What possible output(s) from the given options will NOT be displayed when the following code is executed ? Also, mention, for how many iterations the for loop in the given code will run ?

```python`
`import random a = [1,2,3,4,5,6] for i in range(4): j = random.randrange(i,5) print(a[j],end='-') print()`
``

Q: What possible output(s) from the given options will NOT be displayed when the following code is executed ? Also, mention, for how many iterations the for loop in the given code will run ? ```python import random a = [1,2,3,4,5,6] for i in range(4): j = random.randrange(i,5) print(a[j],end='-') print() ```

Option **(D) 5-1-2-4-** will NOT be displayed, and the `for` loop runs for exactly **4 iterations**. **Why:** `range(4)` gives `i = 0,1,2,3` (4 iterations). For each `i`, `j = random.randrange(i,5)` can only produce values from `i` up to `4` (5 excluded). So the value printed at each position is restricted to a shrinking range as `i` increases, and option D's second value (1) violates the valid range for `i=1`.

Python Libraries (random module) and Iterative Constructs

Random Module and Loop Tracing

Options

(A)3-4-5-4-

(B)2-2-4-5-

(D)5-1-2-4-

Official Answer

Option (D) 5-1-2-4- will NOT be displayed, and the for loop runs for exactly 4 iterations.

Why: range(4) gives i = 0,1,2,3 (4 iterations). For each i, j = random.randrange(i,5) can only produce values from i up to 4 (5 excluded). So the value printed at each position is restricted to a shrinking range as i increases, and option D's second value (1) violates the valid range for i=1.

random.randrange()range()for looplist indexingcode tracingiteration countrandom module

Marking Scheme

11 mark: correctly identifying option (D) '5-1-2-4-' as the output that will NOT be displayed, with valid justification.
21 mark: correctly stating that the for loop runs for 4 iterations.

Hint

Work out the valid range of a[j] separately for each loop iteration (i=0,1,2,3) using randrange(i,5), which excludes 5.

Quick Oral Answer

The loop runs 4 times since range(4) gives i=0 to 3; at each i, randrange(i,5) restricts a[j] to a shrinking set of valid values, and option D fails because its second value, 1, falls outside the valid range for i=1.

Analysis & Explanation

This is a code-tracing question combining the random library with loop control and list indexing.

Loop count

range(4) produces i = 0, 1, 2, 3 — exactly 4 values, so the for loop executes 4 times, printing 4 numbers separated by hyphens.

Range of a[j] at each position

randrange(i, 5) returns an integer from i to 4 inclusive (5 excluded), so:

i=0: j in $\{0,1,2,3,4\}$ → a[j] in $\{1,2,3,4,5\}$
i=1: j in $\{1,2,3,4\}$ → a[j] in {2,3,4,5}
i=2: j in $\{2,3,4\}$ → a[j] in $\{3,4,5\}$
i=3: j in $\{3,4\}$ → a[j] in {4,5}

Checking each option against these ranges

(A) 3-4-5-4-: $3 \in \{1..5\}$ , $4 \in \{2..5\}$ , $5 \in \{3..5\}$ , $4 \in \{4,5\}$ → all valid, CAN be displayed.
(B) 2-2-4-5-: $2 \in \{1..5\}$ , $2 \in \{2..5\}$ , 4∈{3..5}, $5 \in \{4,5\}$ → all valid, CAN be displayed.
(C) 4-3-3-5-: $4 \in \{1..5\}$ , $3 \in \{2..5\}$ , $3 \in \{3..5\}$ , $5 \in \{4,5\}$ → all valid, CAN be displayed.
(D) 5-1-2-4-: $5 \in \{1..5\}$ ✓, but $1 \notin \{2,3,4,5\}$ (second position, i=1) — invalid, so it will NEVER be displayed.

Exam trap

Students often forget that randrange(i,5) EXCLUDES the stop value 5, and separately forget that the valid range of j shrinks as i increases — leading to wrongly validating option D.

Common Mistakes

1Assuming randrange(i,5) can produce 5 itself — forgetting the stop value in randrange() is always excluded.
2Checking only the first value of each option against {1,2,3,4,5} and ignoring that the valid range narrows for later positions (i=1,2,3).
3Miscounting loop iterations as 5 instead of 4 by confusing range(4) with range(1,5).

Interesting Facts

Python's random.randrange(start, stop) mimics the same half-open interval convention as range() — stop is always excluded — a deliberate design choice for consistency across the language.

The random module in CPython uses the Mersenne Twister algorithm internally, which has a period of 2^19937 − 1, making its output sequence statistically excellent for simulations though not cryptographically secure.

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Frequently Asked Questions

Does random.randrange(i,5) ever return 5?

No. Like range(), randrange(start, stop) always excludes the stop value, so randrange(i,5) can return values only from i up to 4.

How many times does the for loop in this code execute?

It executes 4 times because range(4) generates i = 0, 1, 2, 3.