Programming – Pascal: Essential skills 4

 

Learning Outcomes:

• Apply control structures in a solution.
• Sequence, selection and iteration have been introduced in the Compulsory Part.
  
  

Iteration

 

Methods of iteration:

• for … to … do … / for … downto … do …
• repeat … until …
• while … do …

Advanced technique:

• Nested loop

 

for … to … do … / for … downto … do …

Important point: use for loop when the number of loops is known.

for i := 1 to n do                 write (i);

Output: 1234….n              

 

repeat … until …

Important points:

• use repeat loop or while loop when the number of loops cannot be determined before running the program.
• Location of the counter is important.
• The program will run through the content first before checking the condition. i.e. even condition is not met, the content is run at least once.

i := 0; Repeat                 i := i + 3;                 write (i) until i > 10;

i := 0; Repeat                 write (i);                 i := i + 3 until i > 10;

Output: 36912                                                         Output: 0369

 

while … do …

Important points:

• use repeat loop or while loop when the number of loops cannot be determined before running the program.
• Location of the counter is important.
• The program will check the condition first before running through the content i.e. When condition is not met, the content is skipped.

i := 0; while i <10 do begin i := i + 3; write (i) end;

i := 0; while i <10 do begin write (i); i := i + 3 end;

Output: 36912                                                                   Output: 0369

 

Nested loop

Suppose you have an array A[x,y], you want to sum up the elements.

var                 i, j : integer; begin                 for i := 1 to 3 do                 (* Outer loop *)                                 for j := 1 to 2 do                 (* Inner loop *)                                                 sum := sum + A[i,j] end;

Use a table to trace the program.

Outer loop	i	j	Inner loop
	1	1
	1	2
	2	1
	2	2
	3	1
	3	2

The same applies to nested while loop and nested repeat loop.

 

Relevant past paper:

DSE ICT Elect B(SP-2017):  2014 4c*.

CE CIT Elect A(2005-2011): 2005 3a-d. 2006 3cd. 2007 2abc. 2009 1bc, 2bii, 4a. 2010 2a.