Introduction to Control Flow concepts

1
Unit 7

• Introduction to Control Flow concepts
• Sequence
• Repetition
• Conditional
• Introduce values, minimally, at the same time
• See how these can be used to automatically fly
  the rocket

2
Programs must do something

• Concentrated so far on the structure of a model
• the components, the members of each component
• the way in which models evolve
• this is the unique part of the O-O model
• All programs, however, must do something in order
  to be effective
• examine now the different ways of doing
• and how they can be expressed in prog languages

3
We've looked at doing already

• Remember playing with the Fly method from the
  RocketController?
• adjusting or creating a sequence of
  operations/statements
• A statement is a single indivisible unit of doing
• Execution model for a sequence of statements is
  simple
• they are executed, one at a time, from first to
  last
• Matches closely to some typical behaviours we see
  in everyday life
• following a simple step by step cooking recipe,
  or our work plan for the day do this, then
  this, then this

4
Technical/Grammatical details

• A statement is always completed with a semi-colon
• A sequence of statements can be made to look like
  a single statement by
• enclosing it in braces
• this is used all the time, as we'll see later

5
More advanced forms of doing

• Is this sequencing of operations enough?
• No, not typically, because
• It is inflexible
• no opportunity to adjust our behaviour according
  to what we find in our environment
• for example, a simple daily working plan does not
  explicitly tell you what to do if there's a fire
  alarm
• remember the rocket we needed to know the
  environment before setting out
• It is inefficient
• if we want to do something repeatedly, we must
  write the repetitions out in full
• So what do we need?

6
Conditional doing

• Certain doing is performed only if some condition
  is met
• Various ways we express this notion in English
• "I'll take an umbrella if it's raining"
• "If it's raining then I'll take an umbrella"
• "If it's raining or it's cloudy then I'll take an
  umbrella, otherwise I'll take a sunhat"
• "If it's raining then I'll take an umbrella,
  otherwise if it's very sunny then I'll take a
  sunhat
• Notice
• They all express doing, with some condition
  attached
• Sometimes, nothing will be done at all
• Sometimes, we are choosing between alternatives
  something will always be done

7
Stating this in a prog. language

• The statement forms most often found
• "IF a condition holds THEN do something"
• note, if the condition doesn't hold, nothing is
  done
• "IF a condition holds THEN do something ELSE do a
  different thing"
• one or other of the two things must be done
• which one is decided when the condition is
  evaluated see the next slide
• There are others
• we'll leave them for now

8
Notice the condition

• "a condition holds" from previous slide
• used in both forms what does it mean?
• a condition is a factual statement to be verified
  as true or false
• factual statements
• it is raining
• her hair is black
• there is an obstacle ahead
• a condition holds means that the statement was
  verified as being true
• When does the verification take place?
• at the point when the decision must be made
• when the conditional statement is executed
• crucially not until the program is running

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Values, expressions and evaluation

• To avoid confusion between doing statements, and
  factual statements
• the latter are called expressions
• they express an idea or a computation
• which is evaluated to produce a value
• In the case of a conditional expression
• the idea evaluates to 'true' or 'false'
• these are just two values in the great universe
  of values that we'll be exploring

10
Repetitive doing

• A statement of doing is repeated a number of
  times
• sometimes the number is known in advance
• sometimes we only know when to stop at some point
  in the middle of the repetitive activity
• in these cases, again, a condition is used to
  determine when to stop perhaps continue
  repeating the activity as long as the condition
  holds, stopping when it no longer holds

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Consider these statements

• Fill the fridge with beer
• number of repeats known in advance no
• repeated action put another beer in fridge
• condition The fridge is not full
• Change the spark plugs in the car
• number of reps known in advance yes
• repeated action change the next plug
• Do up shirt buttons
• reps known in advance usually not!
• repeated action move hand to next button and do
  it up
• condition Some buttons unfastened
• Add up a series of numbers
• reps known in advance sometimes, sometimes not
• repeated action get next number, add it to
  running total
• condition More numbers available in the series

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How are these expressed?

• We need a formal way of expressing these
• while the fridge is not full, put another beer
  in
• for each cylinder, change spark plug in that
  cylinder for new one
• while there are buttons undone, pick an undone
  one and fasten it
• while there are more numbers in the list, read
  the next one and add it to the total
• Notice the structuring
• while the condition and the repeated activity
• for the number of reps is implicitly specified

13
Step up in understanding

• The flat textual description comes to life only
  when it is executed
• you need to be able to imagine this happening
• the concept seems easy, but the reality is often
  harder to absorb
• For example,
• while the fridge is not full, put another beer
  in
• might result in 100 executions of "put another
  beer in" we don't know from simply looking at
  the text
• The conditionals similarly
• we don't know precisely what will happen until
  they are executed

