# Examples¶

These are some basic examples of use of the package:

using Measurements
a = measurement(4.5, 0.1)
# => 4.5 ± 0.1
b = 3.8 ± 0.4
# => 3.8 ± 0.4
2a + b
# => 12.8 ± 0.4472135954999579
a - 1.2b
# => -0.05999999999999961 ± 0.49030602688525043
l = measurement(0.936, 1e-3);
T = 1.942 ± 4e-3;
g = 4pi^2*l/T^2
# => 9.797993213510699 ± 0.041697817535336676
c = measurement(4)
# => 4.0 ± 0.0
a*c
# => 18.0 ± 0.4
sind(94 ± 1.2)
# => 0.9975640502598242 ± 0.0014609761696991563
x = 5.48 ± 0.67;
y = 9.36 ± 1.02;
log(2x^2 - 3.4y)
# =>  3.3406260917568824 ± 0.5344198747546611
atan2(y, x)
# => 1.0411291003154137 ± 0.07141014208254456


## Measurements from Strings¶

You can construct Measurement objects from strings. Within parentheses there is the uncertainty referred to the corresponding last digits.

measurement("-123.4(56)")
# => -123.4 ± 5.6
measurement("+1234(56)e-1")
# => ->  123.4 ± 5.6
measurement("12.34e-1 +- 0.56e1")
# => 123.4 ± 5.6
measurement("(-1.234 ± 0.056)e2")
# => -123.4 ± 5.6
measurement("1234e-1 +/- 5.6e0")
# => 123.4 ± 5.6
measurement("-1234e-1")
# => -123.4 ± 0.0


## Correlation Between Variables¶

Here you can see examples of how functionally correlated variables are treated within the package:

x = 8.4 ± 0.7
x - x
# => 0.0 ± 0.0
x/x
# => 1.0 ± 0.0
x*x*x - x^3
# => 0.0 ± 0.0
sin(x)/cos(x) - tan(x)
# => -2.220446049250313e-16 ± 0.0
# They are equal within numerical accuracy
y = -5.9 ± 0.2
beta(x, y) - gamma(x)*gamma(y)/gamma(x + y)
# => 0.0 ± 3.979039320256561e-14


You will get similar results for a variable that is a function of an already existing Measurement object:

u = 2x
(x + x) - u
# => 0.0 ± 0.0
u/2x
# => 1.0 ± 0.0
u^3 - 8x^3
# => 0.0 ± 0.0
cos(x)^2 - (1 + cos(u))/2
# => 0.0 ± 0.0


A variable that has the same nominal value and uncertainty as u above but is not functionally correlated with x will give different outcomes:

# Define a new measurement but with same nominal value
# and uncertainty as u, so v is not correlated with x
v = 16.8 ± 1.4
(x + x) - v
# => 0.0 ± 1.979898987322333
v/2x
# => 1.0 ± 0.11785113019775792
v^3 - 8x^3
# => 0.0 ± 1676.4200705455657
cos(x)^2 - (1 + cos(v))/2
# => 0.0 ± 0.8786465354843539


## @uncertain Macro¶

Macro @uncertain can be used to propagate uncertainty in arbitrary real or complex functions of real arguments, including functions not natively supported by this package.

@uncertain (x -> complex(zeta(x), exp(eta(x)^2)))(2 ± 0.13)
# => (1.6449340668482273 ± 0.12188127308075564) + (1.9668868646839253 ± 0.042613944993428333)im
@uncertain log(9.4 ± 1.3, 58.8 ± 3.7)
# => 1.8182372640255153 ± 0.11568300475873611
log(9.4 ± 1.3, 58.8 ± 3.7)
# => 1.8182372640255153 ± 0.11568300475593848


You usually do not need to define a wrapping function before using it. In the case where you have to define a function, like in the first line of previous examples, anonymous functions allow you to do it in a very concise way.

