Structured Problem Solving 2009-2010

www.hope.ac.uk Faculty of Sciences and Social Sciences

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Structured Problem Solving2009-2010

Week 10: Java: Strings

Stewart Blakeway

FML 213

[email protected]


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What we have done already• Seen what an algorithm is– a set of instructions that, if carried out, will lead

to a successful conclusion

• Learned how to represent algorithms in– Structured English– Flow charts

• Used variables to remember

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What we have done already• Applied the top down, stepwise refinement

approach to creating algorithms• Looked at problems more oriented towards

being solved on a computer – stacks, queues, functions, procedures

• Seen how high level languages like Java are used to create programs with the aide of compilers

• The problem solving constructs in Java• Nested for loops

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What we shall do today

• Strings• 3 examples– Reversing the order of characters in a word– Counting the number of words in a sentence– Analysing a URL (e.g. www.hope.ac.uk)

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Strings in computing

• Let Σ be an alphabet, a non-empty finite set.

• Elements of Σ are called symbols or characters.

• A string (or word) over Σ is any finite

sequence of characters from Σ.– Reference: “http://en.wikipedia.org/wiki/String_(computer_science)”

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http://en.wikipedia.org/wiki/String_(computer_science)


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Example

• Suppose Σ = { W, $, Þ,Ŷ, Ǿ}.

• Which of the following are valid strings over this alphabet: – ǾW$$– WORD– Þ– $Ŷw

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Yes

No – ‘O’, ‘R’, ‘D’ not in given alphabet

Yes

No – ‘w’ not in given alphabet


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Strings in Java

• Alphabet includes all ASCII characters– ‘A’..’Z’– ‘a’..’z’– ‘0’..’9’– Punctuation marks– Certain “unprintable” characters like TAB,

NEWLINE are represented by ‘\t’, ‘\n’• Works next term in BlueJ – not here in Java Trainer

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Strings in Java

• Alphabet Σ = {‘A’..’Z’, ‘a’..’z’, ‘0’..’9’, …..}

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Java Strings

String s;

s = "Fred";

System.out.println(s);

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Concatenating strings

Concatenate:Etymology: Middle English, from Late

Latin concatenatus, past participle of concatenare to link together, from Latin com- + catena chain

Date: 15th century

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Concatenating strings with +

String a;String b; String c;

b = "Fred";c = "Bloggs";a = b + c;System.out.println(a);

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Accessing characters in Strings

String s;

s = "Fred";

s[0] is 'F's[1] is 'r's[2] is 'e's[3] is 'd'

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String s;

s = "Fred";

System.out.println(s[0]);

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String s;int i;s = "Fred";for (i=0; i<4; i++) { System.out.println(s[i]); }

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String s;int i;s = "Fred";for (i=0; i<4; i++) { System.out.println(s[i]); }

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String s;int i;s = "Fred";for (i=0; i<4; i++) { System.out.print(s[i]); }

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String s;int i;s = "Fred";for (i=0; i<4; i++) { System.out.print(s[i]); }

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Example 1

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Write out a word in reverse: e.g. “Fred” -> “derF”

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Write out a word in reverse

String s;

int i;

s = "Fred";

for (i=3; i>=0; i--)

{

System.out.print(s[i]);

}

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Reversing a name of any length

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Reversing a word of any length

String s;

int i;

System.in.read(s);

for (i=3; i>=0; i--)

{


}

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We need to be able to find the length of the string

• Java Trainer has a FUNCTION that returns the length of a String

• It is called length

String s;System.in.read(s);int n;n = length(s);

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Reversing a name of any length

String s;

int i;

int n;

System.in.read(s);

n = length(s);

for (i=n-1; i>=0; i--)

{


}

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Initial statement

• Count the number of words in a sentence

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Initial statement

• Count the number of words in a sentence by counting spaces then adding 1

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Initial statement

• Count the number of words in a sentence by counting spaces then adding 1– Assume single space between words– Assume no spaces at beginning or end of

sentence

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First refinement

Get sentence

Initialise count

Count the spaces

Report the final count + 1

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Second refinement

Read Sentence

Count := 0

While (not reached end of sentence)

begin

deal with next character

end

Display Count + 1

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Second refinement

• Do we know before we start how many times we shall go round the loop?

• Yes – since we can use the length function to get the number of characters.

• So, we can use a for loop

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Second refinement

Read Sentence

Count := 0

For each character in the sentence

begin

deal with the character

end

Display Count + 1

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Third refinement

Read Sentence Count := 0For each character in the sentence begin if (character = ' ') begin Count = Count + 1 end endDisplay Count + 1

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Counting the number of words in a sentence: in Java

String sentence;

int count;

int i;

count = 0;

System.in.read(sentence);

for (i=0; i<length(sentence); i++)

{

if (sentence[i] == ' ')

{

count = count + 1;

}

}

System.out.println("There are " + (count+1) + " words.");

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Example 3

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Analysing a URL• URL -> Universal Resource Locator• URL is the basis for communicating locations

of resources (data) on the web. – E.g. http://www.hope.ac.uk

• A URL consists of:– a protocol identifier (e.g. HTTP (hypertext transfer

protocol), FTP (file transfer protocol)– and a protocol-specific syntax further defining the

location.

