ISRO Scientist/Engineer Computer Science 2023 question paper
All 95 questions of ISRO's written test for Scientist/Engineer 'SC' (Computer Science), held on 7 January 2024, with ISRO's answer key. ISRO excluded Q.4, Q.5, Q.12, Q.23, Q.28 and Q.46 from evaluation.
Answers from ISRO's revised and final answer key of 16 February 2024.
- questions · 100 marks
- 95questions · 100 marks
- Part A: Computer Science
- 80Part A: Computer Science
- Part B: aptitude
- 15Part B: aptitude
How the paper is marked
- Part A (Q.1 to Q.80): 1 mark for a correct answer, minus 1/3 for a wrong answer.
- Part B (Q.81 to Q.95): 1 or 2 marks as printed with each question, 20 marks in all, no negative marking.
Source: ISRO's recruitment advertisement ISRO:ICRB:02(EMC):2023 of 25 May 2023, item 16(b).
ISRO CS 2023 answer key
| Question | Part | Marks | Answer |
|---|---|---|---|
| Q.1 | A | 1 | B |
| Q.2 | A | 1 | B |
| Q.3 | A | 1 | A or B |
| Q.4 | A | 1 | Excluded |
| Q.5 | A | 1 | Excluded |
| Q.6 | A | 1 | A |
| Q.7 | A | 1 | A |
| Q.8 | A | 1 | C |
| Q.9 | A | 1 | C |
| Q.10 | A | 1 | D |
| Q.11 | A | 1 | B |
| Q.12 | A | 1 | Excluded |
| Q.13 | A | 1 | D |
| Q.14 | A | 1 | C |
| Q.15 | A | 1 | A |
| Q.16 | A | 1 | C |
| Q.17 | A | 1 | A |
| Q.18 | A | 1 | B |
| Q.19 | A | 1 | B |
| Q.20 | A | 1 | B |
| Q.21 | A | 1 | B |
| Q.22 | A | 1 | C |
| Q.23 | A | 1 | Excluded |
| Q.24 | A | 1 | C |
| Q.25 | A | 1 | B |
| Q.26 | A | 1 | D |
| Q.27 | A | 1 | A |
| Q.28 | A | 1 | Excluded |
| Q.29 | A | 1 | A |
| Q.30 | A | 1 | A |
| Q.31 | A | 1 | B |
| Q.32 | A | 1 | C |
| Q.33 | A | 1 | D |
| Q.34 | A | 1 | A |
| Q.35 | A | 1 | A |
| Q.36 | A | 1 | A |
| Q.37 | A | 1 | C |
| Q.38 | A | 1 | D |
| Q.39 | A | 1 | D |
| Q.40 | A | 1 | D |
| Q.41 | A | 1 | C |
| Q.42 | A | 1 | D |
| Q.43 | A | 1 | C |
| Q.44 | A | 1 | A |
| Q.45 | A | 1 | D |
| Q.46 | A | 1 | Excluded |
| Q.47 | A | 1 | C or D |
| Q.48 | A | 1 | A |
| Q.49 | A | 1 | C |
| Q.50 | A | 1 | C |
| Q.51 | A | 1 | D |
| Q.52 | A | 1 | B |
| Q.53 | A | 1 | C |
| Q.54 | A | 1 | A |
| Q.55 | A | 1 | A |
| Q.56 | A | 1 | B |
| Q.57 | A | 1 | A |
| Q.58 | A | 1 | D |
| Q.59 | A | 1 | B |
| Q.60 | A | 1 | B |
| Q.61 | A | 1 | C |
| Q.62 | A | 1 | D |
| Q.63 | A | 1 | A or B |
| Q.64 | A | 1 | C |
| Q.65 | A | 1 | B |
| Q.66 | A | 1 | A |
| Q.67 | A | 1 | B |
| Q.68 | A | 1 | C |
| Q.69 | A | 1 | A |
| Q.70 | A | 1 | B |
| Q.71 | A | 1 | A |
| Q.72 | A | 1 | A |
| Q.73 | A | 1 | A |
| Q.74 | A | 1 | D |
| Q.75 | A | 1 | C |
| Q.76 | A | 1 | C |
| Q.77 | A | 1 | C |
| Q.78 | A | 1 | B |
| Q.79 | A | 1 | B or C |
| Q.80 | A | 1 | A |
| Q.81 | B | 1 | D |
| Q.82 | B | 1 | D |
| Q.83 | B | 2 | B |
| Q.84 | B | 1 | A |
| Q.85 | B | 1 | A |
| Q.86 | B | 2 | A |
| Q.87 | B | 1 | A |
| Q.88 | B | 1 | C |
| Q.89 | B | 2 | C |
| Q.90 | B | 1 | C |
| Q.91 | B | 1 | D |
| Q.92 | B | 2 | D |
| Q.93 | B | 1 | A |
| Q.94 | B | 1 | C |
| Q.95 | B | 2 | A |
ISRO excluded Q.4, Q.5, Q.12, Q.23, Q.28 and Q.46 from evaluation.
