# The Accumulator Pattern with Lists¶

We can also accumulate values into a list rather than accumulating a single numeric value. Consider, for example, the following program which transforms a list into a new list by squaring each of the values.

Here, we initialize the accumulator variable to be an empty list, on line 2.

We iterate through the sequence (line 3). On each iteration we transform the item by squaring it (line 4).

The update step appends the new item to the list which is stored in the accumulator variable (line 5). The update happens using the .append(), which mutates the list rather than using a reassignment. Instead, we could have written accum = accum + [x], or accum += [x]. In either case, we’d need to concatenate a list containing x, not just x itself.

At the end, we have accumulated a new list of the same length as the original, but with each item transformed into a new item. This is called a mapping operation, and we will revisit it in a later chapter.

Note how this differs from mutating the original list, as you saw in a previous section of this chapter.

exceptions-64: What is printed by the following statements?

alist = [4,2,8,6,5]
blist = [ ]
for item in alist:
blist.append(item+5)
print blist

• (A) [4,2,8,6,5]
• 5 is added to each item before the append is performed.
• (B) [4,2,8,6,5,5]
• There are too many items in this list. Only 5 append operations are performed.
• (C) [9,7,13,11,10]
• Yes, the for loop processes each item of the list. 5 is added before it is appended to blist.
• (D) Error, you cannot concatenate inside an append.
• 5 is added to each item before the append operation is performed.

exceptions-65: What is printed by the following statements?

list= [3,0,9,4,1,7]
new_list=[]
for i in range(len(list)):
new_list.append(list[i]+5)
print new_list

• (A) [8,5,14,9,6]
• 5 is added to each list[i] before the append is performed.
• (B) [8,5,14,9,6,12]
• Yes, the for loop processes each list[i] of the list. 5 is added before list[i] is appended to blist.
• (C) [3,0,9,4,1,7,5]
• There are too many items in this list. Only 5 append operations are performed.
• (D) Error, you cannot concatenate inside an append.
• 5 is added to each list[i] before the append operation is performed.
Next Section - The Accumulator Pattern with Strings