Assuming s1 = 'abc' and s2 = 'cba' are defined in main(), which function will return cbaabccba when result is passed to the print() function?
result = string_it_up(s1, s2)

Question

BlackTom AI · Accepted Answer

Concatenation of strings is performed left-to-right, so 'a' + 'b' produces 'ab'. Here, x = 'abc' and y = 'cba', and we must produce 'cbaabccba'.

1. Relevant concepts
- String concatenation: using + combines strings in the specified order.
- The += operator for strings is equivalent to variable = variable + (other), producing a new string and rebinding the name.
- The returned variable (x or y) determines which built-up string is produced.

2. Step-by-step reasoning
Evaluate each candidate function with x = 'abc' and y = 'cba'.

Correct candidate (option shown in the answer):
def string_it_up(x, y):
  y += x + y
  return(y)

Compute intermediate steps:
- x + y = 'abc' + 'cba' = 'abccba'
- y += x + y means y = y + (x + y) = 'cba' + 'abccba' = 'cbaabccba'
- return(y) yields 'cbaabccba' which matches the required output.

3. Intermediate calculations shown
- x = 'abc'
- y = 'cba'
- x + y = 'abccba'
- y = y + (x + y) = 'cba' + 'abccba' = 'cbaabccba'

4. Why this answer is correct
Because it builds the substring (x+y) first and then prefixes the original y to that result, producing exactly the sequence 'cba' + 'abccba' = 'cbaabccba'. Returning y returns that constructed string.

5. Why the other options are incorrect
- x += y + x → x becomes 'abc' + 'cba' + 'abc' = 'abccbaabc' (starts with 'abc', not 'cba').
- y += y + x → y becomes 'cba' + 'cba' + 'abc' = 'cbacbaabc' (duplicates y first, wrong order).
- x += x + y → x becomes 'abc' + 'abc' + 'cba' = 'abcabccba' (starts with 'abc', wrong prefix).

6. Conclusion
The function def string_it_up(x, y): y += x + y; return(y) returns 'cbaabccba' for s1 = 'abc' and s2 = 'cba', which matches the provided correct answer.

Assuming s1 = 'abc' and s2 = 'cba' are defined in main(), which function will return cbaabccba when result is passed to the print() function? result = string_it_up(s1, s2)单项选择题

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