FUNCTIONS in Python

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FUNCTIONS in Python
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What Are Functions?

• Functions are sub-programs which perform tasks
  which may need to be repeated
• Some functions are bundled in standard
  libraries which are part of any languages core
  package
• Functions are similar to methods, but may not be
  connected with objects
• Programmers can write their own functions

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Why Write Functions?

• Reusability
• Fewer errors introduced when code isnt rewritten
• Reduces complexity of code

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Happy Birthday Example Function Flow

• happy.py
• Simple illustration of functions.
• def happy()
• print "Happy Birthday to you!"
• def sing(person)
• happy()
• happy()
• print "Happy birthday, dear", person "."
• happy()
• def main()
• sing("Fred")
• print
• sing("Lucy")
• print
• sing("Elmer")

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Functions Formal vs. Actual Paramaters (and
Arguments)
Formal Parameters

• moveto.py
• from graphics import
• def moveTo(object, point)
• c object.getCenter()
• dx point.getX() - c.getX()
• dy point.getY() - c.getY()
• object.move(dx,dy)
• def main()
• win GraphWin()
• circ Circle(Point(100,100), 20)
• circ.draw(win)
• p win.getMouse()
• moveTo(circ, p)
• win.close()
• center circ.getCenter()
• print center.getX(), center.getY()
• main()

Function definition
Actual parameters or arguments
Call or invocation of function Arguments must be
in correct order according to function definition
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Functions Return values

• return keyword indicates what value(s) will be
  kicked back after a function has been invoked
• def square(x) return x xThe call
  output square(myNum)

Formal parameter
Return value
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Return value used as argument Example of
calculating a hypotenuse

• num1 10
• num2 14
• Hypotenuse math.sqrt(sum_of_squares(num1,
  num2))
• def sum_of_squares(x,y) t (xx) (y y)
  return t

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Triangle2.py example

• Triangle2.py
• Text of triangle2.py

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Returning more than one value

• Functions can return more than one value
• def hi_low(x,y) if x y return x, y
  else return y, x
• The callhiNum, lowNum hi_low(data1, data2)

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Modifying Parameters How could this be?

• Usually, we ask a function to return a value to
  the calling function. This is how the calling
  function gets data processed.
• But we can also have a function modify a
  parameter used in the calling function.
• This seems counterintuitive, since we said
  earlier that the scope of a variable is limited
  to the function in which it is declared.
• It also seems counterintuitive because we know
  that the VALUE of the parameters is passed to
  other functions, not the parameters themselves.

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Function call that fails to get a value back

• def addInterest(balance, rate)
• newBalance balance (1rate)
• balance newBalance
• def test()
• amount 1000 rate 0.05addInterest(amount,
  rate)
• print amount
• test()

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Function call that gets a return value

• def addInterest(balance, rate) newBalance
  balance (1rate) return newBalance def
  test()amount 1000rate 0.05amount
  addInterest(amount, rate)print amounttest()

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Changing parameter values by reference

• Some programming languages allow a calling
  function to pass parameters by reference (the
  memory address of the variable is passed, not the
  actual value, so the value of parameter in the
  calling function is changed. PYTHON DOES NOT
  ALLOW THIS

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Passing parameters by value

• Python (like all languages) permits passing
  parameter VALUES, not actual parameters, as we
  have already seen. This is called PASSING BY VALUE

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Changing passed parameters

• Python does have a mechanism for having a called
  function alter the value of a parameter passed by
  a calling function.
• The trick is that Python allows OBJECTS to be
  passed, and when this happens the values
  associated with that object CAN be changed by a
  called function.

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Changing parameters passed as objects

• def addInterest(balances, rate)for i in
  range(len(balances))balancesi balancesi
  (1rate)def test()amounts 1000, 2200, 800,
  360rate 0.05addInterest(amounts, 0.05)print
  amountstest()

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Summary passing parameters

• Python always passes VARIABLES (given as
  parameters) by VALUE. In this case, the original
  variables in the calling function are NOT
  changed.
• Functions can RETURN a value which alters a
  variable in the calling function.
• Python allows OBJECTS (lists, graphics, etc.) to
  be passed and altered by functions.

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Program Structure

• Functions serve to reduce complexity in the main
  algorithm of a program, making the program easier
  to understand and maintain.
• Examine futval_graph4.py