A student designing a subway map app draws each station as a dot and each line as a straight colored segment, leaving out street names, true distances, and the curves of the actual track. Which computational thinking practice is the student mainly applying?

Abstraction, by keeping only the details riders need to plan a route

Decomposition, by splitting the subway system into separate lines

Pattern recognition, by noticing that every station is drawn the same way

Algorithm design, by writing ordered steps for traveling between stations

A student proposes writing a program that reads the code of any program together with its input and always reports correctly whether that program will eventually stop. Which response is accurate?

No such program can exist; Turing proved the halting problem is undecidable

Such a program can exist, but only for programs written in compiled languages

Such a program can exist if it runs each program for a very long time limit

Such a program can exist, but it would always require exponential running time

Pseudocode nums = [4, 9, 2, 7] total = 0 for (i = 0; i <= length(nums); i = i + 1) { total = total + nums[i] } print (total) Lists are indexed starting at 0. What happens when the code below runs?

An index-out-of-bounds error occurs when i reaches 4

It prints 18 because the loop skips the first element

It prints 13 because the loop skips the last element

It prints 22 because the loop visits every element once

A grade 7 teacher uses the PRIMM approach for a lesson on loops. Students have just predicted the output of a short working program, run it, and compared the result with their predictions. What should students do next?

Answer targeted questions about how specific lines work, such as tracing or annotating the loop

Take a summative quiz on loops to confirm that the class is ready to move to the next unit

Change the loop's starting value and range to make the program produce a different output

Write an original program from scratch that uses a loop to solve a new problem of their choosing

A student writes separate block scripts to draw a square, a pentagon, and a hexagon, then notices that each one repeats a move-then-turn pair, changing only the number of repeats and the turn angle. Which practice is the student using?

Pattern recognition, by identifying the shared structure across the scripts

Debugging, by comparing scripts to locate an error in one of them

Abstraction, by hiding how the sprite physically moves across the stage

Decomposition, by dividing each shape into separate sides and angles

Problem X can be solved exactly by a known algorithm, but the running time grows exponentially with input size. Problem Y has no algorithm that gives a correct answer for every possible input. How are these problems classified?

X is intractable and Y is undecidable

Both X and Y are undecidable problems

Both X and Y are intractable problems

X is undecidable and Y is intractable

During live coding, a teacher intentionally types a loop that runs one time too many, runs the program, and thinks aloud while locating and fixing the error. What is the primary instructional purpose of this technique?

To model authentic debugging and show that errors are a normal, fixable part of programming

To show students that the language's tools are unreliable and should be avoided when possible

To save preparation time, since the teacher does not need to write any code in advance

To test whether students are paying attention by seeing who notices the mistake first

OSAT Computer Science (181)

Free OSAT Computer Science (181) Practice Questions

10 sample questions for the OSAT Computer Science (181), with the correct answer and an explanation for each. No account needed. Try each one before you open the answer.

Sections covered on the exam: Computational Thinking and Algorithm Design, Programming and Program Development, Computer System Concepts, Learning Environments and Social Impacts of Computing, Pedagogy, Preconditions.

