We use cookies (including Google cookies) to personalize ads and analyze traffic. By continuing to use our site, you accept our Privacy Policy.

SimplyLeet
TagsDisclaimersPrivacyTermsContactAbout Us

Read N Characters Given Read4

Number: 157

Difficulty: Easy

Paid? Yes

Companies: Google, Amazon, Meta

Problem Description

Given a file and an API read4 that reads up to 4 characters from the file at a time, implement a function read that reads exactly n characters and stores them into a buffer. The function should repeatedly call read4 until either n characters have been read or the end of file is reached.

Key Insights

  • The only available API to read characters from the file is read4, which returns a maximum of four characters at each call.
  • The function must keep track of the total number of characters read until it reaches n or EOF.
  • After each call to read4, copy the required number of characters (up to n) from the temporary buffer to the destination buffer.
  • Stop calling read4 when either n characters have been copied or when read4 returns less than 4 characters, indicating the end of the file.

Space and Time Complexity

Time Complexity: O(n) in the worst-case, as the algorithm reads and copies characters up to n. Space Complexity: O(1), aside from the constant-sized temporary buffer used for read4 (of size 4).

Solution

The solution uses a loop to repeatedly call the read4 function, which reads up to 4 characters into a temporary buffer. After each call, copy the relevant number of characters (which might be less than 4 if approaching n or when the end of file is reached) from the temporary buffer to the destination buffer. Use variables to track the total number of characters read so far and stop the process once n characters have been transferred to the destination buffer or when read4 returns less than 4 characters, indicating no more data to read.

Code Solutions

# Function read4 is assumed to be defined externally.
# This is a sample implementation for the read function.
def read(buf, n):
    total_read = 0  # Total number of characters read so far
    eof = False     # End-of-file indicator
    temp = [''] * 4  # Temporary buffer to store characters read by read4
    
    while total_read < n and not eof:
        # Number of characters read in this call
        count = read4(temp)
        # If count is less than 4, we reached end-of-file.
        if count < 4:
            eof = True
        
        # Copy from temp buffer to destination buffer.
        i = 0
        while i < count and total_read < n:
            buf[total_read] = temp[i]
            total_read += 1
            i += 1
    return total_read

# Note: In an actual runtime environment, the read4 API is provided.
← Back to All Questions