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Functions

7. Functions#

Functions can be used to package a set of instructions. These instructions can then be reused for different inputs. Using functions can save you a lot of copying and pasting. Also, if you find a bug in your set of instructions, you only have to fix it once if you use a function instead of copying and pasting.

def fahr_to_celsius(temp_fahr):
    ''' Convert temperature from degrees F to C '''
    temp_celsius = (temp_fahr - 32) * (5/9)
    return temp_celsius

help(fahr_to_celsius)

Help on function fahr_to_celsius in module __main__:

fahr_to_celsius(temp_fahr)
    Convert temperature from degrees F to C

In IPython (and Jupyter notebooks) there is another way of asking for help by typing the function name with a question mark.

fahr_to_celsius?

To use the function:

fahr_to_celsius(32)

0.0

tempF = 212

tempC = fahr_to_celsius(tempF)
print('Temperature in Celsius is:',tempC)

Temperature in Celsius is: 100.0

7.1. Multiple outputs#

def fahr_to_celsius_kelvin(temp_fahr):
    ''' Convert temperature from degrees F to Celsius and Kelvin '''
    temp_celsius = (temp_fahr - 32) * (5/9)
    temp_kelvin = temp_celsius + 273
    return temp_celsius,temp_kelvin

fahr_to_celsius_kelvin(77)

(25.0, 298.0)

7.2. Specifying default values for inputs#

def fahr_to_celsius_kelvin(temp_fahr=32):
    ''' Convert temperature from degrees F to Celsius and Kelvin '''
    temp_celsius = (temp_fahr - 32) * (5/9)
    temp_kelvin = temp_celsius + 273
    return temp_celsius,temp_kelvin

fahr_to_celsius_kelvin()

(0.0, 273.0)

Exercises:

  • Create a function that takes a number as input, and returns the square of that number.

  • Create a function to calculate the standard deviation of an array (not using the np.sqrt function).

7.3. Exercise: temperature conversion#

Create a function that converts temperature from degrees Celsius to degrees Fahrenheit.

Use the function created above to convert the following array of temperature values from degrees C to degrees F. Avoid using a for loop.

temps_c = np.array([50.5, 58.4, 62.3, 49.2])

7.4. Exercise: Stokes law function#

Stokes’ law predicts the settling velocity (or “terminal velocity”) of a sinking sphere, \(v\) (units: m/s). The theoretical velocity is based on the radius of the particle, \(r\) (units: m), the density of the particle \(\rho_p\) (units: kg/m\(^3\)), the density of the ambient fluid \(\rho_f\), the dynamic viscosity of the ambient fluid \(\mu\) (units: kg/(m*s)), and the acceleration due to gravity \(g\) (units: m/s\(^2\)):

\[v = \frac{2}{9}\frac{(\rho_p-\rho_f)}{\mu}g r^2\]

Use the following template to create a function that calculates the velocity predicted by Stokes law:

def stokes_law(r,rho_p,rho_f,mu,g):
    # insert code here
    return v

Use your stokes_law function to estimate the settling velocity of the single-cell alga Phaeocystis globosa, assuming it is spherical. The radius of a typical cell is 46 \(\mu m\) and the typical cell density is 1091 kg/m3. For standard seawater conditions (temperature = 10 deg C, practical salinity = 35) \(\rho_f\) = 1025 kg/m3\(, \)\mu =$ 1.51 × 10−3 kg/(m*s).

Source: L. Peperzak, F. Colijn, R. Koeman, W. W. C. Gieskes, J. C. A. Joordens, Phytoplankton sinking rates in the Rhine region of freshwater influence, Journal of Plankton Research, Volume 25, Issue 4, April 2003, Pages 365–383, https://doi.org/10.1093/plankt/25.4.365

  1. Edit your function to use typical seawater values as defaults for rho_f and mu.

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