Tearing Down a Problem: Turn Every Sentence into a Contract

The output sentence decides what counts as a correct answer

The statement asks you to print the 0-based index of the first occurrence of x, not the value x itself. For the array [4, 2, 7, 2, 1] and x = 2, the value 2 appears at indices 1 and 3. The first occurrence is at index 1, so the required result is 1. Because indexing starts at 0, the first array element has index 0, making the answer 1 rather than 2.

The sample output is meaningful only when you connect it to every word in the required result. Printing 2 would print the target value, not its position. Printing 3 would print the position of a later match. Printing 1 is correct because it is the position of the first matching value under 0-based indexing.

The input format is a map from tokens to variables

Read the first line as two tokens in order: the token 5 becomes n, and the token 2 becomes x. The second line contains exactly n array values, so the five tokens become a[0] = 4, a[1] = 2, a[2] = 7, a[3] = 2, and a[4] = 1.

5 2
4 2 7 2 1

Line breaks explain the input format to a human reader, but they do not change the token order. A typical C++ or Java input reader consumes whitespace, including spaces and newlines, in the same sequence. The program must still read n and x first, then read exactly n array values, because that order is the contract.

Constraints remove invalid assumptions before code exists

The constraint 1 <= n <= 100000 says the array is never empty and may be much larger than the concrete five-element sample. A solution must therefore treat n as the number of values to read, rather than assuming that five values always exist or reserving logic for only the shown sample.

The bounds -1000000000 <= a[i], x <= 1000000000 apply to every array value and to x, not just to the concrete values 4 and 2. Those bounds fit within a standard 32-bit signed integer range, so a C++ int or Java int can represent each input value. The constraint gives a storage fact, while n gives the number of values the input can contain.

A hand trace turns the statement into a coding checklist

Start at index 0 and compare the array value with x. At index 0, a[0] is 4, so the search continues. At index 1, a[1] is 2, which equals x, so the search stops immediately. The later match at index 3 is irrelevant because the first occurrence has already been found.

The concrete input produces the exact output 1. Before writing code, turn the statement into four checks: read n and x in that order, read exactly n array values, stop at the first value equal to x, and print its 0-based index. If no value equals x, the required output is -1. The sample values cannot replace any of these rules, because a different valid input may have a different length, target, or matching position.