9.8. The Accumulator Pattern with Strings¶

Strings are immutable, so they can not be changed. But it is possible to create entirely new strings. Combining string concatenation using + with a loop is the accumulator pattern with strings. With it we can create new strings from existing strings.

Remember that the accumulator pattern allows us to keep a “running total”. With strings, we are not accumulating a numeric total. Instead we are accumulating characters onto a string.

def repeat(text: str, times: int) -> str:

    repeated_text: str

    repeated_text = ''

    for i in range(0, times, 1):

        repeated_text = repeated_text + text

    return repeated_text

​

def main() -> None:

    print("I love CS" + repeat("!", 10))

    return None

​

main()

​

The accumulator variable, repeated_text, is intiialized on line 3 to be the empty string. Every iteration of the loop, text is concatenated onto the end of repeated_text so it gets longer with each iteration. Step through the function using codelens to see the accumulator variable grow.

Check your understanding

s: str
r: str
i: int
s = "ball"
r = ""
for i in range(0, len(s), 1):
    r = s[i] + r
print(r)