Fundamentals of Software Development IProgramming patterns involving iteration1 Overview LoopsOverview Loops We’ll also see how loops are often combined

Fundamentals of Software Development I

Programming patterns involving iteration

1

Programming patterns Programming patterns involving iterationinvolving iteration

• Overview LoopsOverview Loops• We’ll also see how loops are often We’ll also see how loops are often

combined with arrays (officially your combined with arrays (officially your next topic), e.g., --next topic), e.g., --



2

Programming patterns Programming patterns involving iterationinvolving iteration

• We’ll review loops – We’ll review loops – – some of the important variations, and some of the important variations, and – ways you can use them toward certain ends.ways you can use them toward certain ends.

• We’ll also see how loops are often We’ll also see how loops are often combined with arrays (officially your next combined with arrays (officially your next topic), e.g., --topic), e.g., --– Processing every member of an arrayProcessing every member of an array– Searching for things in an arraySearching for things in an array– Loops within loops for multi-dimensional Loops within loops for multi-dimensional

tablestables



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LoopsLoops• A A looploop is: is:

– a block of code that executes a block of code that executes repeatedly while some condition repeatedly while some condition holds true.holds true.

• Java provides three forms for Java provides three forms for explicit loops:explicit loops:– whilewhile– forfor– do..whiledo..while

• The The conditionsconditions in loops are in loops are written as in written as in ifif statements statements

• The The breakbreak statement breaks statement breaks out of the loop that it is withinout of the loop that it is within

• Some loop examples followSome loop examples follow

while (while (conditioncondition) {) { statement; statement; … … statement;statement;}}

for (start; for (start; condition; stepcondition; step) {) { statement; statement; … … statement;statement;}}

do {do { statement;statement; … … statement;statement;} while (} while (conditioncondition););



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forfor loops versus loops versus whilewhile loops loops• for (for (int i = 0int i = 0; ; i < 7i < 7; ; i++i++) {) { System.out.println (i + " " + i*i); System.out.println (i + " " + i*i);}} is equivalent tois equivalent to

• int i = 0int i = 0;;while (while (i < 7i < 7) {) { System.out.println (i + " " + i*i); System.out.println (i + " " + i*i); i++i++;;}}

• Typically we use:Typically we use:– forfor when we know in advance how many times the loop will when we know in advance how many times the loop will

executeexecute– whilewhile when something that happens in the loop determines when when something that happens in the loop determines when

the loop exitsthe loop exits– do..whiledo..while when we want a when we want a whilewhile loop that always does at least loop that always does at least

one iterationone iteration



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Loop patternsLoop patterns

• The next few slides present loop The next few slides present loop patternspatterns that are often useful, that are often useful, including:including:– Do N timesDo N times

• Do N times, changing per loop variableDo N times, changing per loop variable

– CountCount– Sum, minimum, maximumSum, minimum, maximum– Find-firstFind-first– Do-foreverDo-forever– Wait-untilWait-until



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Loop pattern: “do N Loop pattern: “do N times”times”

Example: Example: print print kk and and kk55 to the to the consolconsole e windowindow a w a given given numbenumber of r of times:times:

for (int k = 0; k < 5; k++) {for (int k = 0; k < 5; k++) { System.out.print(k + ″ System.out.print(k + ″ ″″);); System.out.println( Math.pow((double) k, 5.0) );System.out.println( Math.pow((double) k, 5.0) );

String s; int start, stop;String s; int start, stop;

s = JOptionPane.showInputDialog(″Start at?”)s = JOptionPane.showInputDialog(″Start at?”)start = Integer.parseInt(s);start = Integer.parseInt(s);

s = JOptionPane.showInputDialog(″Stop after?”)s = JOptionPane.showInputDialog(″Stop after?”)stop = Integer.parseInt(s);stop = Integer.parseInt(s);

for (int k = start; k <= stop; k++) {for (int k = start; k <= stop; k++) { System.out.print(k + ″ ″);System.out.print(k + ″ ″); System.out.println(System.out.println( Math.pow((double) k, 5.0)) Math.pow((double) k, 5.0))};};



