## DEV Community

HHMathewChan

Posted on • Originally published at rebirthwithcode.tech

# Python exercise 12: find the hidden ball

## Question

• Create a function that returns the letter position where the ball final position is once the swapping is finish.

### description

• There are three cups on a table, at positions A, B, and C. At the start, there is a ball hidden under the cup at position B. • There will be several swap perform, represented by two letters.
• For example, if I swap the cups at positions A and B, this can be represented as `AB` or `BA`.

### Examples

``````cup_swapping(["AB", "CA"]) ➞ "C"

cup_swapping(["AC", "CA", "CA", "AC"]) ➞ "B"

cup_swapping(["BA", "AC", "CA", "BC"]) ➞ "A"
``````

## My solution

• 1. initialise a current position "B"
• 2. iterate over the list of swap combination
• 2.1 iterate over each swap combination
• 3. check the ball have been swap
• 3.1 if current position is exist in the swap combination, it is swapped
• 3.2 update the current position
• 3.2.1 if current_position equals to the first letter
• 3.2.2 then final letter is the current position
• 3.3.1 if current_position not equal the first letter
• 3.3.2 then first letter is the current position
• 4. print the final position
``````def cup_swapping(swaps):
current_position = "B"
for move in swaps:
if current_position in move:
if current_position == move:
current_position = move
else:
current_position = move
return current_position
``````

## Solution by others

### Method 1

• shorten version of my answer
``````def cup_swapping(swaps):
current_position = "B"
for move in swaps:
if current_position in move:
current_position = move if move == current_position else move
return current_position
``````

### Method 2

``````def cup_swapping(swaps, current_position="B"):
for move in swaps:
current_position = move.replace(current_position, "") if current_position in move else current_position
return current_position
``````
• key point
• iterate over each swap
• in the swap, there is two letter
• if the current position appear in the swap, replace that letter with empty string
• i.e. replace "AB" to "A" if current position is "A"
• and also update the current position to the remaining single letter
• if the current position do not appear in the swap, keep the current position

## My reflection

• It is a good feeling to solve a problem without hint. First I got stuck when just write the code, then I try to solve the question by writing the algorithm down, it sudden feel much easier to think. Thus, I will make the habit the write down the algorithm first rather coding at first. Beside, I learn new way to shorten the if else statement

## Credit

challenge found on edabit