Repetition Statements.  Often it is necessary to repeat statements many times  Java has two ways of doing this  while statements  for statements.

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Presentation transcript:

Repetition Statements

 Often it is necessary to repeat statements many times  Java has two ways of doing this  while statements  for statements

While statements  while statements have the following syntax:  while ( )  The statements execute 0 or more times based on the boolean expression  If the expression is true, the statements will be executed and then the expression will be tested again.  When the expression evaluates to false, the loop is exited

While statements  int sum = 0, number = 1;  while (number <= 100){ sum += number; number++; } int sum = 0, number = 1;

While statements  int sum = 0, number = 1;  while (number <= 100){ sum += number; number++; } int sum = 0, number = 1; number <= 100 True False

While statements  int sum = 0, number = 1;  while (number <= 100){ sum += number; number++; } int sum = 0, number = 1; number <= 100 True False

While statements  int sum = 0, number = 1;  while (number <= 100){ sum += number; number++; } int sum = 0, number = 1; number <= 100 True False sum += number; number++;

While statements  int sum = 0, number = 1;  while (number <= 100){ sum += number; number++; } int sum = 0, number = 1; number <= 100 True False sum += number; number++;

While statements  int sum = 0, number = 1;  while (number <= 100){ sum += number; number++; } int sum = 0, number = 1; number <= 100 True False sum += number; number++;

While statements  int sum = 0, number = 1;  while (number <= 100){ sum += number; number++; } condition True False Statements

While statements  Questions??

While statements  InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”); while (x < 0) x = in.getInteger(“Enter a positive number”); InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”);

While statements  InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”); while (x < 0) x = in.getInteger(“Enter a positive number”); x < 0 InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”);

While statements  InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”); while (x < 0) x = in.getInteger(“Enter a positive number”); x < 0 InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”); False

While statements  InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”); while (x < 0) x = in.getInteger(“Enter a positive number”); x < 0 InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”); False x = in.getInteger(“Enter a positive number”); True

While statements  InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”); while (x < 0) x = in.getInteger(“Enter a positive number”); x < 0 InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”); False x = in.getInteger(“Enter a positive number”); True

while statements Write a method that adds the numbers x through y

do while statements  do while loops are a variation of the while loop  Syntax: do { } while ( );  This is the same as a while loop, except the statements are always executed at least once.

do while statements while (<boolean expression) do{ } while ( ); condition True False Statements condition False Statements True

While statements  InputBox in = new InputBox(mw); int x = in.getInteger(“Enter a positive number”); while (x < 0) x = in.getInteger(“Enter a positive number”);  int x; InputBox in = new InputBox(mw); do{ x = in.getInteger(“Enter a positive number”); } while (x < 0); while loop do while loop

While statements  Questions?

for statements  for statements have four parts  Initalization – executed once (initially)  Condition – boolean expression  Statements – executed based on condition  Increment – executed after statements and before condition is reevaluated  They have the following syntax:  for( [ initialization ]; [ condition ]; [increment] )  Example:  sum = 0;  for( int i = 0; i < 100; i++ ) sum = sum + i;

for statements for( [ initialization ]; [ condition ]; [increment] ) initalization condition false statements true increment

for statements  sum = 0;  for( int i = 0; i < 100; i++ ) sum = sum + i;

for statements  sum = 0;  for( int i = 0; i < 100; i++ ) sum = sum + i; sum = 0;

for statements  sum = 0;  for( int i = 0; i < 100; i++ ) sum = sum + i; sum = 0; int i = 0;

for statements  sum = 0;  for( int i = 0; i < 100; i++ ) sum = sum + i; sum = 0; int i = 0; i < 100 true false

for statements  sum = 0;  for( int i = 0; i < 100; i++ ) sum = sum + i; sum = 0; int i = 0; i < 100 true false sum = sum + i;

for statements  sum = 0;  for( int i = 0; i < 100; i++ ) sum = sum + i; sum = 0; int i = 0; i < 100 true false sum = sum + i; i++;

for statements Questions????

Nested for statements  Nested for statements are just like regular for statements  They are very useful for many processing tasks  Multiplication Table Example:  OutputBox out = new OutputBox(mw);  out.show();  for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); }

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); }

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j 1

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j 1

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j 1 1 _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j 2 1 _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j 2 1 _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 1 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 2 j _

Nested for statements OutputBox out = new OutputBox(mw); out.show(); for(int i = 1; i < 10; i++){ for(int j = 1; j < 10; j++){ out.print( (i * j) + “ “); } out.skipLine(1); } i 10 j At the end, you get this >

Nested for statements Questions???

Loops  There are two ways loops can be used  Sentinel controlled loops  Loop is exited based on a sentinel value  while (x != 0) // do something  Count controlled  Loop is exited when a certain count is reached  for (x = 0; x < 10; x++) // do something

Pitfalls  Infinite loops  Make sure loops can exit!  Real number addition 1.0/ / /3.0 =? 1.0  for( x = 0; x != 1.0; x+= )  In general avoid using real numbers as counter variables  Use, >= and not !=  for(x = 0; x < 3; x++);

Pitfalls  Off by one errors  for(int i = 0; i <= 10; i++)  // this will execute 11 times  for(int i=1; i < 10; i++)  // this will execute 9 times  for(int i = 1; i <= 10; i++)  // this will execute 10 times  for(int i =0; I < 10; i++)  // this will execute 10 times

Pitfalls  Off by one errors  for(int i = 0; i <= 10; i++)  // this will execute 11 times  for(int i=1; i < 10; i++)  // this will execute 9 times  for(int i = 1; i <= 10; i++)  // this will execute 10 times  for(int i =0; I < 10; i++)  // this will execute 10 times Use zero based indexing

Pitfalls  Make sure the loop body contains a statement that will eventually cause the loop to terminate  Make sure the loop repeats exactly the correct number of times  If you want to execute the loop body N times, then go 0 to < n or 1 to <= n