Part A: Computer Science Q.1 to Q.80
- Q.11 markOfficial answer: B
Tap to open full sizeData Structures and Algorithms › Graph algorithms
Full solutionA minimum spanning tree of a connected graph with vertices always has edges. Kruskal's algorithm builds one by taking the lightest edges that do not form a cycle.
- Q.21 markOfficial answer: B
Tap to open full sizeData Structures and Algorithms › Graph algorithms
Full solutionMatch each algorithm to its defining problem: Floyd-Warshall solves all pair shortest paths, Kruskal's builds a minimum spanning tree, and Dijkstra and Bellman-Ford are single-source methods. Fix the two unambiguous matches first and use them to eliminate options.
- Q.31 markOfficial answer: A or B
Tap to open full sizeEarlier, ISRO's key gave A.
Data Structures and Algorithms › Trees & heaps
Full solutionCompute subtree heights and the best path through each node in a single post-order traversal, so each node is handled once in constant time.
- Q.41 markExcluded from evaluation by ISRO
Tap to open full sizeEarlier, ISRO's key gave B.
- Q.51 markExcluded from evaluation by ISRO
Tap to open full sizeEarlier, ISRO's key gave B.
- Q.61 markOfficial answer: A
Tap to open full sizeProbability and Statistics › Counting & axioms
Full solutionA simple graph on labelled vertices is fixed by choosing, for each of the vertex pairs, whether the edge is present, giving graphs.
- Q.71 markOfficial answer: A
Tap to open full sizeCompiler Design › Lexical analysis
Full solutionRead an NFA as a regular expression piece by piece. An alternation is two parallel branches joined by -edges. An -edge from the join back to an earlier final state is a Kleene star. The edge before that state is a prefix that appears only once.
- Q.81 markOfficial answer: C
Tap to open full sizeTheory of Computation › CFG & PDA
Full solutionCount parse trees by choosing the root production, which fixes which operator is applied last. Then count the trees of the two sub-strings. With no precedence in the grammar, each operator can be the root.
- Q.91 markOfficial answer: C
Tap to open full sizeOperating Systems › Memory management
Full solutionNext Fit continues searching from the point of the last allocation, so consecutive requests are placed in adjacent memory. Data allocated together sits together, which is spatial locality.
- Q.101 markOfficial answer: D
Tap to open full sizeCompiler Design › Lexical analysis
Full solutionEach compiler phase takes the output of the one before it. Tokens come before the parse tree, the parse tree before semantic checks, and the checked tree before intermediate code. Optimization works on that intermediate code, so it comes after it.
- Q.111 markOfficial answer: B
Tap to open full sizeFull solutionRead LL(k) as: first L = Left-to-right scan of the input, second L = Leftmost derivation, k = number of lookahead symbols. LR parsers instead give a Rightmost derivation in reverse.
- Q.121 markExcluded from evaluation by ISRO
Tap to open full sizeEarlier, ISRO's key gave C.
- Q.131 markOfficial answer: D
Tap to open full sizeComputer Networks › Data link & MAC
Full solutionCSMA/CD requires , so the rate that fits the minimum frame to the round trip exactly (and is the highest allowed) is .
- Q.141 markOfficial answer: C
Tap to open full sizeComputer Networks › Layering & switching
Full solutionEvery layer adds its own header, so the overhead is (number of layers x header size). Wasted bandwidth is that overhead divided by the total bytes transmitted (payload plus headers).
