate
results // are.
7
7. Defensive Coding

• Use assertions to verify assumptions in your code
• void myFunc (int ptr)
• // Dont ever call myFunc with a NULL ptr!
• if (ptr 1)
• include ltassert.hgt
• void myFunc (int ptr)
• assert (ptr ! NULL)
• if (ptr 1)
• Exits program if ptr is NULL (condition not true)
• Better than a comment
• Checked at runtime gives useful error message
• gt assert failed on line 208 of file myFunc.cpp
  ptr ! NULL

8
What If I Need That Last Bit of Speed?

• define NDEBUG // Just turned off assertion
  checking
• // Make sure this line is in
  front of
• // include ltassert.hgt
• include ltassert.hgt
• void myFunc (int ptr)
• ptr NULL
• assert (ptr ! NULL) // Not checked ? wont
  fire.
• Can turn off assertion checking in release build
• Avoids any speed overhead
• Leave on for debug build for extra checking
• And maybe leave on in release too if your program
  not very time critical

9
7. Defensive Coding

• B. Set deleted / invalid pointers to NULL
• delete (ptr)
• ptr NULL // Now well crash if we try to
  use it ? good!
• ptr-gtvalue 1 // Will seg fault immediately
• C. Self-checking code (advanced technique)
• program_ok validate_key_data_structure
  (my_struct)

10
Should have used assert or validity checkers!
11
Find the Bug!

• const int NUM_WEIGHTS 20 // 1. Constant
  variable
• define WEIGHT_ZERO 0 // 2. Pre-processor
  constant
• enum WtReturn HAS_NEG -1, HAS_ZERO 0,
  ALL_POS 1
• // 3. make an enumeration (list) of int
  constants
• int checkWeights (int weightsNUM_WEIGHTS)
  for (int i 0 i lt NUM_WEIGHTS i)
• if (weightsi 0)
• return (HAS_ZERO)
• if (weightsi lt 0)
• return (HAS_NEG)
• return (ALL_POS)

Test program, find a failing case

• weights -1, 2, 3, 4, 5, 6, 20
• checkWeights returns ALL_POS

12
8. Use Compiler Warnings

• gt g -Wall weights.cpp
• gt weights.cpp In function int
  checkWeights(int)
• gt weights.cpp11 warning suggest parentheses
  around assignment used as truth value

int checkWeights (int weightsNUM_WEIGHTS)
for (int i 0 i lt NUM_WEIGHTS i)
if (weightsi 0) return (HAS_ZERO)
if (weightsi lt 0) return
(HAS_NEG) return (ALL_POS)
Line 11
13
8. No Warnings

• Dont have any warnings in your code
• Warnings flag potentially problematic code
• If you leave some warnings in, hard to see new
  ones
• Fix right away stay at 0 warnings!
• Tell compiler to generate all useful warnings
  (more than default)
• Command line g Wall ltgt
• Netbeans
• File Project Properties C Compiler More
  Warnings

14
Over-Arching Rules

• Code will be read more than written
• By you
• By others
• Make it readable!
• Code will be modified many times
• Make small functions avoid repeated code
• Test as you develop
• Keep the tests with the code
• Documentation should be in the code comments!

15
Code Reviews

• If you do one thing for code quality, do code
  reviews
• Altera 4 reviewers read code written by one team
  member
• Significant amount 400 1000 lines
• Write down thoughts
• Then meet to discuss ? readable, clear,
  efficient?
• Google every commit reviewed and approved

16
Code Reviews

• This course you will read another teams
  milestone1 code submission
• Both teams have the same TA
• Write a short (1 page) code review
• Can have an appendix with code examples
• 2 of your final mark
• Does not affect the other teams grade
• TA already reviewed their code for style
• Same TA will read your code review
• Code sent to you week of Feb. 2
• Review due Monday, Feb. 9

17
Intro to Graphics
18
Graphics APIs
myProg.exe
Low level APIs Different for different platforms
win32 API
PostScript
x11 API
19
Graphics APIs
myProg.exe
Solution another layer Higher level API Can
target multiple low-level APIs
win32 API
PostScript
x11 API
20
Graphics APIs
myProg.exe
This course EasyGL (simple, cross-platform
graphics)
win32 API
PostScript
x11 API
21
EasyGL Overview

• include graphics.h
• In any file where you want to make graphics calls
• Need to include 3 files in your project, and some
  libraries in your build step
• See EasyGL quick start guide and example
  code/makefile
• First call set up window
• init_graphics (Some example graphics, WHITE)
• Second call choose your coordinate system
• set_visible_world (xleft, ybottom, xright, ytop)

Background colour
Window title
22
EasyGL Overview

• Set drawing attributes
• setcolor (int color_index) // E.g. BLUE ( 9)
• setcolor (t_color (red, green, blue))
• // red, green and blue are 8-bit integers
• // e.g. t_color (0, 0, 255) is also blue
• setlinewidth (3) // 3 pixels wide
• setlinestyle (DASHED)
• sticky affect all subsequent drawing until
  changed
• Draw primitives
• drawline (x1, y1, x2, y2)
• fillrect (lower_left_pt, upper_right_pt)
• ...

23
Issue Interaction?
myProg.exe
How to pass this information to myProg.exe?
This course EasyGL (simple, cross-platform
graphics)
Graphics drawing myProg.exe calls functions
Hardware receives the input and X11 knows there
is an event
x11 API
User resizes window or
User clicks on a button
24
Solution Callbacks
EasyGL checks the event queue and calls the
appropriate callback ? now myProg.exe can handle
it.
myProg.exe
This course EasyGL (simple, cross-platform
graphics)
myProg.exe registers callback functions for
various events
Hardware receives the input and X11 inserts event
into event queue
x11 API
Then hands control to EasyGL
User resizes window or clicks on a button