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Loops

5. Loops#

Loops are a way of repeating commands over and over again.

A while loop is an indefinite loop. It runs indefinitely until a certain condition is met. Be careful with while loops. If the condition is never met, the while loop will try to run forever.

import numpy as np
i = 0
while i<5:
    i = i + 1
    print(i)
print('done with while loop')

1
2
3
4
5
done with while loop

A for loop is a definite loop. It iterates over a list, array or other variable types that are groups of values or other variables. It definitely stops when it gets to the end of the list.

name_list = ['Bob','Jane','Mary']
for name in name_list:
    print("Hello " + name)
print('done')         

Hello Bob
Hello Jane
Hello Mary
done

An example of looping through a sequence of numbers created with the np.arange() function.

import numpy as np
print(np.arange(5))

[0 1 2 3 4]

print(np.arange(2,5))

[2 3 4]

print(np.arange(2,5,0.5))

[2.  2.5 3.  3.5 4.  4.5]

for i in np.arange(5):
    print(i*2)

0
2
4
6
8

Exercise:

Write a for loop that prints out the cumulative sum of an array.

For example, the cumulative sum of the array

[1,3,6,4,7]

would be:

[1,4,10,14,21]

Write out your algorithm first. An algorithm is a process or set of rules followed in calculations or problem solving. It helps to write or sketch these instructions first, then translate the instructions to Python. This strategy helps break a coding problem up into smaller, more manageable parts.

Use the following code as a template (replace the dashes —– with code).

x = [1,3,6,4,7]

cumsum = 0
for val in x:
    cumsum = -------
    print(cumsum)

This should produce the output:

1
4
10
14
21
done

5.1. For loop - cumulative sum 2#

Write a for loop that stores the cumulative sum of an array in another array. Use the np.append() function - see help(np.append) - and make sure that you have run import numpy as np at the top of your program in order to use this function.

x = [1,3,6,4,7]

cumsum = 0
cumsum_array = []
for val in x:
    cumsum = -------
    cumsum_array = np.append(------,--------)

print(cumsum_array)

This should produce the output:

[ 1.  4. 10. 14. 21.]

5.2. Fibonacci sequence (if time)#

Write a for loop that prints the first n numbers of the Fibonacci sequence: 1,1,2,3,5,8,13… Write out your algorithm first, and start with the following minimal code.

n = 10

# insert code here
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