Looping Patterns With For Loops

When using for loops, a few patterns show up frequently. If you learn these patterns — how to recognize them and how to code them — it can speed up your programming.

The Action Pattern

In the action pattern, we want to do something to (or with) each element of a list.

Here is the pseudocode template:

possible setup code
for each x in xx:
    do somethign using x

Example: Printing the Elements of a List

for x in xx:
    print(x)

Example: Printing the Elements of a List and Their Squares

for x in xx:
    print(f"{x}, {x*x}")

Reading Elements from a File a Printing Them

with open('foo.txt') as file:
    for x in file.readlines()
        print(x)

Writing Elements of a List to a File

with open('foo.txt','w') as file:
    for x in xx:
        file.write(f"{x}\n")

The Accumulator Pattern

In the accumulator pattern, we want to build up a pattern using a collection of data.

Here is the pseudocode template:

initialize the accumulator
for each x in xx:
    modify accumulator using x
final result is in accumulator

Example: Counting the Elements of a List

For this, count is the accumulator, and at each step we add one to it.

count = 0
for x in xx:
    count = count + 1

Example: Summing the Elements of a List

For this, sum is the accumulator, and at each step we add x to it.

sum = 0
for x in xx:
    sum = sum + x

Example: Taking the Maximum of the Elements of a List

For this, themax is the accumulator, and at each step we update it with x using the built-in funciton max.

themax = xx[0]
for x in xx[1:]:
    themax = max(themax,x)

Note here we initialize themax to the first element of the list, and iterate over the rest of the elements.

Example: Create a Dictionary with the Elements of a List and Their Squares

For this, d is the accumulator, and at each step we update it by inserting key x with value x*x.

d = {}
for x in xx:
    d[x] = x * x

Your turn!

Try to write Python code to

Take the product of the elements of a list
Take the minimum of the elements of a list