The macro works with functions calling C/Fortran functions as well. For example, Cuba.jl package performs numerical integration by wrapping the C Cuba library. You can define a function to numerically compute with Cuba.jl the integral defining the error function and pass it to @uncertain macro. Compare the result with that of the erf function, natively supported in Measurements.jl package

using Cuba
cubaerf(x::Real) =
2x/sqrt(pi)*cuhre((t, f) -> f = exp(-abs2(t*x)), 1, 1)
@uncertain cubaerf(0.5 ± 0.01)
# => 0.5204998778130466 ± 0.008787825789336267
erf(0.5 ± 0.01)
# => 0.5204998778130465 ± 0.008787825789354449


Also here you can use an anonymous function instead of defining the cubaerf function, do it as an exercise.

Tip

Note that the argument of @uncertain macro must be a function call whose arguments are Measurement objects. Thus,

@uncertain zeta(13.4 ± 0.8) + eta(8.51 ± 0.67)


will not work because here the outermost function is +, whose arguments are zeta(13.4 ± 0.8) and eta(8.51 ± 0.67), that however cannot be calculated. Once more, wrap this expression in an (anonymous) function:

@uncertain ((x, y) -> zeta(x) + eta(y))(13.4 ± 0.8, 8.51 ± 0.67)
# => 1.9974303172187315 ± 0.0012169293212062773


The type of all the arguments provided must be Measurement. If one of the arguments is actually an exact number (so without uncertainty), convert it to Measurement type:

atan2(10, 13.5 ± 0.8)
# => 0.6375487981386927 ± 0.028343666961913202
@uncertain atan2(10 ± 0, 13.5 ± 0.8)
# => 0.6375487981386927 ± 0.028343666962347438


In addition, the function must be differentiable in all its arguments. For example, the scaled first derivative of the Airy Ai function $$\text{airyx}(1, z) = \exp((2/3) z \sqrt{z})\text{Ai}'(z)$$ is not differentiable in the first argument, not even the trick of passing an exact measurement would work because the first argument must be an integer. You can easily work around this limitation by wrapping the function in a single-argument function

@uncertain (x -> airyx(1, x))(4.8 ± 0.2)
# => -0.42300740589773583 ± 0.004083414330362105


## Complex Measurements¶

Here are a few examples about uncertainty propagation of complex-valued measurements.

u = complex(32.7 ± 1.1, -3.1 ± 0.2)
v = complex(7.6 ± 0.9, 53.2 ± 3.4)
2u+v
# => (73.0 ± 2.3769728648009427) + (47.0 ± 3.4234485537247377)im
sqrt(u*v)
# => (33.004702573592 ± 1.0831254428098636) + (25.997507418428984 ± 1.1082833691607152)im
gamma(u/v)
# => (-0.25050193836584694 ± 0.011473098558745594) + (1.2079738483289788 ± 0.133606565257322)im


You can also verify the Euler’s formula

cis(u)
# => (6.27781144696534 ± 23.454542573739754) + (21.291738410228678 ± 8.112997844397572)im
cos(u) + sin(u)*im
# => (6.277811446965339 ± 23.454542573739754) + (21.291738410228678 ± 8.112997844397572)im


## Arbitrary Precision Calculations¶

If you performed an exceptionally good experiment that gave you extremely precise results (that is, with very low relative error), you may want to use arbitrary precision (or multiple precision) calculations, in order not to loose significance of the experimental results. Luckily, Julia natively supports this type of arithmetic and so Measurements.jl does. You only have to create Measurement objects with nominal value and uncertainty of type BigFloat.

Tip

As explained in the Julia documentation, it is better to use the big string literal to initialize an arbitrary precision floating point constant, instead of the BigFloat and big functions. See examples below.

For example, you want to measure a quantity that is the product of two observables $$a$$ and $$b$$, and the expected value of the product is $$12.00000007$$. You measure $$a = 3.00000001 \pm (1\times 10^{-17})$$ and $$b = 4.00000001 \pm (1\times 10^{-17})$$ and want to compute the standard score of the product with stdscore(). Using the ability of Measurements.jl to perform arbitrary precision calculations you discover that

a = big"3.00000001" ± big"1e-17"
b = big"4.00000001" ± big"1e-17"
stdscore(a*b, 12.00000007)
# => -7.25510901439718980095468884170649047384323406887854411581099003148365616351548


the measurement significantly differs from the expected value and you make a great discovery. Instead, if you used double precision accuracy, you would have wrongly found that your measurement is consistent with the expected value:

stdscore((3.00000001 ± 1e-17)*(4.00000001 ± 1e-17), 12.00000007)
# => 0.0


and you would have missed an important prize due to the use of an incorrect arithmetic.

