GATE CSE 2021 Set 1 | Question: 40

Question

Define $R_n$ to be the maximum amount earned by cutting a rod of length $n$ meters into one or more pieces of integer length and selling them. For $i>0$, let $p[i]$ denote the selling price of a rod whose length is $i$ meters. Consider the array of prices:

$$\text{p}[1]=1,\text{p}[2]=5,\text{p}[3]=8,\text{p}[4]=9,\text{p}[5]=10,\text{p}[6]=17,\text{p}[7]=18$$Which of the following statements is/are correct about $R_7$?

• A. $R_7=18$
• B. $R_7=19$
• C. $R_7$ is achieved by three different solutions
• D. $R_7$ cannot be achieved by a solution consisting of three pieces

Comments

It is a DP problem of cutting a rod.

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any logic behind selling 2 meters rod two times ???

Answer 1

A & C should be correct

• $1^{st}$ Solution $: p[2];p[3];p[2] = 5+8+5 = 18$
• $2^{nd}$ Solution $: p[7] = 18$
• $3^{rd}$ Solution $: p[6]; p[1] = 17+1 = 18$

Comments

cutting a rod into 1 piece ? doesn’t sound ambiguous

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In question it says that Rn is achieved by selling the peices how P[2] can be sell two times in solution to obtain R7 as 18

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@debmalyaSEN Cutting a Rod into 1 piece = keeping the rod as a whole single piece. Hence it’s valid.

@unnayansharma p[2] just says the price of a piece of length 2. Imagine the rod is cut into 3 pieces of lengths : (2 units, 3 units, 2 units)

Answer 2

ANSWER : A,C

1st : directly from p[7]= 18

2nd  : p[6] + p[1]= 18 

3rd : p[2] + p[3] + p[2]= 5+8+5= 18

and these are the only 3 ways you can try other combinations but these only are gives maximum .