Q.Construct matrix $A = [a_{ij}]$ of order $2\times 3$ where $a_{ij} = \dfrac{(i+2j)^2}{2}$.

Matrix Representation Order — The Intuition First

Imagine you're a teacher taking attendance. You have a list of 5 students and you call out their names one by one. The order matters — "Ravi, Priya, Anjali, Vikram, Sneha" is a specific sequence. If you wrote that sequence down, you'd have an ordered list.

Now imagine you're arranging those same 5 students into rows and columns for a class photo. You decide: 2 rows, 3 columns (one spot empty). The moment you fix how many rows and how many columns, you've chosen a matrix shape. But here's the key: the order in which you fill those slots — row by row, or column by column — changes who ends up where.

That's the core idea of matrix representation order: given a list of numbers (or objects), the same list can be arranged into a matrix in different ways depending on whether you fill it row-wise or column-wise.

The Precise Statement

A matrix of size $m \times n$ has $m$ rows and $n$ columns, so it holds $m \times n$ entries. If you have a sequence of $m \times n$ elements, there are two standard ways to map them into the matrix:

Let the sequence be $a_1, a_2, a_3, \dots, a_{mn}$. In row-major order, the element at position $(i, j)$ — meaning row $i$, column $j$ — is:

In column-major order, the element at $(i, j)$ is:

These formulas are not something to memorise blindly. They come from counting: in row-major, you skip $(i-1)$ full rows (each of $n$ elements) and then take the $j$-th element of that row. In column-major, you skip $(j-1)$ full columns (each of $m$ elements) and then take the $i$-th element of that column.

A Concrete Example

Take the sequence: $1, 2, 3, 4, 5, 6$. Put it into a $2 \times 3$ matrix.

Row-major (fill rows first):

Column-major (fill columns first):

Same six numbers, completely different matrices. The order of filling changes the arrangement.

Why This Matters

In Indian exam contexts (JEE, CUET, board exams), you rarely need to compute these formulas explicitly. But the concept appears in:

• Programming: C stores 2D arrays in row-major order; Fortran uses column-major. This affects memory layout and performance.
• Linear algebra: When you write a matrix as a list of column vectors or row vectors, you're implicitly choosing a representation order.
• Data interpretation: Some problems give you a list of numbers and ask you to form a matrix "by filling row-wise" or "column-wise". The answer changes.

The One Thing to Remember

Matrix representation order is about the sequence of filling, not the shape. The shape $m \times n$ is fixed. The order — row-major or column-major — decides which number goes where. Once you see that, the formulas are just counting.