Of course, you can perform any mathematical operation supported in Measurements.jl using arbitrary precision arithmetic:

hypot(a, b)
# => 5.000000014000000000399999998880000003119999991353600023834879934652928178154746 ± 9.999999999999999999999999999999999999999999999999999999999999999999999999999967e-18
log(2a)^b
# => 1.030668110995484938037006520012324656386442805506891265153048683619922226691323e+01 ± 9.744450581349821315555305038012032439062183433587962363526314884889736017119502e-17


## Operations with Arrays and Linear Algebra¶

You can create arrays of Measurement objects and perform mathematical operations on them in the most natural way possible:

A = [1.03 ± 0.14, 2.88 ± 0.35, 5.46 ± 0.97]
B = [0.92 ± 0.11, 3.14 ± 0.42, 4.67 ± 0.58]
exp(sqrt(B)) - log(A)
# => 3-element Array{Measurements.Measurement{Float64},1}:
#     2.5799612193837493 ± 0.20215123893809778
#     4.824843081566397 ± 0.7076631767039828
#     6.982522998771525 ± 1.178287422979362
cos(A).^2 + sin(A).^2
# 3-element Array{Measurements.Measurement{Float64},1}:
#     1.0 ± 0.0
#     1.0 ± 0.0
#     1.0 ± 0.0


If you originally have separate arrays of values and uncertainties, you can create an array of Measurement objects by providing measurement with those arrays:

C = measurement([174.9, 253.8, 626.1], [12.2, 19.4, 38.5])
# => 3-element Array{Measurements.Measurement{Float64},1}:
#     174.9 ± 12.2
#     253.8 ± 19.4
#     626.1 ± 38.5
sum(C)
# => 1054.8000000000002 ± 44.80457565918909
mean(C)
# => 351.6000000000001 ± 14.93485855306303


Tip

prod and sum (and mean, which relies on sum) functions work out-of-the-box with any iterable of Measurement objects, like arrays or tuples. However, these functions have faster methods (quadratic in the number of elements) when operating on an array of Measurements than on a tuple (in this case the computational complexity is cubic in the number of elements), so you should use an array if performance is crucial for you, in particular for large collections of measurements.

Some linear algebra functions work out-of-the-box, without defining specific methods for them. For example, you can solve linear systems, do matrix multiplication and dot product between vectors, find inverse, determinant, and trace of a matrix, do QR factorization, etc.

A = [(14 ± 0.1) (23 ± 0.2); (-12 ± 0.3) (24 ± 0.4)]
b = [(7 ± 0.5), (-13 ± 0.6)]
# Solve the linear system Ax = b
x = A \ b
# => 2-element Array{Measurements.Measurement{Float64},1}:
#      0.763072 ± 0.0313571
#     -0.160131 ± 0.0177963
# Verify this is the correct solution of the system
A * x # This should be equal to b
# => 2-element Array{Measurements.Measurement{Float64},1}:
#       7.0 ± 0.5
#     -13.0 ± 0.6
dot(x, b)
# 7.423202614379084 ± 0.5981875954418516
det(A)
# => 611.9999999999999 ± 9.51262319236918
trace(A)
# => 38.0 ± 0.4123105625617661
A * inv(A) ≈ eye(A)
# => true
qrfact(A)
# => Base.LinAlg.QR{Measurements.Measurement{Float64},Array{Measurements.Measurement{Float64},2}}(Measurements.Measurement{Float64}[-18.4391 ± 0.209481 -1.84391 ± 0.522154; -0.369924 ± 0.00730266 33.1904 ± 0.331267],Measurements.Measurement{Float64}[1.75926 ± 0.00836088,0.0 ± 0.0])


In order to propagate the uncertainties, Measurements.jl keeps track of the partial derivative of an expression with respect to all independent measurements from which the expression comes. The package provides two convenient functions, Measurements.derivative and Measurements.gradient, that return the partial derivative and the gradient of an expression with respect to independent measurements.