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Loop pattern: “count”Loop pattern: “count”• Counts how many times a given character Counts how many times a given character

appears in a string:appears in a string:

public static int charCount(String s, char c) {public static int charCount(String s, char c) {int count = 0;int count = 0;

for (k = 0; k < s.length(); k++) {for (k = 0; k < s.length(); k++) {if (s.charAt(k) == c) {if (s.charAt(k) == c) {

++ count;++ count;}}

}}

return count;return count;}}



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Loop pattern: “sum”Loop pattern: “sum”• Sums the digits in a string of digits:Sums the digits in a string of digits:

public static int digitSum(String s) {public static int digitSum(String s) { int digit;int digit;

int sum = 0;int sum = 0;

for (k = 0; k < s.length(); k++) {for (k = 0; k < s.length(); k++) { digit = Integer.parseInt(s.substring(k, k+1));digit = Integer.parseInt(s.substring(k, k+1)); sum = sum + digit;sum = sum + digit; }}

return sum;return sum;}}



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Loop pattern: Loop pattern: “maximum”“maximum”• Finds the largest digit in a string of digits:Finds the largest digit in a string of digits:

public static int digitMax(String s) {public static int digitMax(String s) { int digit;int digit;

int max = Integer.parseInt(s.substring(0, 1));int max = Integer.parseInt(s.substring(0, 1));

for (k = 1; k < s.length(); k++) {for (k = 1; k < s.length(); k++) { digit = Integer.parseInt(s.substring(k, k+1));digit = Integer.parseInt(s.substring(k, k+1)); if (digit > max) {if (digit > max) { max = digit;max = digit; }} }}

return max;return max;}}



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Loop Pattern: “find-first”Loop Pattern: “find-first”

• Find the first place where a given character appears in Find the first place where a given character appears in a string.a string.Return -1 if the character does not appear.Return -1 if the character does not appear.

• public int findChar(String s, char c) {public int findChar(String s, char c) { int i = 0;int i = 0;

while (i < s.length()) {while (i < s.length()) { if (s.charAt(i) == c) {if (s.charAt(i) == c) { return i;return i; }} i++;i++; }} return -1;return -1; // Not found// Not found

}}

for (i=0; i < s.length(); i++) {for (i=0; i < s.length(); i++) {if (s.charAt(i) == c) {if (s.charAt(i) == c) {

return i;return i;}}

}}



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Loop Pattern: “do-Loop Pattern: “do-forever”forever”

• Our instruction-followers often Our instruction-followers often go “forever”:go “forever”:

while (true) {while (true) { System.out.println( System.out.println(""hihi"");); ... ... } }

for (; true;) {for (; true;) {System.out.println(System.out.println(""hihi");");

}}



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Loop pattern: “break in Loop pattern: “break in middle”middle”

while (true) {while (true) {

......

if (...) {if (...) {

break;break;

}}

......

}}



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How loops are combined How loops are combined with arrayswith arrays

• This is a preview of what you’ll really This is a preview of what you’ll really be studying next!be studying next!

• Example: Processing every member Example: Processing every member of an array:of an array:

for (int k = 0; k < maxDepth; k++) {for (int k = 0; k < maxDepth; k++) {

myArray[k] = 0; // Clear out whole myArray[k] = 0; // Clear out whole array!array!

}}



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• Preview, cntd:Preview, cntd:• Searching for things in an arraySearching for things in an arrayint k;int k;for (k = 0; k < maxDepth; k++) {for (k = 0; k < maxDepth; k++) {

if (myArray[k] = 0) {if (myArray[k] = 0) { break; // Find an empty slot in break; // Find an empty slot in array!array! }} }}



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• Preview, cntd:Preview, cntd:• Loops within loops for multi-dimensional Loops within loops for multi-dimensional

tablestables

for (int k = 0; k < maxDepth; k++) {for (int k = 0; k < maxDepth; k++) {

for (int m=0; m<maxWidth; m++) {for (int m=0; m<maxWidth; m++) {

myTable[k][m] = 0; // Clear out 2-dim myTable[k][m] = 0; // Clear out 2-dim table!table!

}}

}}

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Fundamentals of Software Development IProgramming patterns involving iteration1 Overview LoopsOverview Loops We’ll also see how loops are often combined