- Q.151 markOfficial answer: A
Tap to open full sizeComputer Networks › Data link & MAC
Full solutionRemember the peak rate of each 802.11 amendment: b = 11 Mbps, a/g = 54 Mbps, ac = about 3.5 Gbps in the usual textbook figure, ad = about 7 Gbps. Fix the three firm ones first and the last pair follows.
- Q.161 markOfficial answer: C
Tap to open full sizeComputer Networks › Data link & MAC
Full solutionParity and CRC only detect errors. Reed-Solomon and LDPC carry enough structured redundancy to correct them.
- Q.171 markOfficial answer: A
Tap to open full sizeData Structures and Algorithms › Linear structures
Full solutionApply precedence first, then place every operator after its two operands: the two products are converted separately and the
+goes last. - Q.181 markOfficial answer: B
Tap to open full sizeComputer Organization and Architecture › I/O, interrupts & DMA
Full solutionIn a vectored interrupt the device sends the vector, which tells the processor where its service routine is, so no polling is needed.
- Q.191 markOfficial answer: B
Tap to open full sizeComputer Organization and Architecture › Pipelining
Full solutionExecution time is . A super-scalar design issues several instructions per cycle, which lowers CPI below 1 and shortens the run for the same instruction count.
- Q.201 markOfficial answer: B
Tap to open full sizeComputer Organization and Architecture › Pipelining
Full solutionInstructions flow through a pipeline in program order, so the buffer holding them between stages must be first-in, first-out (an instruction prefetch queue).
- Q.211 markOfficial answer: B
Tap to open full sizeComputer Organization and Architecture › ALU & control unit
Full solutionAn operation performed on data held in registers is a micro-operation. Each machine instruction is carried out as a sequence of such micro-operations.
- Q.221 markOfficial answer: C
Tap to open full sizeComputer Networks › Data link & MAC
Full solutionParity and XOR checks are blind to certain even-numbered error patterns, while a CRC with a suitably chosen generator polynomial detects every double-bit error within a frame shorter than the generator's period.
- Q.231 markExcluded from evaluation by ISRO
Tap to open full sizeEarlier, ISRO's key gave A.
- Q.241 markOfficial answer: C
Tap to open full sizeData Structures and Algorithms › Complexity
Full solutionCompare growth rates by dividing out common factors or by taking logarithms: is smaller than any power , a function is super-polynomial but still below because .
- Q.251 markOfficial answer: B
Tap to open full sizeData Structures and Algorithms › Sorting & hashing
Full solutionRadix sort makes one pass per digit, and each pass handles every item against the possible digit values, so the count is with , , .
- Q.261 markOfficial answer: D
Tap to open full sizeData Structures and Algorithms › Trees & heaps
Full solutionIf a node has a left subtree, its inorder predecessor is the rightmost (largest) node of that left subtree: go left once, then right as far as possible.
- Q.271 markOfficial answer: A
Tap to open full sizeProgramming in C › Pointers & arrays
Full solutionAdding to a pointer advances it by elements, so the address changes by (element type) bytes.
- Q.281 markExcluded from evaluation by ISRO
Tap to open full sizeEarlier, ISRO's key gave D.
- Q.291 markOfficial answer: A
Tap to open full sizeData Structures and Algorithms › Sorting & hashing
Full solutionHeap sort builds the heap in and then performs extractions, each a sift-down of at most levels. The total is for every input, and it needs no extra array.
- Q.301 markOfficial answer: A
Tap to open full sizeDatabase Management and Warehousing › SQL
Full solutionEach new row is built from the current maxima, which always belong to the latest row. So the values follow , giving , while .
- Q.311 markOfficial answer: B
Tap to open full sizeDatabase Management and Warehousing › SQL
Full solutionA comma-separated list of tables in FROM with no condition is the Cartesian product, which is what CROSS JOIN means.
