Lecture 7

Review

1. How many times does this while loop execute? num = float(input("Enter a number: "))
   while num < 0:
       print("Only the square root of positive numbers can be calculated")
       num = float(input("Enter a number: "))
   
2. How many times does this while loop execute? repeat = 'no'
   while repeat != 'no':
       repeat = input("Do you want to repeat the loop? ")
   
3. What value prints out at the end? total = 0
   i = 0
   while i < 5:
       total += i
   print(total) What is the accumulator variable in the program?

Nested Loops

Just like nested if statements, it is possible to nest loops inside of other loops. For each time through the outer loop, the inner loop loops completely:

Why do you think minutes is set to zero inside the first while loop? Why isn't it:

for Loops

The other kind of loop Python supports is the for loop. Python's for loop is more like a for-each loop (which some programming languages, like Java and PHP, support) than the standard for loop. It will execute the body of the for loop for each element in a collection. The only collection type we've seen so far is the string (it's a collection of characters), but we'll see others very soon.

Here is the basic form of a for loop:

The variable is the loop's iterator variable. It iterates through each element in the collection. In other words, it takes on each value in the collection.

Here is an example of using a for loop. In this example, the loop counts the number of lowercase vowels in a user-provided string.

The range function

What if you want to loop a set number of times? You could write a while loop like this:

However, for loops are much better for looping a set number of times (this is true across programming languages). In Python, this is accomplished with the help of the range function. It creates a special collection of integers from a specified lower-bound to a specified upper-bound (not including this upper-bound). For example:

Notice that the numbers start at 10 and go up to, but not including 20.

Range is a function that takes between 1 and 3 arguments. We haven't seen many functions yet that can do this. The number of arguments you give it changes how it behaves.

1. If you give it one argument, it is the upper-bound and the lower-bound is assumed to be 0. >>> for i in range(5):
           print(i)
   0
   1
   2
   3
   4
   
2. If you give it two arguments, the first is the lower-bound and the second is the upper-bound. >>> for i in range(3, 5):
           print(i)
   3
   4
   
3. If you give it three arguments, the first is the lower-bound, the second is the upper-bound, and the third is the step size to take: >>> for i in range(3, 13, 2):
           print(i)
   3
   5
   7
   9
   11
   

Using the range function, we can rewrite the while loop above to be:

Notice how much shorter it is!

You can combine the range and len functions to iterate over the indices of a collection:

How is this an improvement over the original version of this code? It's not. In most cases, you will want to iterate over the elements in the collection (as done in the example earlier). However, it you want to compare two elements in the loop, you will most often need to know their indices and the only way to get them is through range(len(collection)). For example, to count the number of consecutive vowels in a string:

Why is it range(len(value)-1) and not range(len(value))? In other words, why do we need to subtract 1 from len(value)?

A couple of notes about the range function:

• It does not work with floating-point numbers, only with integers.
• If you want to count down instead of up, you must specify the step size and it must be negative: >>> for i in range(5, 0):
          print(i)
  
  >>> #notice that nothing prints out
  >>> for i in range(5, 0, -1):
          print(i)
  
  5
  4
  3
  2
  1
  >>> #why wasn't 0 printed out?
• In Python 2, the range function would produce a list. For large ranges, this could take up a lot of memory so for Python 3, the range function produces a range object instead. If you will be using Python 2 in the future, be aware of this minor difference.

while loops and for loops

The while loop executes while a condition is True. The for loop executes once for each element in a collection. In general:

• If you want to access each element in a collection, you should use the for loop.
• If you know how many times a loop should execute, you probably want to use the for loop (with the range function).
• If you want a loop to execute an unknown number of times, then you probably want to use the while loop.

Lists and Tuples

Sometimes, it is very useful to be able to store a list of values, but so far all of the datatypes we've seen can only hold one value. A float can only hold one floating-point number, an int can only hold one integer, and a string can only hold one text. If you want to hold multiple values, you would need multiple variables, but what if you don't know how many values you'll need? This is where lists can help! (In other programming languages, lists are called arrays.) A list is one variable that can hold multiple values. (Unlike other programming languages, python does not require all data in the list to be the same type.)

To create a list in Python, you can use either the square brackets or the list function:

Indexing

When you store elements in a list, the order is preserved. Just like strings, the first position in a list is at index 0 and the last position is at index len(list_name) - 1. To access an element in a list, use the index notation (just like with strings.

Getting elements from the end of the list is a common task, so the Python language developers tried to make it more convenient than requiring programmers to always type list_name[len(list_name)-1]. You can use negative indices to indicate starting at the end of the list. The trick is to just drop the len(list_name) part of the index. For example:

This is also true with string indexing:

With lists, you can change the value of an element by assigning a new value to the list at that index, e.g.:

Note: this does not work with strings or with tuples.

Slicing

The slicing operation creates a new list, taking elements from the first list that the programmer specifies. You again use the square brackets, but now specify two or three values instead of just one. Separate the values with the colon (:). The general form for slicing is:

or

The start_index is the index to start slicing at. Stop slicing at the stop_index (will not include this index). The optional step_size indicates how many steps to take before picking the next element; if it is not specified, the default is 1.

Some examples:

You can leave out the start_index or the stop_index (although you still need the colons). If you leave out start_index, it defaults to 0. If you leave out stop_index, it defaults to len(list_name).

Some examples:

Slicing makes a new list and puts the elements into that new list. So, if you change the new list, the old list will not change.

Just like with indexing, you can do assignments to a slice. It replaces the specified part of the list with the given value.

You can also replace parts of a list with another list: