EmSAT Computer Science

Question 1

In Python, what is the output of the following code?

x = [10, 20, 30, 40, 50]
print(x[-2])

Accepted Answer

40

Answer

Negative indices in Python count from the end of the list. Index -1 is the last element (50), so index -2 is the second-to-last element, which is 40.

Question 2

Which data structure follows the Last-In, First-Out (LIFO) principle?

Accepted Answer

Stack

Answer

A stack operates on the LIFO principle — the last element pushed onto the stack is the first one to be popped off. A queue uses FIFO (First-In, First-Out) instead.

Question 3

A UAE university's IT department assigns the network address 192.168.10.0/24 to a campus lab. How many usable host IP addresses are available on this subnet?

Accepted Answer

254

Answer

A /24 subnet contains 2^8 = 256 addresses. Two are reserved: one for the network address (192.168.10.0) and one for the broadcast address (192.168.10.255). That leaves 256 − 2 = 254 usable host addresses.

Question 4

In a relational database, which constraint ensures that a column's value uniquely identifies each row and cannot be NULL?

Accepted Answer

PRIMARY KEY

Answer

A PRIMARY KEY combines two guarantees: uniqueness (no two rows can share the same value) and NOT NULL (the value must always be present). A UNIQUE constraint allows NULLs; a FOREIGN KEY references another table's key; CHECK validates a condition.

Question 5

Which layer of the OSI model is responsible for end-to-end data delivery, error detection, and flow control between two hosts?

Accepted Answer

Transport Layer (Layer 4)

Answer

The Transport Layer (Layer 4) handles end-to-end communication using protocols such as TCP (reliable, connection-oriented) and UDP (faster, connectionless). TCP provides error recovery and flow control. The Network Layer routes packets between networks; the Data Link Layer handles node-to-node delivery within the same network.

EmSAT Practice Test

EmSAT Practice Test

EmSAT Computer Science