- Q.321 markOfficial answer: C
Tap to open full sizeDatabase Management and Warehousing › SQL
Full solutionWhen the condition for a DELETE depends on another table, delete from the target table and filter it with
IN (SELECT ...)on the related table. - Q.331 markOfficial answer: D
Tap to open full size - Q.341 markOfficial answer: A
Tap to open full sizeDatabase Management and Warehousing › Normal forms
Full solutionEach normal form is tied to a kind of dependency. Functional dependencies lead to 2NF, 3NF and BCNF. Multivalued dependencies lead to 4NF. Join dependencies lead to 5NF, also called Project-Join Normal Form.
- Q.351 markOfficial answer: A
Tap to open full sizeDigital Electronics › Combinational
Full solutionA multiplexer picks one of many inputs. A 1-to- demultiplexer needs select lines. An -to- encoder needs only OR gates, one per output bit.
- Q.361 markOfficial answer: A
Tap to open full sizeDigital Electronics › Sequential
Full solutionA serial shift register delays data by one clock period per stage, so total delay = (number of bits) × (clock period).
- Q.371 markOfficial answer: C
Tap to open full sizeDigital Electronics › Combinational
Full solutionA full adder adds three bits (two operands plus carry-in) and gives a two-bit result: sum and carry-out. A half adder has only two inputs because it has no carry-in.
- Q.381 markOfficial answer: D
Tap to open full sizeDigital Electronics › Boolean logic
Full solutionA NAND gate with its inputs tied together is an inverter. A NAND of two inverted signals is an OR gate, by De Morgan's theorem.
- Q.391 markOfficial answer: D
Tap to open full sizeEarlier, ISRO's key gave C.
Digital Electronics › Boolean logic
Full solutionXOR outputs LOW when its two inputs are equal and HIGH when they differ. XNOR is the opposite: HIGH when the inputs are equal.
- Q.401 markOfficial answer: D
Tap to open full sizeDigital Electronics › Boolean logic
Full solutionJoining both inputs of a NAND or NOR gate gives a single-input gate. Because and , the output is , an inverter.
- Q.411 markOfficial answer: C
Tap to open full sizeFull solutionIf one row of a square matrix is a scalar multiple of another, the rows are linearly dependent, the matrix is singular and its determinant is . Look for such a relation before expanding.
- Q.421 markOfficial answer: D
Tap to open full sizeDiscrete Mathematics › Graph theory
Full solutionHandshaking lemma gives degree sum ; the radius is the smallest eccentricity, so some vertex always attains it; girth is the number of edges in the shortest cycle; equal degrees mean a regular graph, not a multigraph.
- Q.431 markOfficial answer: C
Tap to open full sizeDiscrete Mathematics › Graph theory
Full solutionEuler's formula for a connected planar graph, , gives the missing count when two of the three are known. The outer (unbounded) region counts as a face.
- Q.441 markOfficial answer: A
Tap to open full sizeProbability and Statistics › Counting & axioms
Full solutionWith equally likely outcomes, probability is favourable count over total count. "Not white" can be counted directly (10 of 15) or found as 1 minus the probability of white.
- Q.451 markOfficial answer: D
Tap to open full sizeData Structures and Algorithms › Complexity
Full solutionCount the entries of the result () and multiply by the length of the inner sum for each entry (): three nested loops, each running once over its own dimension.
- Q.461 markExcluded from evaluation by ISRO
Tap to open full sizeEarlier, ISRO's key gave C.
- Q.471 markOfficial answer: C or D
Tap to open full sizeEarlier, ISRO's key gave C.
Discrete Mathematics › Mathematical logic
Full solutionEvery variable doubles the number of possible assignments, so variables give rows.
- Q.481 markOfficial answer: A
Tap to open full sizeOperating Systems › Processes & threads
Full solutionLinux creates every thread with clone(), and each thread is a separate kernel task that the scheduler sees. That makes the threading model one-to-one.
- Q.491 markOfficial answer: C
Tap to open full sizeOperating Systems › Synchronisation
Full solutionA lock holder temporarily inherits the priority of the highest-priority task waiting for the lock, so a medium-priority task cannot preempt it and delay the waiter indefinitely.
- Q.501 markOfficial answer: C
Tap to open full sizeOperating Systems › Memory management
The question and its official answerA segment number is an index into the process's segment table. The compiler decides what the segments are (code, data, stack, and so on). The loader builds the segment table when the program is loaded, so it is the loader that numbers the segments and fills in each one's base and limit.
