Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 13 | C++
13.1 Write a method to print the last K lines of an input file using C++.
pg 76
SOLUTION
One brute force way could be to count the number of lines (N) and then print from N-10 to
Nth line. But, this requires two reads of the file – potentially very costly if the file is large.
We need a solution which allows us to read just once and be able to print the last K lines. We
can create extra space for K lines and then store each set of K lines in the array. So, initially,
our array has lines 0 through 9, then 1 through 10, then 2 through 11, etc (if K = 10). Each
time that we read a new line, we purge the oldest line from the array. Instead of shifting the
array each time (very inefficient), we will use a circular array. This will allow us to always find
the oldest element in O(1) time.
Example of inserting elements into a circular array:
Code:
step 1 (initially): array = {a, b, c, d, e, f}. p = 0
step 2 (insert g): array = {g, b, c, d, e, f}. p = 1
step 3 (insert h): array = {g, h, c, d, e, f}. p = 2
step 4 (insert i): array = {g, h, i, d, e, f}. p = 3
1 string L[K];
2 int lines = 0;
3 while (file.good()) {
4 getline(file, L[lines % K]); // read file line by line
5 ++lines;
6 }
7 // if less than K lines were read, print them all
8 int start, count;
9 if (lines < K) {
10 start = 0;
11 count = lines;
12 } else {
13 start = lines % K;
14 count = K;
15 }
16 for (int i = 0; i < count; ++i) {
17 cout