Question 1 · Computational Thinking and Algorithm Design
A student is reading a memory dump and needs to rewrite the 8-bit binary value 10110110 in hexadecimal. Which hexadecimal value is equivalent?
- A.6B
- B.B5
- C.D6
- D.B6
Show answer and explanation
Answer: D. B6
Split the byte into 4-bit groups: 1011 = 11 = B and 0110 = 6, so the value is B6. Writing 6B reverses the two nibbles, and D6 comes from misreading 1011 as 1101.
Question 2 · Computational Thinking and Algorithm Design
A student designing a subway map app draws each station as a dot and each line as a straight colored segment, leaving out street names, true distances, and the curves of the actual track. Which computational thinking practice is the student mainly applying?
- A.Decomposition, by splitting the subway system into separate lines
- B.Abstraction, by keeping only the details riders need to plan a route
- C.Pattern recognition, by noticing that every station is drawn the same way
- D.Algorithm design, by writing ordered steps for traveling between stations
Show answer and explanation
Answer: B. Abstraction, by keeping only the details riders need to plan a route
Abstraction means filtering out details that do not matter for the purpose at hand; riders need connections and order of stations, not street names or exact track curves. Decomposition would involve breaking the problem into smaller subproblems, which is not what removing detail from the map does.
Question 3 · Computational Thinking and Algorithm Design
A student proposes writing a program that reads the code of any program together with its input and always reports correctly whether that program will eventually stop. Which response is accurate?
- A.Such a program can exist, but only for programs written in compiled languages
- B.Such a program can exist if it runs each program for a very long time limit
- C.Such a program can exist, but it would always require exponential running time
- D.No such program can exist; Turing proved the halting problem is undecidable
Show answer and explanation
Answer: D. No such program can exist; Turing proved the halting problem is undecidable
Turing showed by contradiction that no algorithm can decide halting correctly for every program-input pair, so the problem is undecidable, not merely slow. A time limit fails because a program that has not stopped yet might still stop later, so the checker cannot distinguish slow programs from infinite loops.
Question 4 · Computational Thinking and Algorithm Design
Pseudocodenums = [4, 9, 2, 7] total = 0 for (i = 0; i <= length(nums); i = i + 1) { total = total + nums[i] } print(total)
Lists are indexed starting at 0. What happens when the code below runs?
- A.It prints 18 because the loop skips the first element
- B.It prints 13 because the loop skips the last element
- C.It prints 22 because the loop visits every element once
- D.An index-out-of-bounds error occurs when i reaches 4
Show answer and explanation
Answer: D. An index-out-of-bounds error occurs when i reaches 4
The list has 4 elements with valid indexes 0 through 3, but i <= length(nums) lets i reach 4. Reading nums[4] goes past the end, causing an out-of-bounds runtime error before anything is printed. Using i < length(nums) would visit each element once and print 22.
Question 5 · Computational Thinking and Algorithm Design
A grade 7 teacher uses the PRIMM approach for a lesson on loops. Students have just predicted the output of a short working program, run it, and compared the result with their predictions. What should students do next?
- A.Take a summative quiz on loops to confirm that the class is ready to move to the next unit
- B.Change the loop's starting value and range to make the program produce a different output
- C.Write an original program from scratch that uses a loop to solve a new problem of their choosing
- D.Answer targeted questions about how specific lines work, such as tracing or annotating the loop
Show answer and explanation
Answer: D. Answer targeted questions about how specific lines work, such as tracing or annotating the loop
PRIMM stands for Predict, Run, Investigate, Modify, Make, so after running the code students investigate it through focused questions, tracing, and annotation. Changing the loop's range is the Modify stage, which comes after investigation, and writing an original program is the final Make stage.
Question 6 · Computer System Concepts
A file permission is shown as the octal number 345 in a system log. A student needs the same value in decimal for a worksheet. What is octal 345 written in base ten?
- A.837
- B.239
- C.229
- D.217
Show answer and explanation
Answer: C. 229
Octal place values are 64, 8, and 1, so 3 × 64 + 4 × 8 + 5 × 1 = 192 + 32 + 5 = 229. The value 837 results from reading 345 as hexadecimal (3 × 256 + 4 × 16 + 5).
Question 7 · Computational Thinking and Algorithm Design
A student writes separate block scripts to draw a square, a pentagon, and a hexagon, then notices that each one repeats a move-then-turn pair, changing only the number of repeats and the turn angle. Which practice is the student using?
- A.Pattern recognition, by identifying the shared structure across the scripts
- B.Debugging, by comparing scripts to locate an error in one of them
- C.Abstraction, by hiding how the sprite physically moves across the stage
- D.Decomposition, by dividing each shape into separate sides and angles
Show answer and explanation
Answer: A. Pattern recognition, by identifying the shared structure across the scripts
Noticing that different problems share a common structure is pattern recognition; it often leads next to abstraction, such as a single drawPolygon(sides) procedure. Decomposition is tempting, but the student is comparing whole solutions for similarities, not breaking one problem into parts.
Question 8 · Computational Thinking and Algorithm Design
Problem X can be solved exactly by a known algorithm, but the running time grows exponentially with input size. Problem Y has no algorithm that gives a correct answer for every possible input. How are these problems classified?
- A.Both X and Y are undecidable problems
- B.Both X and Y are intractable problems
- C.X is undecidable and Y is intractable
- D.X is intractable and Y is undecidable
Show answer and explanation
Answer: D. X is intractable and Y is undecidable
An intractable problem is solvable in principle but not in reasonable (polynomial) time for large inputs, while an undecidable problem, such as the halting problem, has no algorithm at all that is always correct. Reversing the labels confuses being too slow with being impossible.
Question 9 · Computational Thinking and Algorithm Design
Pseudocodenums = [4, 7, 1, 9, 3] for (i = 1; i < length(nums); i = i + 1) { nums[i] = nums[i] + nums[i - 1] } print(nums[3])
Lists are indexed starting at 0. The loop below changes the list as it runs. What value is printed after the loop ends?
- A.21
- B.24
- C.12
- D.10
Show answer and explanation
Answer: A. 21
Each element becomes itself plus the already-updated element before it, so the list becomes [4, 11, 12, 21, 24]. nums[3] is 9 + 12 = 21. The answer 10 uses the original neighbor (9 + 1), forgetting that nums[2] was already changed to 12.
Question 10 · Learning Environments and Social Impacts of Computing
During live coding, a teacher intentionally types a loop that runs one time too many, runs the program, and thinks aloud while locating and fixing the error. What is the primary instructional purpose of this technique?
- A.To show students that the language's tools are unreliable and should be avoided when possible
- B.To save preparation time, since the teacher does not need to write any code in advance
- C.To model authentic debugging and show that errors are a normal, fixable part of programming
- D.To test whether students are paying attention by seeing who notices the mistake first
Show answer and explanation
Answer: C. To model authentic debugging and show that errors are a normal, fixable part of programming
Live coding with deliberate mistakes makes an expert's debugging process visible, including reading output, forming hypotheses, and fixing off-by-one errors, and normalizes errors for novices. Catching inattentive students may be a side effect, but the technique's purpose is modeling the problem-solving process.

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