- Q.511 markOfficial answer: D
Tap to open full sizeComputer Organization and Architecture › I/O, interrupts & DMA
Full solutionHardware interrupts are asynchronous and come from external devices. Traps, exceptions and system calls are synchronous and are raised by the running program (software).
- Q.521 markOfficial answer: B
Tap to open full sizeOperating Systems › CPU scheduling
Full solutionLong term scheduler: admits jobs and so controls the degree of multiprogramming. Medium term scheduler: swaps processes in and out. Short term scheduler: runs most often, so it must be the fastest.
- Q.531 markOfficial answer: C
Tap to open full sizeOperating Systems › Synchronisation
Full solutionA counting semaphore both counts filled slots and blocks or wakes threads, so
wait(full)makes the consumer sleep on an empty buffer andsignal(full)from the producer wakes it. - Q.541 markOfficial answer: A
Tap to open full sizeOperating Systems › Memory management
Full solutionMemory protection hardware catches any access outside a process's own allowed memory and raises a trap, which hands control to the OS in kernel mode.
- Q.551 markOfficial answer: A
Tap to open full sizeTheory of Computation › Finite automata
Full solutionIdentify the dead state, then follow the only path that avoids it. Here that path is one from the start, followed by any number of 's looping on the final state.
- Q.561 markOfficial answer: B
Tap to open full sizeTheory of Computation › Finite automata
Full solutionThe NFA accepts strings whose second-to-last symbol is , so a DFA must remember the last two symbols, which gives distinguishable states (Myhill-Nerode).
- Q.571 markOfficial answer: A
Tap to open full sizeTheory of Computation › CFG & PDA
Full solutionSplit the grammar into its two parts. generates with , and wraps it with matching pairs of and . So the number of 's exceeds the number of 's by exactly .
- Q.581 markOfficial answer: D
Tap to open full sizeTheory of Computation › Turing machines
Full solutionRecursive languages are closed under complement. A language is recursive exactly when both it and its complement are recursively enumerable. Recursively enumerable languages are not closed under complement.
- Q.591 markOfficial answer: B
Tap to open full sizeTheory of Computation › Context-free languages
Full solutionContext-free languages are closed under union and under intersection with a regular language (product of a PDA and a DFA). The result of that intersection is context-free in general, not regular.
- Q.601 markOfficial answer: B
Tap to open full sizeTheory of Computation › CFG & PDA
Full solutionUnroll the grammar to get a closed form. The rule pairs two 's with one , and gives one followed by any number of 's. So , which has an odd number of 's, all 's first, and at least 's.
- Q.611 markOfficial answer: C
Tap to open full sizeArtificial Intelligence › Logic
Full solutionThe connectives of First Order Logic are , , , and . Any other symbol, such as
<>, is not one. - Q.621 markOfficial answer: D
Tap to open full sizeArtificial Intelligence › Uncertainty
Full solution"Hybrid" means the network mixes the two variable types: continuous nodes (often Gaussian) and discrete nodes, linked by conditional distributions such as conditional Gaussians or threshold models.
- Q.631 markOfficial answer: A or B
Tap to open full sizeEarlier, ISRO's key gave A.
- Q.641 markOfficial answer: C
Tap to open full size - Q.651 markOfficial answer: B
Tap to open full sizeFull solutionClustering methods are either partitional (one flat set of disjoint clusters, as in K-Means) or hierarchical (a nested tree, built top-down by divisive or bottom-up by agglomerative methods).
- Q.661 markOfficial answer: A
Tap to open full size - Q.671 markOfficial answer: B
Tap to open full size - Q.681 markOfficial answer: C
Tap to open full size - Q.691 markOfficial answer: A
Tap to open full size - Q.701 markOfficial answer: B
Tap to open full size - Q.711 markOfficial answer: A
Tap to open full size - Q.721 markOfficial answer: A
Tap to open full size - Q.731 markOfficial answer: A
Tap to open full size - Q.741 markOfficial answer: D
Tap to open full size - Q.751 markOfficial answer: C
Tap to open full size - Q.761 markOfficial answer: C
Tap to open full size - Q.771 markOfficial answer: C
Tap to open full size - Q.781 markOfficial answer: B
Tap to open full size - Q.791 markOfficial answer: B or C
Tap to open full sizeEarlier, ISRO's key gave C.
