How to use a for loop? in Python

The for loop has 2 main components:

1. an iterating variable e.g. i

2. a list of desired values e.g. range(3) will be three values i = 0, i = 1, i = 2

These loops are great when you want to repeat the same command multiple times. You can use the variable to update your command.

Consider this for loop, the iterating variable is i and the desired values are range(3):

for i in range(3):
    print('i =', i)

i = 0
i = 1
i = 2

The for loop reassigns a variable e.g. i each time it finishes the commands that are tabbed.

for <variable> in <desired_values

\(\rightarrow\)     <run these commands>

You can accomplish the same output without a for-loop by copy-pasting the print command:

print('i = ', 0)
print('i = ', 1)
print('i = ', 2)

i =  0
i =  1
i =  2

Use a for loop to calculate a sum

The np.sum will sum the values of an array, but if you have a Python list inside [ and ] it will return an error. You can use a for loop to calculate this value (if possible use NumPy though!). Here is the process:

1. define the vector, v

2. initialize the value for your sum, sum_v

3. create a for-loop that assigns your variable to each value of v, for vi in v:

4. inside the for-loop, add each value to sum_v, sum_v += vi

v = [1, 2, 3, 10]

sum_v = 0

for vi in v:
    sum_v += vi
print('sum of vectors components in v is', sum_v)

sum of vectors components in v is 16

Wrapping up

In this notebook, you built some for-loops to print and add variables from desired values. You can use the for loops to repeat the same command multiple times while changing the the iterating variable.