An Introduction to Control Structures

1
An Introduction to Control Structures
2
Objectives

• After completing this overview, you will be able
  to
• Explain the sequence, selection, and repetition
  structures
• Write simple algorithms using the sequence,
  selection, and repetition structures

3
Defining Control Structures

• All computer programs, no matter how simple or
  how complex, are written using one or more of
  three basic structures
• Sequence Selection
• Repetition
• These structures are called control structures or
  logic structures, because they control the flow
  of a programs logic
• In most programs, you will use both the selection
  and repetition structures

4
The Sequence Structure

• The sequence structure in a computer program
  directs the computer to process the program
  instructions, one after another, in the order
  listed in the program
• You can observe how the sequence structure works
  by programming a mechanical man named Rob
• Like a computer, Rob has a limited instruction
  setin other words, Rob can understand only a
  specific number of instructions, also called
  commands

5
The Sequence Structure

• The four instructions shown in Figure 1-1 are
  called an algorithm
• An algorithm is simply the set of step-by-step
  instructions that accomplish a task

6
The Repetition Structure

• When used in a program, the repetition structure,
  also referred to as a loop, directs the computer
  to repeat one or more instructions until some
  condition is met, at which time the computer
  should stop repeating the instructions
• Notice that the algorithm contains both the
  sequence and repetition structures
• Rather than writing the walk instruction 50
  times, this algorithm uses the repeat 50 times
  instruction to direct Rob to walk 50 times before
  he turns and then sits

7
The Repetition Structure

• Notice that the instruction to be repeatedin
  this case, walkis indented below the repeat50
  timesinstruction
• Indenting in this manner indicates the
  instructions that are part of the repetition
  structure, and therefore, are to be repeated

8
The Repetition Structure

• But what if you do not know precisely how many
  steps there are between Rob and the chair?
• You simply must change the repetition structures
  condition
• The repetition structure shown in the algorithm
  in Figure 1-3 ends when Rob is standing
  directly in front of the chair

9
The Selection Structure

• Like the sequence and repetition structures, you
  already are familiar with the selection
  structure, also called the decision structure
• The selection structure makes a decision, and
  then takes an appropriate action based on that
  decision
• The selection structure also provides the
  appropriate action to take based on the result of
  that decision

10
Robs New Instruction Set

• Notice that the instructions to be followed when
  the balloon is redin this case, drop the balloon
  in the red boxand the instruction to be followed
  when the balloon is not redin this case, drop
  the balloon in the yellow boxare indented

11
An Example of the Selection Structure

• Notice that the algorithm shown in Figure 1-5
  contains all three control structures sequence,
  selection, and repetition

12
Summary Concept Lesson

• An algorithm is the set of step-by-step
  instructions that solve a problem
• The algorithms for all computer programs contain
  one or more of the following three control
  structures sequence, selection, and repetition
• The sequence structure directs the computer to
  process the instructions, one after another, in
  the order listed in the program

13
Summary Concept Lesson

• The repetition structure directs the computer to
  repeat one or more instructions until some
  condition is met
• The selection structure, also called the decision
  structure, directs the computer to make a
  decision, and then selects an appropriate action
  to take based on that decision
• The sequence structure is used in all programs
• Most programs also contain both the selection and
  repetition structures