- Q.801 markOfficial answer: A
Tap to open full sizeDatabase Management and Warehousing › ER model
Full solutionAn E-R model captures structure: entities, attributes and relationships. It is a static view. Processes (functional view) and behaviour over time (dynamic view) are modelled with other diagrams.
Part B: Aptitude Q.81 to Q.95
Part B is the same aptitude test as in the ISRO Scientist/Engineer Electronics 2023 paper, held the same day, with the same question numbers. Its solutions are listed under ISRO EC 2023.
- Q.811 markOfficial answer: D
Tap to open full sizeFull solutionIn picture analogies, find the relation in the complete row (material to the tool that applies it) and carry the same relation to the incomplete row.
- Q.821 markOfficial answer: D
Tap to open full sizeEarlier, ISRO's key gave B.
The question and its official answerIn a figure analogy, list every change in the example pair separately (outline shape, shading direction, small symbol's fill, orientation and corner) and apply each one exactly as drawn, not as you expect it to behave.
- Q.832 marksOfficial answer: B
Tap to open full sizeFull solutionWork out which item each operator acts on. Both operators here change only the 1st item, so the 2nd and 3rd items pass through untouched.
- Q.841 markOfficial answer: A
Tap to open full size - Q.851 markOfficial answer: A
Tap to open full sizeFull solutionTo show a survey underestimates a group, find the option that moves members of that group out of the reported count (here, dishonest people who reported being honest). Anything that adds people to the count, or only changes the degree of those already in it, cannot cause an underestimate.
- Q.862 marksOfficial answer: A
Tap to open full sizeFull solutionIn a circular seating puzzle, fix one person, number the seats, and decide which way a person's right hand points from the direction they face (towards the anticlockwise side when facing the centre). Then place people with the direct conditions and test each possible position for the others against the negative conditions.
- Q.871 markOfficial answer: A
Tap to open full sizeFull solutionThe three groups of dominoes alternate between counting down and counting up. The last group must count down again, so with at its end the missing dominoes are and .
- Q.881 markOfficial answer: C
Tap to open full sizeFull solutionIn number-figure puzzles, test the simplest relation first: the sum of the outer numbers compared with the centre. Confirm that one rule fits every complete figure before applying it to the incomplete one.
- Q.892 marksOfficial answer: C
Tap to open full sizeThe question and its official answerAssign one operation to each symbol using the chains where it appears together with symbols already decoded. Then apply the operations to the query tile in order. Track three things separately: colour, the direction of the diagonal, and the position of the straight line.
- Q.901 markOfficial answer: C
Tap to open full sizeFull solutionCombine an "all" statement with a "some" statement. The members picked out by "some" also fall under the "all" group, which gives a new "some" statement. A conclusion that brings in an idea the facts never mention (such as "better" or "simple") is not forced.
- Q.911 markOfficial answer: D
Tap to open full sizeFull solutionA course of action must be a sensible, proportionate response to the problem in the statement. An unverified allegation calls for an inquiry, not punishment, and the regulator's directive is not the thing at fault.
- Q.922 marksOfficial answer: D
Tap to open full sizeFull solutionWhen each term is a multiple of the previous one plus a growing amount, find the rule from the early terms, then test whether the later terms continue from it. The wrong term is the one that fails while the terms after it still agree with the rule applied to the correct value.
- Q.931 markOfficial answer: A
Tap to open full sizeGeneral Aptitude › Data interpretation
Full solutionRead each point off the line graph, take the absolute difference for each pair of consecutive days given, and compare. A steep segment of the line marks a large change.
- Q.941 markOfficial answer: C
Tap to open full sizeGeneral Aptitude › Data interpretation
Full solutionPercentage change is the change divided by the starting value, times 100. Here the starting year is 1983.
- Q.952 marksOfficial answer: A
Tap to open full sizeFull solutionFor a fixed perimeter, the rectangle of largest area is a square: with fixed, .