source("http://macosa.dima.unige.it/r.R")    # If I have not already loaded the library
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S 04 Lenghts and Areas

# areaF(f, a,b, N) (or areaFun) calculates the area of the N-gon that approximates the
# graph of f.
# Examples for N = 2, 4, 10 thousand, 100 thousand for a certain f
BF=3; HF=2
f <- function(x) sqrt(1-x^2)
PLANE(-1,1, 0,1); areaF(f,-1,1,2)    # 1
PLANE(-1,1, 0,1); areaF(f,-1,1,4)    # 1.3660254
PLANE(-1,1, 0,1); areaF(f,-1,1,1e4)  # 1.5707947

 
# Use areaF2 if (for great N) you want a faster computation
 
PLANE(-1,1, 0,1); areaF2(f,-1,1,1e5)  # 1.5707963 (=π/2)
 
# Another example (graphs on the left):
Plane(0,3,0,3); areaF(floor, 0,3, 2)      # 3.75
Plane(0,3,0,3); areaF(floor, 0,3, 1000)   # 3.0015
Plane(0,3,0,3); areaF2(floor, 0,3, 1e4)   # 3.00015
    
# The areas are signed: positive/negative if they are above/below the x axis
# (graphs above on the right)
f1 <- function(x) -x^2+1;  f2 <- function(x) -f1(x)
BF=3; HF=3
PLANE(-1,1, -1,1); areaF(f1,-1,1,2000)    # 1.333333
PLANE(-1,1, -1,1); areaF(f2,-1,1,2000)    # -1.333333
#
# I can calculate the area of the polygon that is under the graph of a piecewise linear
# function (with coordinates in x and y), using the areaPol command, if I define the
# function AREA (but I can use another name) in this way:
AREA = function(x,y) areaPol( c(x[length(x)],x[1],x), c(0,0,y) )
# Example:
                    
x = c(10,13, 17, 19); y = c(7,1,9,8)
Plane(min(x),max(x), 0,max(y)); polyl(x,y,"blue")
AREA = function(x,y) areaPol( c(x[length(x)],x[1],x), c(0,0,y) )
AREA(x,y)    # 49
#
# areaPar(x,y, a,b, N), areaPolar(r, a,b, N) calculate the area enclosed by a curve
# in parametric form and polar form (see S01). To calculate the length of a curve (when
# it is finite), I use lengFun(f,a,b,N), lengPar(x,y,a,b,N) or lengPolar(r,a,b,N)
# depending on how it is defined. lengPar3(x,y,z,a,b,N) works analogously in the case
# of a curve in the space.

# Examples for calculating the length of the semicircle (in various ways):
           3.141593              length = 24
  area  = 6 π
# as a function (input: f, A,B, N) f = function(x) sqrt(1-x^2) # in a parametric representation (input: f,g, A,B, N) x = function(t) cos(t); y = function(t) sin(t) # as a polar equation (input: r, A,B, N) # (in this particular case r does not depend on ang) r = function(ang) 1 lengFun(f, -1,1, 100); lengFun(f, -1,1, 10000); lengFun(f, -1,1, 100000) # 3.140761 3.141592 3.141593 lengPar(x,y, 0,pi, 100); lengPar(x,y, 0,pi, 1000); lengPar(x,y, 0,pi, 10000) # 3.141463 3.141591 3.141593 lengPolar(r, 0,pi, 100); lengPolar(r, 0,pi, 1000); lengPolar(r, 0,pi, 10000) # 3.141463 3.141591 3.141593 # Length and area of asteroid # asteroid of "radius" 4 f = function(t) 4*cos(t)^3; g = function(t) 4*sin(t)^3 PLANE(-4,4, -4,4); param(f,g, 0,2*pi, "blue") # (graph above on the right) lengPar(f,g,0,2*pi, 5000) # 24 A = areaPar(f,g, 0,2*pi,5000); A # 18.84957 A = areaPar(f,g, 0,2*pi,10000); A # 18.84956 A/pi # 6 # For the length of three-dimensional curves, see here. # # See here for areas enclosed by curves expressible only in Cartesian form. # ( startP() xClick yClick ) # # See here for a continuous function in a finite closed interval that has an infinite # length graph and neither right nor left derivative. # and for ... # and a continuous function that is not derivable at any point Other examples of use