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Conditional statements in C

• Single option conditional statement
• (do nothing if condition doesn't hold)
• This is a template. The words in italics are
  expanded out in use to be a valid expression and
  statement. Remember that a sequence of
  statements can be viewed as one statement with
  braces
• Conditional with two alternatives
• The condition decides which of two actions to
  perform
• To execute these statements
• condition is evaluated first. The appropriate
  statement is then executed, if any, depending on
  the result

if( Condition ) Statement
if( Condition ) Statement else Statement
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Repetitive Statements in C

• We will look at just one for now
• typically used when we don't know in advance how
  many repetitions required
• To execute this statement
• first the condition is evaluated
• if it is true then
• the contained statement is executed
• we start executing the whole while statement from
  the top again
• else
• the condition is false and we complete execution
  of the whole while statement. The contained
  statement is NOT executed

while( Condition ) Statement
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See it in action - AdvancedRocketry

• Run ExecuteMission again
• this should be using AutoRocketController
• when the landing sequence engaged message comes
  up, the Land method is executing
• Before examining that method closely
• how would you try to land the rocket safely?
• in this rocket version, safely means
• upright, nose pointing upwards
• but could be travelling at any speed down or
  sideways
• (it's a sturdy rocket, this one!)

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Getting to the ground how?

• Requirements
• Must be upright
• Must be dropping downwards
• Must stop once we've reached the ground
• Speed sideways or downwards unimportant
• So?
• To be dropping downwards but upright
• Engines must be turned off, let gravity do the
  work
• As we coast downwards, must check periodically to
  see whether we've reached the ground
• Procedure
• Turn engines off
• Get upright
• Get to ground

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ARocket.EnginesOff() // Get upright
again while( ARocket.IsTilting()
) ARocket.TurnLeft( 1 ) there's more

• Examine the Land method
• Engines off is easy enough simply a method call
• Getting upright is trickier
• Don't know the starting attitude
• Hence continually test to see if the rocket is
  tilting
• if it is, then nudge it round by 1 degree
• eventually, after enough repeats, the rocket will
  be upright
• Note that IsTilting returns a true or false value
• known as a Boolean, or bool for short

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// Get down to the ground// The condition in the
while loop tests if we're above groundwhile(
AArena.NearestBelow( ARocket.GetPosition() ) gt 0
) ARocket.Coast( 1 )

• Finally, to get to the ground
• Must ask the Arena how close we are to the ground
• while we are still above it, we must continue to
  Coast with engines off hence fall downwards
• How do we ask the Arena about positioning
• NearestBelow method returns distance to the
  nearest object below the supplied position. This
  distance is a number
• We supply the position of the rocket, using
  GetPosition
• The distance returned by NearestBelow is then
  tested to see if it is greater than zero (ie
  we're above the ground
• This final test gives us the true/false boolean
  value we need for the while statement

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Try a delicate rocket

• This rocket didn't care about landing speed.
• In ExecuteMission, at the bottom, change
• new AdvancedRocket
• to
• new DelicateAdvRocket
• Rerun it to see what happens
• this rocket crashes if landing speed downwards is
  greater than 20 and sideways motion is greater
  than 10

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See if you can do better

• Go to SmartLander.cs
• a derived class of AutoRocketController
• overriding ONLY the Land method
• initially it is the same as AutoRocketController
• Adjust ExecuteMission, so it uses this controller
  instead of AutoRocketController
• change needed at bottom of file
• See if you can work out how to slow the rocket
  gently so it doesn't crash hard
• Take the following hints one at a time, working
  to get an answer on each before going to the next

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Hints

• You can brake the descent
• by firing the engines for a short time
• Do you always need to brake the descent?
• If not always
• what is the condition for firing the rocket?
• There is a testing method FallingFasterThan
• you pass a number it returns true if the rocket
  is falling faster than this value
• The answer is on the next slide
• only go on if you are totally stuck
• just try something out

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General plan

• Inside the repetitive loop taking the rocket to
  the ground
• you need to check how fast the rocket is falling,
  if it's too fast, fire the rockets
• sounds like a conditional statement
• what is the condition?
• what is the action
• if the condition is true?
• if the condition is false?
• code on next slide

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// Get down to the groundwhile(
AArena.NearestBelow( ARocket.GetPosition() ) gt 0
) if( ARocket.FallingFasterThan( 20 )
) ARocket.IncThrustLevel( 10 ) // Turn
engines on to ARocket.Coast( 1 ) // brake
the fall ARocket.EnginesOff() else
ARocket.Coast( 1 ) // fall with no
brakes

• Try this out, if you didn't get there
• Try also now to slow the sideways motion
• methods available to test speed sideways
• similar technique should be used
• when will you do the sideways braking?
• I suggest before this while statement, and after
  you've got the rocket back upright
• if you need it, there's some useful code in spare
  code.txt

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Summary

• Explored the structure inside methods, the code
  that does something
• the statement
• sequences of statements
• conditional statements - if
• repetitive statements - while
• Necessarily looked at
• expressions, values and evaluation
• We'll explore these more in the next unit