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http://www.hope.ac.uk/


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Analysing a URL

• In this example we shall analyse a HTTP protocol URL to extract information about the location of the host. – E.g. http://www.hope.ac.uk

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Analysing a URL

• In this example we shall analyse a HTTP protocol URL to extract information about the location of the host. – E.g. http://www.hope.ac.uk

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Host name



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Analysing a URL

• http://www.hope.ac.uk

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Country



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Analysing a URL


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Classification



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Analysing a URL


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Organisation



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Analysing a URL


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Service



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Analysing a URL


• Country = uk (United Kingdom)

• Classification = ac (Academic)• Organisation = hope (Liverpool

Hope)• Service = World Wide Web (Hope’s web

server)

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Analysing a URL

• http://moodle.hope.ac.uk

• Country = uk (United Kingdom)

• Classification = ac (Academic)• Organisation = hope (Liverpool Hope)• Service = moodle (Hope’s

VLE)

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http://moodle.hope.ac.uk/


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Analysing a URL

• Country– uk (United Kingdom)– de (Germany)– fr (France)

• Classification– ac (academic)– gov (government)– org (organisation)

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Analysing a URL – .COM is different

• http://www.microsoft.com

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USA (or an organisation based

anywhere)



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Organisation



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Service



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Analysing a URL


• Organisation based anywhere• Organisation = microsoft• Service = world wide web

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http://www.microsoft.com/


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First we design a solution

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Initial statement

Analyse a URL

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First refinement

Get URL

Analyse the URL

Report the results

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Getting the URL

Get URL

Analyse the URL

Report the results

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We can store the URL in a variable called URL.

No further refinement necessary


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Second refinement – what output do we expect

Get URLAnalyse the URLif (URL has .com) then begin display .com report endElse begin display country report end

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Knowing what the final output should be is a key guide to determining what the analysis should be finding out


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Third refinement – what output do we expect

display .com report

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display “Organisation based anywhere”

display “Organisation: “, Organisation

display “Service: “, Service

We need the analysis to record Organisation and Service


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Third refinement – what output do we expect

display country report

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display “Country: ”, Country

display “Organisation: “, Organisation

display “Service: “, Service

We need the analysis to record Country, Organisation and Service


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Second refinement – analyse the URL

• We keep two examples in front of us:– http://moodle.hope.ac.uk case 1– http://www.microsoft.com case 2

• We need to extract the separate parts of the URLs

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Second refinement – analyse the URL

• We keep two examples in front of us:– http://moodle.hope.ac.uk case 1– http://www.microsoft.com case 2

• We identify the words separated by full stops: ‘.’• Start from the back and stop at the first ‘/’– In case 1: uk, ac, hope, moodle (4 words)– In case 2: com, microsoft, www (3 words)

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We now know what the analysis needs to do

Identify the parts of the URL as separate wordsif (last word is “com”) then begin copy the other words into the variables Organisation and

Service endelse begin Copy the words into the variables Country, Classification, Organisation and Service end

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Nearly there

• We have got most of the algorithm into a level of detail that we can translate directly into Java code

• The only part remaining is how to split the URL into separate words.

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Identify the parts of the URL as separate words – in Java

i = length(url) - 1;

word1 = "";

while (url[i] != '.')

{

word1 = url[i] + word1;

i--;

}

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First word


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Identify the parts of the URL as separate words – in Java

i--;

word2 = "";


{


i--;

}

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Second word


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The whole program in 6pt. Copy and paste the two parts into Java Trainer

String word1;

String word2;

String word3;

String word4;

String url;

String country;

String classification;

String organisation;

String service;

int i;

url = "http://www.hope.ac.uk";

i = length(url) - 1;

word1 = "";


{


i--;

}

i--;

word2 = "";


{


i--;

}

i--;

word3 = "";

while ((url[i] != '.') && (url[i] != '/'))

{


i--;

}

i--;

word4 = "";

while ((url[i] != '.') && (url[i] != '/'))

{


i--;

}

if (word1 == "com")

{

organisation = word2;

service = word3;

}

else

{

country = word1;

classification = word2;

organisation = word3;

service = word4;

}

if (word1 == "com")

{

System.out.println("Organisation based anywhere");

System.out.println("Organisation: " + organisation);

System.out.println("Service: " + service);

}

else

{

System.out.println("Country: " + country);

System.out.println("Classification: " + classification);

System.out.println("Organisation: " + organisation);

System.out.println("Service: " + service);

}

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Structured Problem Solving 2009-2010