Intro to Computer Science Practice Questions

These are 12 practice questions from Encodr's free Intro to Computer Science deck, taken from 10 of the course's 16 units. They are the kind an intro Python course tests: read a short program, predict what it prints or which exception it raises, and know the one rule that explains why. Each question comes with the answer and the reason.

How to use this set

Cover the answer, write down your prediction, and only then read on. Do this on paper, with the value of each variable after each line, before you look at the options. Trying to answer before you see the solution is the testing effect at work, and it tends to work better than rereading the same material. If you get one wrong, find the exact line where your trace and Python disagreed. Then run the code and check.

A note on the first question: the answer comes from the fact that Python stores floats in binary. If you want to see base 2 for yourself, the number base converter shows the place-value working for any whole number.

The questions

1. Expressions and operators

What does this program print?

``python print(0.1 + 0.2) ``

• A. 0.3
• B. 0.30
• C. 0.30000000000000004
• D. 0.29999999999999993

Answer: C. 0.30000000000000004

Why: The binary approximations of 0.1 and 0.2 sum to a value just above 0.3, and print shows all the digits needed to tell it apart.

2. Objects, variables and types

What does this print?

``python grid = [[0] * 2] * 2 grid[0][0] = 9 print(grid) ``

• A. [[9, 0], [0, 0]]
• B. [[9, 0], [9, 0]]
• C. [[9, 9], [0, 0]]
• D. [[0, 0], [0, 0]]

Answer: B. [[9, 0], [9, 0]]

Why: [[0] 2] 2 makes two references to the same row list, so setting grid[0][0] also shows up in grid[1][0].

3. Decisions

True or false? The condition if x == 3 or 4: is true only when x is 3 or 4.

``python x = 7 if x == 3 or 4: print("yes") ``

Answer: False

Why: It parses as (x == 3) or 4, and 4 is truthy, so the condition is always true.

4. Loops

A loop else clause runs only if the loop ends without hitting _____.

Answer: break

Why: A loop else clause runs when the loop finishes without hitting break, including when the loop body never ran. Only break (or return or an exception) skips it.

5. Functions and scope

What does this program print?

``python def add(x, lst=[]): lst.append(x) return lst a = add(1) b = add(2) print(a is b, a) ``

• A. False [1]
• B. False [1, 2]
• C. True [1, 2]
• D. True [2]

Answer: C. True [1, 2]

Why: Both calls return the same default list, so a is b is True, and by the time it prints it holds both items.

6. Functions and scope

Which exact exception class does f() raise?

``python x = 5 def f(): print(x) x = 10 f() ``

• A. NameError
• B. TypeError
• C. ValueError
• D. UnboundLocalError

Answer: D. UnboundLocalError

Why: The assignment x = 10 makes x local for the whole function, so print(x) reads a local that has no value yet.

7. Strings

Why is "10" < "9" True in Python?

``python print("10" < "9") ``

• A. The first differing character decides, and "1" < "9"
• B. Python compares the numbers 10 and 9, and 10 is smaller
• C. Python converts both strings to integers before comparing
• D. Longer strings are always considered smaller than shorter

Answer: A. The first differing character decides, and "1" < "9"

Why: String comparison is by characters, not numeric value. The first characters "1" and "9" differ and decide it.

8. Dictionaries, sets and tuples

What does this print?

``python d = {"a": 1, "a": 2} print(d, len(d)) ``

• A. {'a': 2} 1
• B. {'a': 1} 1
• C. {'a': 1, 'a': 2} 2
• D. SyntaxError

Answer: A. {'a': 2} 1

Why: The later "a": 2 overwrites the earlier pair, leaving one key.

9. Recursion

What happens when this program runs?

``python def total_rec(n): return 0 if n == 0 else n + total_rec(n - 1) print(total_rec(10000)) ``

• A. It prints 50005000
• B. It prints 0
• C. A RecursionError is raised
• D. An OverflowError is raised

Answer: C. A RecursionError is raised

Why: Each call adds a stack frame, and about 1000 nested calls is the default limit, so 10000 raises RecursionError.

10. Recursion

True or false? Python removes tail calls, so a function whose last action is the recursive call never hits the recursion limit.

Answer: False

Why: Python has no tail-call elimination; count(5000) style recursion still raises RecursionError.

11. Searching, sorting and Big-O

A binary search for 3 is run on the unsorted list [5, 1, 9, 3, 7]. The middle item is 9, which is larger than 3, so it keeps only the left half and continues. What is the outcome?

• A. It finds 3 at index 3
• B. It finds 3 but takes more comparisons than usual
• C. It raises an IndexError
• D. It reports not found, even though 3 is in the list

Answer: D. It reports not found, even though 3 is in the list

Why: Discarding the right half on a hunch about order threw away the target, so the result is "not found". Unsorted data breaks the precondition.

12. Files and exceptions

Which exception does this raise?

``python int("3.5") ``

• A. TypeError
• B. ValueError
• C. IndexError
• D. NameError

Answer: B. ValueError

Why: The string "3.5" is the right type but is not a valid integer literal, so int() raises ValueError.

Where to go from here

Count how many you got right, then look at which units the misses came from. A cluster of misses in one area, such as aliasing or scope, means that chapter needs another pass, not the whole course.

• The full set is in the free Intro to Computer Science deck, which spaces your reviews so the traps you miss come back sooner.
• What's in Intro to Computer Science lists all 16 units in order.
• How to study for Intro to Computer Science covers tracing, the common traps and how to practice for the written exam.
• The binary search question above sits in the Big-O unit; Big-O notation explained for beginners walks through the idea from scratch.
• Build a plan around the exam date with how to build a study schedule before an exam.