6.4. The Accumulator Pattern¶

If you want to multiply two numbers together, the most basic approach is to think of it as repeating the process of adding one number to itself. The number of repetitions is where the second number comes into play. For example, if we wanted to multiply three and five, we could think about it as adding three to itself five times. Three plus three is six, plus three is nine, plus three is 12, and finally plus three is 15. Generalizing this, if we want to implement the idea of squaring a number, call it n, we would add n to itself n times.

Do this by hand first and try to isolate exactly what steps you take. You’ll find you need to keep some “running total” of the sum so far, either on a piece of paper, or in your head. Remembering things from one step to the next is precisely why we have variables in a program. This means that we will need some variable to remember the “running total”. It should be initialized with a value of zero. Then, we need to update the “running total” the correct number of times. For each repetition, we’ll want to update the running total by adding the number to it.

In words we could say it this way. To square the value of n, we will repeat the process of updating a running total n times. To update the running total, we take the old value of the “running total” and add n. That sum becomes the new value of the “running total”.

toSquare = 10 runningtotal = 0 for counter in range(toSquare): runningtotal = runningtotal + toSquare print("The result of", toSquare, "squared is", runningtotal)

In the program above, notice that the variable runningtotal starts out with a value of 0. Next, the iteration is performed x times. Inside the for loop, the update occurs. runningtotal is reassigned a new value which is the old value plus the value of x.

This pattern of iterating the updating of a variable is commonly referred to as the accumulator pattern. We refer to the variable as the accumulator. This pattern will come up over and over again. Remember that the key to making it work successfully is to be sure to initialize the variable before you start the iteration. Once inside the iteration, it is required that you update the accumulator.

Here is the same program in codelens. Step through the program and watch the “running total” accumulate the result.

Check your understanding

func-4-1: Consider the following code:

x = int(input("Enter a positive integer: "))
runningtotal = 0
for counter in range(x):
    runningtotal = runningtotal + x
print("The square of", x, "is", runningtotal)

What happens if you put the initialization of runningtotal (the line runningtotal = 0) inside the for loop as the first instruction in the loop?

• (A) The square program will print x instead of x * x
• The variable runningtotal will be reset to 0 each time through the loop. However because this assignment happens as the first instruction, the next instruction in the loop will set it back to x. When the loop finishes, it will have the value x, which is what is returned.
• (B) The square program will cause an error
• Assignment statements are perfectly legal inside loops and will not cause an error.
• (C) The square program will work as expected and print x * x
• By putting the statement that sets runningtotal to 0 inside the loop, that statement gets executed every time through the loop, instead of once before the loop begins. The result is that runningtotal is 'cleared' (reset to 0) each time through the loop.
• (D) The square program will print 0 instead of x * x
• The line runningtotal = 0 is the first line in the for loop, but immediately after this line, the line runningtotal = runningtotal + x will execute, giving runningtotal a non-zero value (assuming x is non-zero).

        func-4-2: Rearrange the code statements so that the program will add up the first n odd numbers where n is provided by the user.n = int(input('How many odd numbers would
you like to add together?'))
thesum = 0
oddnumber = 1
---
for counter in range(n):
---
   thesum = thesum + oddnumber
   oddnumber = oddnumber + 2
---
print(thesum)