Writing Solid Code

• Introduction

• Programming principles

Juin-Der Lee

2003/03/25

Introduction

◆Why this topic

1.Trend:

a. Propose new algorithm together with code.(Jagathe)

b. Collect related algorithms into single software package.

(SPM, EEGLAB, FMRLAB…)

2. Program is equally important as algorithm. (Incorrect program produces incorrect result no matter how good the algorithm is.)

1. Our clients: doctors, psychologists, experimenter.

For them, though algorithm accuracy is important, program availability is more important. (In my point of view, that’s why most people use AIR to perform registration, though AIR may not be the most accurate algorithm.) They don’t want to waste time coding.

2. Internet makes resource sharing become easy.

3. Open software.(Linux)

Why this trend

Lab 1Implement 1 …

Stat sinica Algorithm Implement s (program)

Lab 2Implement 2

Lab nImplement n

Everybody codes

Lab 1Implement s …

Stat sinica Algorithm Implement s

Lab 2Implement s

Lab nImplement s

Save them from whole night coding

Lab 1Implement s

Implement s…

Stat sinica Algorithm Implement s

Lab 2Implement x

Lab nImplement s

UCSD Algorithm Implement x

…

They could have multiple choices

Mathematics, Statistics

Algorithms

Program

publish

Position of program in most research groups

Mathematics, Statistics

Algorithms

Program

publish

Position of program in future research groups

Conclusion

• Program is very important.

• We should pay more attention on programming.

• Improve our programming skills.

Programming Principles

• Correct（正確性）• Readability （可讀性）• Easy to modify （易於修改）• Reusability （可重用性）• Extendibility （擴充性）• Efficiency （效率）• Memory （所需記憶體的大小）• Code size （執行檔的大小）• Portability （可移植性）

………….

if( ischecked( orientU) ) VOL_ORIENT=OBLIQUE;

if( ischecked( orientS) ) VOL_ORIENT=SAGITTAL;

if( ischecked( orientA) ) {VOL_ORIENT=AXIAL;NEURO_AXIAL=False;}

if( ischecked( orientNA)) {VOL_ORIENT=AXIAL;NEURO_AXIAL=True;}

if( ischecked( orientC) ) {VOL_ORIENT=CORONAL;GE_COR=False;}

if( ischecked( orientG) ) {VOL_ORIENT=CORONAL;GE_COR=True;}

…………...

Bad Style

Code pieces from Mri3dX:

fuzzy logic!

………….

if(ischecked(orientU)) VOL_ORIENT=OBLIQUE;

else if(ischecked(orientS)) VOL_ORIENT=SAGITTAL;

else if(ischecked(orientA)) {VOL_ORIENT=AXIAL;NEURO_AXIAL=False;}

else if(ischecked(orientNA)) {VOL_ORIENT=AXIAL;NEURO_AXIAL=True;}

else if(ischecked(orientC)) {VOL_ORIENT=CORONAL;GE_COR=False;}

else if(ischecked(orientG)) {VOL_ORIENT=CORONAL;GE_COR=True;}

…………...

Good Style

After modification:

Use if-else-if statement to reveal the exclusive property of each situation.

Bad Style

function f(a,b,c)

%set default valueif( nargin ==0) a=1; b=2; c=3;end

if( nargin ==1) b=2; c=3;End

if( nargin ==2) c=3;end

A common matlab function:

1. Redundant test.

2. Have to take lots effort to modify (what if change default value of c to be 4?)

3. Redundant code.

Better Style

function f(a,b,c)

%set default value

if( nargin ==0)

a=1;

b=2;

c=3;

elseif( nargin ==1)

b=2;

c=3;

elseif( nargin ==2)

c=3;

end

A common matlab function:

1. Redundant test.

2. Have to take lots effort to modify (what if change default value of c to be 4?)

3. Redundant code.

Better Style

function f(a,b,c)

Default-a-value=1;

default-b-value=2;

default-c-value=3;

%set default value

if( nargin ==0)

a = default-a-value;

b = default-a-value;

c = default-a-value;

elseif( nargin ==1)

b = default-a-value;

c = default-a-value;

elseif( nargin ==2)

c = default-a-value;

end

1. Redundant test.

2. Have to take lots effort to modify (what if change default value of c to be 4?)

3. Redundant code.

Better Style

function f(a,b,c)

default-a-value=1;

default-b-value=2;

default-c-value=3;

%set default value

if( nargin <1) a=default-a-value; end

if( nargin <2) b=default-a-value; end

if( nargin <3) c=default-a-value; end

1. Redundant test.

2. Have to take lots effort to modify (what if change default value of c to be 4?)

3. Redundant code.

Simple is beautiful!

Use named constant to improve readability and make code easy to modify(avoid magic number)

fid = fopen(‘a.ima’,’rb’);

%skip header

fseek(fid,6144,’bof’);

%start reading data

…

What is 6144?

Magic number

fid = fopen(‘a.ima’,’rb’);

%skip header

%6144: header size

fseek(fid,6144,’bof’);

%start reading data

…

What if header size is changed?

Use named constant to improve readability and make code easy to modify(avoid magic number)

fid = fopen(‘a.ima’,’rb’);

%skip header

header_size=6144;

fseek(fid,header_size,’bof’);

%start reading data

…

What if header size is changed?

named constant

Use named constant to improve readability and make code easy to modify(avoid magic number)

Named constant not only plays the role of comment but also makes future modification easier.

Improve efficiency by simple rearrangement of code

% find how many boys and girls are in the class

for i=1:NumStudent,

if( student(i).sexual==‘Boy’)

NumBoy = NumBoy + 1;

elseif( student(i).sexual==‘Girl’)

NumGirl = NumGirl + 1;

end

end

Boy: 1 test + 1 assignment

Girl: 2 tests + 1 assignment

50 students:

1. 40 boys, 10 girls:

60 tests + 50 assignments

2. 10 boys, 40 girls:

90 tests + 50 assignments

Put more frequency situation in the first if statement.

Programming Principles

• Correct（正確性）• Readability （可讀性）• Easy to modify （易於修改）• Reusability （可重用性）• Extendibility （擴充性）• Efficiency （效率）• Memory （所需記憶體的大小）• Code size （執行檔的大小）• Portability （可移植性）

• Free our mind from heavy labor

• Code for human

our-self

colleague

people around the world

End