Read N Characters Given Read4
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ArraysStringsTwo Pointers
The API: int read4(char *buf) reads exactly 4 characters from the file. The return value is the actual number of characters read. For example, it returns 3 if there is only 3 characters left in the file. By using the read4 API, implement the function int read(char *buf, int n) that reads n characters from the file.
Note:
The read function may be called multiple times.
Example 1:
Input: buf = "abc", n = 4 Output: "abc" Explanation: The actual number of characters read is 3, which is less than n.
Example 2:
Input: buf = "abcde", n = 5 Output: "abcde"
Note:
- Consider that you cannot modify the file.
- Is there really a file?
- The file is represented by an object that can be passed only to your solution.
- You will not be able to test your solution with the judge program.
Solution
Clarifying Questions
When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:
- What is the size of the input buffer `buf` and what is its initial state (empty or potentially containing data)?
- What should I return if `read4` returns 0 before `n` characters have been read (e.g., end of file reached)?
- Can `n` be 0, negative, or larger than the size of `buf`?
- Does `read4` always return 4 characters unless it reaches the end of the file?
- Is the `buf` passed to `read` guaranteed to be large enough to hold `n` characters?
Brute Force Solution
Approach
The goal is to read a specific number of characters from a source that can only read in chunks of four. The brute force strategy involves repeatedly reading chunks and then carefully selecting the characters we need until we've read the target amount or the source is exhausted.
Here's how the algorithm would work step-by-step:
- Try to read the first chunk of four characters from the source.
- Check how many characters were actually read in that chunk.
- Take the characters we need from that chunk, up to the number of characters we still need to read overall.
- If we've read enough characters in total, or if the chunk read was shorter than four (meaning the source is finished), then we're done.
- Otherwise, try to read the next chunk of four characters, and repeat the process of taking what we need from it.
- Keep going until we have read the target amount of characters or there are no more characters left in the source to read.
Code Implementation
def read_n_characters(read4, buffer, number_of_characters_to_read):
characters_read = 0
temporary_buffer = [''] * 4
while characters_read < number_of_characters_to_read:
# Read up to 4 characters at a time from the read4 function
characters_from_read4 = read4(temporary_buffer)
if characters_from_read4 == 0:
break
# Determine how many characters to copy to the buffer
characters_to_copy = min(number_of_characters_to_read - characters_read, characters_from_read4)
# Copy characters from the temporary buffer to the output buffer
for i in range(characters_to_copy):
buffer[characters_read] = temporary_buffer[i]
characters_read += 1
return characters_readBig(O) Analysis
Time Complexity
O(n) – The algorithm reads at most n characters. The `read4` function is called repeatedly, but the total number of characters read across all calls is bounded by n, the number of characters we want to read. The loop iterates until we have read n characters or `read4` returns less than 4, indicating the end of the input. Therefore, the time complexity is directly proportional to n, resulting in O(n).
Space Complexity
O(1) – The algorithm reads chunks of four characters and selects from them. It doesn't store all the read characters at once in a new data structure proportional to the input size N. It uses a fixed-size buffer to read four characters at a time from Read4, and variables to track progress, but the size of this buffer is constant, independent of N. Thus, the auxiliary space used is constant, resulting in O(1) space complexity.
Optimal Solution
Approach
The challenge is to read characters from a file using a provided helper function that reads only a limited number of characters at a time. Our goal is to efficiently manage the characters read from the helper function to fulfill the requested number of characters, handling cases where the file has fewer characters than requested.
Here's how the algorithm would work step-by-step:
- Use the helper function to read up to four characters from the file.
- Keep track of how many characters were actually read by the helper function in the previous step.
- Copy the characters you've read into the provided output buffer, up to the required number of characters.
- If the helper function read fewer than four characters, it means you've reached the end of the file. Stop reading and return the number of characters you've successfully copied into the output buffer.
- Repeat the reading and copying process until either you've read the desired number of characters or you've reached the end of the file.
Code Implementation
def read_n_characters(buffer, number_of_characters_to_read):
characters_read = 0
temporary_buffer = [''] * 4
while characters_read < number_of_characters_to_read:
# Read up to 4 characters using the helper function.
characters_from_read4 = read4(temporary_buffer)
# Check if end of file is reached.
if characters_from_read4 == 0:
break
# Copy characters from temp buffer to output buffer.
characters_to_copy = min(number_of_characters_to_read - characters_read, characters_from_read4)
for i in range(characters_to_copy):
buffer[characters_read] = temporary_buffer[i]
characters_read += 1
return characters_readBig(O) Analysis
Time Complexity
O(n) – The dominant operation is the while loop that iterates until we've read 'n' characters or the read4 function returns less than 4 characters indicating the end of the file. Inside the loop, we call read4 which takes constant time. We then copy characters from the read4 buffer to the output buffer, which takes at most 4 units of time in each iteration. Since we iterate at most n/4 times (each read4 reads 4 characters), the time complexity is proportional to n. Therefore, the overall time complexity is O(n).
Space Complexity
O(1) – The solution uses a fixed-size buffer (4 characters) provided by the read4 function internally. Besides this buffer, the solution primarily uses a few integer variables to track the number of characters read and copied. The space used by these variables is constant and does not depend on the input size N, which represents the number of characters to read. Therefore, the auxiliary space complexity is O(1).
Edge Cases
| Case | How to Handle |
|---|---|
| n is zero | Return 0 immediately, as no characters need to be read. |
| read4 returns 0 (end of file) before reading n characters | Stop reading and return the number of characters actually read. |
| n is larger than the total characters available from the file | Read until read4 returns 0 and return the actual number of characters read. |
| read4 returns fewer than 4 characters, but more than 0 | Copy the available characters and continue reading if needed until n characters are read or EOF is reached. |
| Repeated calls to read with overlapping buffers and shared read4 buffer | The read4 buffer must be handled correctly between calls to read, ensuring no data corruption. |
| Memory allocation failure when creating the buffer (out of memory) | Throw an exception or return an error code to indicate memory allocation failure. |
| read4 returns a negative value or throws an exception | Handle the error appropriately (e.g., throw an exception or return an error code). |
| read4 buffer contains unicode characters | Ensure that the character encoding is handled correctly when copying from the read4 buffer to the output buffer. |
n is zero
How to Handle:
Return 0 immediately, as no characters need to be read.
read4 returns 0 (end of file) before reading n characters
How to Handle:
Stop reading and return the number of characters actually read.
n is larger than the total characters available from the file
How to Handle:
Read until read4 returns 0 and return the actual number of characters read.
read4 returns fewer than 4 characters, but more than 0
How to Handle:
Copy the available characters and continue reading if needed until n characters are read or EOF is reached.
Repeated calls to read with overlapping buffers and shared read4 buffer
How to Handle:
The read4 buffer must be handled correctly between calls to read, ensuring no data corruption.
Memory allocation failure when creating the buffer (out of memory)
How to Handle:
Throw an exception or return an error code to indicate memory allocation failure.
read4 returns a negative value or throws an exception
How to Handle:
Handle the error appropriately (e.g., throw an exception or return an error code).
read4 buffer contains unicode characters
How to Handle:
Ensure that the character encoding is handled correctly when copying from the read4 buffer to the output buffer.
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