x = 98.1 ± 12.7
y = 105.4 ± 25.6
z = 78.3 ± 14.1
Measurements.derivative(2x - 4y, x)
# => 2.0
Measurements.derivative(2x - 4y, y)
# => -4.0
Measurements.gradient(2x - 4y, [x, y, z])
# => 3-element Array{Float64,1}:
#      2.0
#     -4.0
#      0.0  # The expression does not depend on z


Tip

The Measurements.gradient function is useful in order to discover which variable contributes most to the total uncertainty of a given expression, if you want to minimize it. This can be calculated as the Hadamard (element-wise) product between the gradient of the expression with respect to the set of variables and the vector of uncertainties of the same variables in the same order. For example:

w = y^(3//4)*log(y) + 3x - cos(y/x)
# => 447.0410543780643 ± 52.41813324207829
(Measurements.gradient(w, [x, y]) .* uncertainty([x, y])).^2
# => 2-element Array{Any,1}:
#     1442.31
#     1305.36


In this case, the x variable contributes most to the uncertainty of w. In addition, note that the Euclidean norm of the Hadamard product above is exactly the total uncertainty of the expression:

vecnorm(Measurements.gradient(w, [x, y]) .* uncertainty([x, y]))
# => 52.41813324207829


## stdscore Function¶

You can get the distance in number of standard deviations between a measurement and its expected value (this can be with or without uncertainty) using stdscore:

stdscore(1.3 ± 0.12, 1)
# => 2.5000000000000004
stdscore(4.7 ± 0.58, 5 ± 0.01)
# => -0.5172413793103445 ± 0.017241379310344827


## weightedmean Function¶

Calculate the weighted and arithmetic means of your set of measurements with weightedmean and mean respectively:

weightedmean((3.1±0.32, 3.2±0.38, 3.5±0.61, 3.8±0.25))
# => 3.4665384454054498 ± 0.16812474090663868
mean((3.1±0.32, 3.2±0.38, 3.5±0.61, 3.8±0.25))
# => 3.4000000000000004 ± 0.2063673908348894


## value and uncertainty Functions¶

Use value and uncertainty to get the values and uncertainties of measurements. They work with real and complex measurements, scalars or arrays:

value(94.5 ± 1.6)
# => 94.5
uncertainty(94.5 ± 1.6)
# => 1.6
value([complex(87.3 ± 2.9, 64.3 ± 3.0), complex(55.1 ± 2.8, -19.1 ± 4.6)])
# => 2-element Array{Complex{Float64},1}:
#     87.3+64.3im
#     55.1-19.1im
uncertainty([complex(87.3 ± 2.9, 64.3 ± 3.0), complex(55.1 ± 2.8, -19.1 ± 4.6)])
# => 2-element Array{Complex{Float64},1}:
#     2.9+3.0im
#     2.8+4.6im


## Use with SIUnits.jl and Unitful.jl¶

You can use Measurements.jl in combination with a third-party package in order to perform calculations involving physical measurements, i.e. numbers with uncertainty and physical unit. The details depend on the specific package adopted. Such packages are, for instance, SIUnits.jl and Unitful.jl. You only have to use the Measurement object as the value of the SIQuantity object (for SIUnits.jl) or of the Quantity object (for Unitful.jl). Here are a few examples.

using Measurements, SIUnits, SIUnits.ShortUnits
hypot((3 ± 1)*m, (4 ± 2)*m) # Pythagorean theorem
# => 5.0 ± 1.7088007490635064 m
(50 ± 1)Ω * (13 ± 2.4)*1e-2*A # Ohm's Law
# => 6.5 ± 1.20702112657567 kg m²s⁻³A⁻¹
2pi*sqrt((5.4 ± 0.3)*m / ((9.81 ± 0.01)*m/s^2)) # Pendulum's  period
# => 4.661677707464357 ± 0.1295128435999655 s

using Measurements, Unitful
hypot((3 ± 1)*u"m", (4 ± 2)*u"m") # Pythagorean theorem
# => 5.0 ± 1.7088007490635064 m
(50 ± 1)*u"Ω" * (13 ± 2.4)*1e-2*u"A" # Ohm's Law
# => 6.5 ± 1.20702112657567 A Ω
2pi*sqrt((5.4 ± 0.3)*u"m" / ((9.81 ± 0.01)*u"m/s^2")) # Pendulum's period
# => 4.661677707464357 ± 0.12951284359996548 s