Computer Science 1620

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Computer Science 1620

• Default Parameter Values

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• Default Parameters
• If a projectile is launched vertically with
  velocity v0, the maximum height it will reach is
  given by
• g is the downward gravitational acceleration
• on Earth, g is roughly 9.81 m/s2 at sea level
• write a function that takes an initial velocity,
  and returns the height of the projectile

g
v0
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• include ltiostreamgt
• using namespace std
• double height(double v0)
• const double G 9.81
• return v0 v0 / (2 G)
• int main()
• cout ltlt "v0 5 m/s, height " ltlt height(5.0)
  ltlt "m" ltlt endl
• cout ltlt "v0 10 m/s, height " ltlt
  height(10.0) ltlt "m" ltlt endl
• cout ltlt "v0 15 m/s, height " ltlt
  height(15.0) ltlt "m" ltlt endl
• cout ltlt "v0 20 m/s, height " ltlt
  height(20.0) ltlt "m" ltlt endl
• cout ltlt "v0 25 m/s, height " ltlt
  height(25.0) ltlt "m" ltlt endl
• return 0

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• Suppose your boss wants this function to work on
  the moon as well
• gravitational acceleration on moon 1.62 m/s

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• include ltiostreamgt
• using namespace std
• double height(double v0)
• const double G 1.62
• return v0 v0 / (2 G)
• int main()
• cout ltlt "v0 5 m/s, height " ltlt height(5.0)
  ltlt "m" ltlt endl
• cout ltlt "v0 10 m/s, height " ltlt
  height(10.0) ltlt "m" ltlt endl
• cout ltlt "v0 15 m/s, height " ltlt
  height(15.0) ltlt "m" ltlt endl
• cout ltlt "v0 20 m/s, height " ltlt
  height(20.0) ltlt "m" ltlt endl
• cout ltlt "v0 25 m/s, height " ltlt
  height(25.0) ltlt "m" ltlt endl
• return 0

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• Suppose your boss wants to be able to calculate
  for both moon and Earth in same program
• Solution 1 write two separate functions

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• include ltiostreamgt
• using namespace std
• double earth_height(double v0)
• const double G 9.81
• return v0 v0 / (2 G)
• double moon_height(double v0)
• const double G 1.62
• return v0 v0 / (2 G)
• int main()
• cout ltlt "v0 5 m/s, earth " ltlt
  earth_height(5.0) ltlt "m" ltlt endl
• cout ltlt "v0 5 m/s, moon " ltlt
  moon_height(5.0) ltlt "m" ltlt endl
• cout ltlt "v0 28 m/s, earth " ltlt
  earth_height(28.0) ltlt "m" ltlt endl

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• This works, but
• repeated code
• suppose your boss wants it to work for any planet
  in the solar system, plus all of their moons
• way too many functions
• Solution 2 send the gravitational constant as a
  second parameter

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• include ltiostreamgt
• using namespace std
• const double EARTH_G 9.81
• const double MOON_G 1.62
• const double MARS_G 3.69
• double height(double v0, double G)
• return v0 v0 / (2 G)
• int main()
• cout ltlt "v0 28 m/s, earth " ltlt
  height(28.0, EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 28 m/s, moon " ltlt
  height(28.0, MOON_G) ltlt "m" ltlt endl
• cout ltlt "v0 28 m/s, mars " ltlt
  height(28.0, MARS_G) ltlt "m" ltlt endl
• return 0

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applepie g -o default default.cc applepie
./default v0 28 m/s, earth 39.9592m v0 28
m/s, moon 241.975m v0 28 m/s, mars
106.233m applepie
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• The previous solution works fine
• but what if the majority of the time, our
  calculations are on Earth?

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• include ltiostreamgt
• using namespace std
• const double EARTH_G 9.81
• const double MOON_G 1.62
• const double MARS_G 3.69
• double height(double v0, double G)
• return v0 v0 / (2 G)
• int main()
• cout ltlt "v0 5 m/s, earth " ltlt height(5.0,
  EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 24 m/s, earth " ltlt
  height(24.0, EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 39 m/s, earth " ltlt
  height(39.0, EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 46 m/s, earth " ltlt
  height(46.0, EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 59 m/s, earth " ltlt
  height(59.0, EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 104 m/s, earth " ltlt
  height(104.0, EARTH_G) ltlt "m" ltlt endl

Repeated code.
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• Default Parameter Value
• a programmer can give a parameter in a function a
  default value
• if the function call omits an argument for this
  parameter, then the default value is used instead
• use the assignment operator to indicate a default
  parameter values

