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lab46
while loops

The variables in a loop

Background: We have seen for-loops:

python maintains an index-variable which ranges over 0,1,2,… up 'til the end of the range; The index-variable is always ¹ an int.
you maintain a “so-far” variable:
- initialize it before the loop starts;
- updating it in the body of the loop, as needed for your task.
Your function's answer is often the same as the so-far variable. (The so-far variable might be an int, or a string, or a boolean, or whatever.)

A growing problem

On Day 0, the virus mutated, and one person was infected. “It's only one person who wants to eat brains — how bad can it be?”, the researchers consoled themselves. But Patient Zed bit another researcher and fled. The next morning, that researcher was also infected, and now there were two Zeta-Omega Mutated Bioform Interferon Entities (which the ridiculously, in their unscientific sensationalism, shortened to "zombie").

Each day, every victim bites one more non-infected person, doubling the number of victims the next morning. Computer modeling can be very helpful, in determining the course of the zombie apocalypse.

How many zombies are there after 10 days? (or 20, or 30) Write a function to find out. (After giving it a try, you can compare your code with a solution.)

It turns out, we can re-write the same function above using a while-loop, instead of a for-loop:

# zombiePopulation2 : int -> int
# Given a number of days, return the number of zombies at the end of that many days.
#
def zombiePopulation2( days ):
    zombiesSoFar = 1
    dayNum = 0
    while (dayNum < days):
        zombiesSoFar = zombiesSoFar * 2
        dayNum = dayNum + 1   # Put this at the *end* of the body, to act like the for-loop does!

        # For illustration/understanding only:
        print( "At end of loop: dayNum=" + str(dayNum) + " and zombiesSoFar=" + str(zombiesSoFar) )

    return zombiesSoFar

What the while loop does: it manages the index-variable manually!

We have to initialize the index variable dayNum to 0 ourselves.
We have to update the index variable dayNum = dayNum + 1.

So it seems like the while loop is only more tedious… but it also gives us more flexibility: If we want to initialize our index-variable to 20 or -5 instead of zero, we can! If we want to increment by 3 or by -1 each time, we can!

while-loops are a bit more flexible than for-loops — even more than we've arleady mentioned: you can use while-loops even if you don't know the range in advance.

Consider: How many days until there are at least 10,000 infected?
Since we don't know how many days it takes, we don't know what range to use! No problem — use a while-loop to keep doubling our zombiesSoFar, and continue while the zombiesSoFar is less than 10,000 (or, any target population-size):

def daysTilExceed( popSize ):
    zombiesSoFar = 1
    dayNum = 0
    while (zombiesSoFar < popSize):
        zombiesSoFar = zombiesSoFar * 2
        dayNum = dayNum + 1

        # For illustration/understanding only:
        print( "At end of loop: dayNum=" + str(dayNum) + " and zombiesSoFar=" + str(zombiesSoFar) )

    return dayNum

You may have gleaned the syntax of while by now:
while (condition): statements…
The semantics:
- If the condition evaluates to true, then do the body (the statements…), and repeate this whole process.
- If the condition is false, then we are done with the loop, and we proceed to the next statement after the loop (indented at the same level as the word “while”).

Warning: It is suddenly possible to write infinite loops! Consider:

i = 0
while (i != 10):
    print("Whoops, I won't hit my stop-condition exactly!  (Hit 'stop' button at center-right.)"
    i = i+3
    print("The value of i is: " + str(i) )

j = 0
while (j < 10):
    print("Whoops, I am forgetting to update my index variable!  (Hit 'stop' button at center-right.)")
    print("The value of j is: " + str(j) )

YOUR TASK: (not to be checked off — but this will be on a quiz) Re-write countVowels2, which is just like countVowels from lab44—strings as arrays: so-far variables revisited, except that you need use a while loop instead of a for-loop.

Remember: in a while-loop, you need to include the work that python did for you in a for-loop: You'll need to initialize the index-variable (once), and update it (each time through the loop).

¹ Well, in every example we've made, the index-variable is an int. But really, in theory you could sequence over any range, not just a range of numbers. For example, for monthLen in (31,28,31,30): is valid python, as is for hue in (black, red, green, makeColor(20,30,40), magenta):. Neat! ↩

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©2014, Ian Barland, Radford University
Last modified 2014.Apr.28 (Mon) Please mail any suggestions
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lab46 while loops

The variables in a loop

A growing problem

lab46
while loops