Iteration

Jon Reades
Let's Get This Party Started

it·er·a·tion

/itə’rāSHən/

Noun

The repetition of a process or utterance.

  • repetition of a mathematical or computational procedure applied to the result of a previous application, typically as a means of obtaining successively closer approximations to the solution of a problem.
  • a new version of a piece of computer hardware or software. plural noun: iterations

Two Types of Iteration

‘For’ loops

  • Used with finite lists of definite length
  • For each item in this list do something…

‘While’ loops:

  • Used with unknown or non-finite lists
  • While a condition is still True, do something to the list…

Making the Difference Memorable

For Loops

This ‘simple’ loop allows us to print out every element of the list in turn:

geographers = ['Rose','Massey','Jefferson']
for g in geographers:
  print(g)

Notice the format:

for x in list:
  # ...do something using the current value of x...

While Loops

This loop does the same thing, but differently:

geographers = ['Rose','Massey','Jefferson']
g = 0
while g < len(geographers):
  print( geographers[g] )
  g += 1

Notice the format:

while <some condition is true>:
  # ...do something...

Nesting Loops

We can use one loop ‘inside’ another loop! What do you think this might print?

geographers = ['Rose','Massey','Jefferson']
for g in geographers:
  for h in g:
    print(h)

Huh??? Let’s puzzle this out…

Debugging

When you see something completely new, it’s often good to:

  1. Add print(...) statements to see how the values of a variable are changing.
  2. Comment out the parts you don’t understand so that you can focus on the parts you do
  3. Then iteratively add complexity!

Step 1: The ‘Outer Loop’

So I would start off with:

geographers = ['Rose','Massey','Jefferson']
for g in geographers:
  print(g)
#   for h in g:
#   print(h)

This prints:

'Rose'
'Massey'
'Jefferson'

Step 1: Summing Up

OK, so now we know:

  1. That g is the name of a geographer.
  2. The ‘outer’ loop sets g to the name of a new geographer on each iteration.
  3. So if g is set to 'Rose' what does for h in g: do?

Step 2: The ‘Inner’ Loop

We know change it like this:

for g in geographers:
  print(g)
  for h in g:
    print(h)
  break # <-- Notice this!

This prints:

Rose
R
o
s
e

Step 2: Summing Up

And now we know that:

  1. h is looping over the string, meaning that a string can be treated as a list!
  2. And break is a really useful way to control a loop while you’re testing your code!

Recap

  1. for iterates once over a collection items (e.g. a list).
  2. while keeps going until a condition is False.

Test Yourself

What will this code print? I’d suggest that you don’t run it!

geographers = ['Rose','Massey','Jefferson']
g = 0
while g < len(geographers):
  print( geographers[g] )

Test Yourself (Tricksy Version)

Here’s a really tricky one! The following two blocks of code produce the same output, how are they different?

geographers = ['Rose','Massey','Jefferson']
geographers.reverse()
for g in geographers:
  print(g)

And:

geographers = ['Rose','Massey','Jefferson']
g = len(geographers)-1
while g >= 0:
  print( geographers[g] )
  g -= 1

One More Thing…

Let’s go back to the Lists examples for a second:

female_geographers = ['Rose','Valentine','Massey','Jefferson']
male_geographers = ['Von Humboldt','Harvey','Hägerstrand']
all_geographers = []
all_geographers.append(female_geographers)
all_geographers.append(male_geographers)

Have a think about how this code works:

for ag in all_geographers:
  for g in ag:
    print(g)

Resources

We don’t cover the concept of recursion, but it’s quite a powerful idea and links nicely with Iteration:

Iteration • Jon Reades