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• Default Parameter Value
• Example our height function

If the user does not send a second argument to
height, it uses the value 9.81 for G.
double height(double v0, double G 9.81)
return v0 v0 / (2 G)
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• include ltiostreamgt
• using namespace std
• const double EARTH_G 9.81
• const double MOON_G 1.62
• const double MARS_G 3.69
• double height(double v0, double G EARTH_G)
• return v0 v0 / (2 G)
• int main()
• cout ltlt "v0 28 m/s, moon " ltlt
  height(28.0, MOON_G) ltlt "m" ltlt endl
• cout ltlt "v0 5 m/s, earth " ltlt height(5.0)
  ltlt "m" ltlt endl
• return 0

For this function call, parameter G gets the
value 1.62, since the calling function sent it a
value.
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• include ltiostreamgt
• using namespace std
• const double EARTH_G 9.81
• const double MOON_G 1.62
• const double MARS_G 3.69
• double height(double v0, double G EARTH_G)
• return v0 v0 / (2 G)
• int main()
• cout ltlt "v0 28 m/s, moon " ltlt
  height(28.0, MOON_G) ltlt "m" ltlt endl
• cout ltlt "v0 5 m/s, earth " ltlt height(5.0)
  ltlt "m" ltlt endl
• return 0

For this function call, parameter G did not
receive a value from the calling function.
Therefore, it defaults to 9.81.
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• include ltiostreamgt
• using namespace std
• const double EARTH_G 9.81
• const double MOON_G 1.62
• const double MARS_G 3.69
• double height(double v0, double G)
• return v0 v0 / (2 G)
• int main()
• cout ltlt "v0 5 m/s, earth " ltlt height(5.0,
  EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 24 m/s, earth " ltlt
  height(24.0, EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 39 m/s, earth " ltlt
  height(39.0, EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 46 m/s, earth " ltlt
  height(46.0, EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 59 m/s, earth " ltlt
  height(59.0, EARTH_G) ltlt "m" ltlt endl
• cout ltlt "v0 104 m/s, earth " ltlt
  height(104.0, EARTH_G) ltlt "m" ltlt endl

Repeated code.
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• include ltiostreamgt
• using namespace std
• const double EARTH_G 9.81
• const double MOON_G 1.62
• const double MARS_G 3.69
• double height(double v0, double G EARTH_G)
• return v0 v0 / (2 G)
• int main()
• cout ltlt "v0 5 m/s, earth " ltlt height(5.0)
  ltlt "m" ltlt endl
• cout ltlt "v0 24 m/s, earth " ltlt
  height(24.0) ltlt "m" ltlt endl
• cout ltlt "v0 39 m/s, earth " ltlt
  height(39.0) ltlt "m" ltlt endl
• cout ltlt "v0 46 m/s, earth " ltlt
  height(46.0) ltlt "m" ltlt endl
• cout ltlt "v0 59 m/s, earth " ltlt
  height(59.0) ltlt "m" ltlt endl
• cout ltlt "v0 104 m/s, earth " ltlt
  height(104.0) ltlt "m" ltlt endl

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• Default Parameters
• this is how getline is able to take two or three
  parameters
• the third parameter of getline has a default
  value of '\n'
• therefore, if you don't send it a stopping
  character, it defaults to this value

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• Default Parameter Value Rules
• 1) in the function header
• if you specify a default parameter value for a
  parameter, then all of the parameters to the
  right of that parameter must also have a default
  parameter
• this means that all default parameters are on the
  right

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• Example

double height(double v0, double G EARTH_G)
return v0 v0 / (2 G)
OK!
double height(double v0 1.0, double G)
return v0 v0 / (2 G)
Error!
double height(double v0 1.0, double G
EARTH_G) return v0 v0 / (2 G)
OK!
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• Default Parameter Value Rules
• 2) in the function call
• if you send a value to a default parameter, then
  you must send a value to all of the parameters to
  the left of that value

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• Example (using a round function)

double round(double d 1.234567, int digits 3)
return static_castltintgt(d pow(10.0,
digits) 0.5) / pow(10.0, digits) int main()
cout ltlt round(1.234567, 3) ltlt endl cout
ltlt round(1.234567) ltlt endl cout ltlt round(, 3)
ltlt endl cout ltlt round() ltlt endl return
0
OK!
OK!
Compiler error!
OK!
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• Default Parameter Rules
• default values can be constants, global
  variables, or function calls

double f() return 3.0 double height(double v0
f(), double G EARTH_G) return v0 v0 /
(2 G)