While loops

Programming for Geoscientists Data Science and Machine Learning for Geoscientists

while loops repeat instructions until certain condition is False (while the condition is True). The syntax is as follows:

    while condition:
        # Some code to be executed repeatedly as long as condition is True
        
    # Some more code to be executed once the while loop is over, executed once

Some while loops are infinite, where the condition never becomes False. A while loop, like if statements, needs the code to be indented.

Example

counter = 0  # Set counter
print("Initial counter is: ", counter)

print("Entering while loop")
while counter <= 10:
    # Increase counter by 1 as long as it is <= 10
    counter += 1
    
    print(counter)
    
print("While loop finished.")
print("Final counter is: ", counter)

Initial counter is:  0
Entering while loop
1
2
3
4
5
6
7
8
9
10
11
While loop finished.
Final counter is:  11

Exercises

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• Write a program that prints all Fibonacci numbers smaller than 50. An example of a Fibonacci sequence is the following:

\[ F_0 = 0,\quad F_1 = 1, \quad F_{n}=F_{n-1}+F_{n-2}.\]

Answer

first = 0
second = 1

print(first)

while second <= 50:
    print(second)
    temp = first + second
    first = second
    second = temp

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• Create a while loop to find Celsius temperatures for Fahrenheit temperatures between 20\(^\circ\)F and 80\(^\circ\)F at 5\(^\circ\)F increments. The Fahrenheit-Celsius conversion formula is:

\[ T_{Celsius} = \frac{5}{9}*(T_{Fahrenheit}-32). \]

Answer

temp_F = 20           # Initial temperature in Fahrenheit
dF = 5                # Increment in Fahrenheit

while temp_F <= 80:
    temp_C = 5./9.*(temp_F-32)  # Calculate the temperature in Celsius

    # A nicely formatted f-string
    print(f"{temp_F:.2f} degrees Fahrenheit is {temp_C:.2f} degrees Celsius.")

    # Add increment to the temperature for the next iteration
    temp_F = temp_F + dF