COMP 116: Introduction to Scientific Programming

Lecture 28: Midterm #2 Review

AnnouncementsMidterm in SN014

Topics

Conditional LogicWriting Simple FunctionsVariables, workspaces, scopeLoopsStrings

Conditional Logic

Conditional Programming

◦When you want to run some code only if some test (“condition”) is satisfied

◦Making choices between 2 or more options

◦if-else-end

Relational OperatorsTests relationship between two objects or arrays

What is the data type of the result of these operators?

Name Operators Examples

Equivalence

Equality

Inequality

Binary Operators

Less Than

Less Than or Equal

Greater Than or Equal

Greater Than

Relational OperatorsTests relationship between two objects or arrays

Result is always a logical data type (or an array of logical data types)

Name Operators Examples

Equivalence

Equality == 5 == 5, x == y

Inequality ~= 5 ~= 5, z == (x^2 + y^2)

Binary Operators

Less Than < 5 < 3, sin(2) < cos(y)

Less Than or Equal

<= 4 <= 4,

Greater Than or Equal

>= 7 >= 10

Greater Than > 10 > 7

Logical OperatorsBoolean operatorsName Operators Examples

Unary Operators

Logical Negation (NOT)

Binary Operators

Logical And (AND)Short circuit (AND)

Logical Or (OR)Short circuit (OR)

Exclusive Or (XOR)

Logical OperatorsBoolean operatorsName Operators Examples

Unary Operators

Logical Negation (NOT)

~ (3 == 5) = 1 (true)

Binary Operators

Logical And (AND)Short circuit (AND)

& or &&

5 & 7 = 1 (true)

Logical Or (OR)Short circuit (OR)

| or ||

0 | 1 = 1 (true)

Exclusive Or (XOR) XOR xor(8, 5) = 0 (false)

Operates on logical data types, also returns a logical result.

Other Logical ObjectsLogical Constants: true, falseLogical Functions: and(), or(), xor(), not()

Predicate Logic: any(), all()Conversion Function: logical()Test functions (is* functions)

◦Examples: isvarname(), iskeyword()String Comparison functions:

◦strcmp(), strcmpi(), strncmp(), strncmpi()

Two options for logical indexing

Use a logical vector ◦x = rand(1, 100);◦x(x > 0.5); % returns only those elements in x which satisfy test

Use the find() function◦y = find( x > 0. 5 ); ◦x( y ) % returns elements in x whose indices are in y

Conditional ExecutionMultiple chained tests

if <Test1> commands1; % T1 trueelseif <Test2> commands2; % T1 false T2 true

elseif <Test3> commands3; % T1,T2 false T3 true

else commands4; % all falseend

Text Output

disp(msg)- displays an array or text string◦disp(‘Derp');

error(msg) - displays an error message and aborts current function or script◦error('Value Out of Range');

Writing simple functionsfunction [o1, o2] = funcName( i1, i2 )

% function comments… % body (implementation)end• Can have multiple inputs and multiple outputs

function [] = funcName()function o1 = funcName()function o1 = funcName( i1 )function o1 = funcName( i1, i2 )function [o1, o2] = funcName( i1, i2,

i3)

ScopeFunctions run in their own

‘workspaces’

MATLAB

sq.m x =4 x2 =16

foo =4 x2 =5 bar =16

Scripts vs. Functions

Loops: for loop statementthe counted loop solution

for <varindex> = <start>:<stop><Body: do some work>

end

for <idx> = <start>:<step>:<stop><Body: do some work>

end

Loops: while loop statementthe conditional loop solution

while <test> <Body: do some work> <Update: make progress towards exiting loop>end• While loops are great if we don’t know how many times

we need to loop, but if we can write a test for when we’re done

• For this to work properly, the test needs to evaluate to a logical value

• The while loop will run as long as test evaluates to true

• The while loop does not have a built-in counter like the for-loop (if you want to count something, you need to implement the counter yourself)

Loops: Common pitfalls

While-loops:◦Counters not initialized

◦While-loop never terminates or gets never executed

◦Counter does not count all the way to the desired value: e.g., x<5 instead of x<=5

Common Idioms: Minimum value within a vector

minVal.m Remarks

function ret = minVal(v)ret = v(1);

for i=2:length(v) if (v(i)<ret) ret = v(i); endend

Initialize return value

Now loop through all the remaining onesCheck if current value is smaller It’s smaller, so this is the new minimum value

Common Idioms: Check if value is contained within vector

isInVector.m Remarks

function ret = InVector(v, val)ret = false;for i=1:length(v) if (v(i)==val) ret = true; endendend

Initialize return value to default: ‘false’Now loop over all candidate valuesCheck if current value is the desired oneYes, so change the return value

Practice: Rewrite this using a while loop

isInVector.m Remarks

function ret = InVector(v, val)ret = false;c = 1;while (~ret & i<=length(v)) if ( v(i)==val ) ret = true; end i = i+1;endend

Initialize return value to default: ‘false’Initialize counter for while loopStop looping when value was foundCheck if current value is the desired oneYes, so change the return valueIncrement the counter, otherwise may loop forever!

Common Idioms: Check if value is contained within vector

Note the two conditions, understand why this works

Common Idioms: Looping over a matrix• Use a nested for loop:

function ret = findMaxElement( A )sz = size(A);ret = A(1,1);

for i=1:sz(1) for j=1:sz(2) if ( A(i,j)>ret ) ret = A(i,j); end endend

Strings as a vector of charsCan be manipulated like any other

vector

s1 = 'The quick brown fox 's2 = 'jumped over the lazy dog'

s = [s1 s2] % concatenate stringss(5) % ans = qs(17:19) % ans = foxjIdx = find( s == 'j' )jStr = s(jIdx:jIdx+3) % ans = jump

String Comparison

Avoid normal comparison operators!◦s1 == s2, s1 < s3, s1 >= s3◦Operators work element by element (on

characters)◦Thus, strings (i.e., the vector of chars) must

be same length

Use string comparison functions instead◦strcmp(), string comparison◦strcmpi, string comparison while ignoring

case◦strncmp, strncmpi:

Similar, but compares first n characters only

String Searching

strfind◦Search for a string inside another

string◦returns indices to start of each

instancestrVal = [‘with great power comes great responsibility.’];

strfind( strVal, ‘great’)

% ans = [6 24]

String Replacement

strrep

strVal = [‘with great power comes great responsibility.’];

strrep( strVal, ‘great’, ‘no’)

Summary

Practice, practice, practice◦Work through the examples done in

class