matrices cannot be multiplied: column count of the first mat

Error message

matrices cannot be multiplied: column count of the first matrix must match row count of the second matrix

What it means

Multiply returns this error when the inner dimensions disagree: m1.Columns() != m2.Rows(). Matrix multiplication is only defined when the first matrix's column count equals the second matrix's row count.

Source

Thrown at math/matrix/multiply.go:18

// multiply.go
// description: Implementation of matrix multiplication
// time complexity: O(n^3) where n is the number of rows in the first matrix
// space complexity: O(n^2) where n is the number of rows in the first matrix

package matrix

import (
	"context"
	"errors"
	"sync"
)

// Multiply multiplies the current matrix (m1) with another matrix (m2) and returns the result as a new matrix.
func (m1 Matrix[T]) Multiply(m2 Matrix[T]) (Matrix[T], error) {
	// Check if the matrices can be multiplied.
	if m1.Columns() != m2.Rows() {
		return Matrix[T]{}, errors.New("matrices cannot be multiplied: column count of the first matrix must match row count of the second matrix")
	}

	// Create a new matrix to store the result.
	var zeroVal T
	result := New(m1.Rows(), m2.Columns(), zeroVal)

	ctx, cancel := context.WithCancel(context.Background())
	defer cancel() // Make sure it's called to release resources even if no errors

	var wg sync.WaitGroup
	errCh := make(chan error, 1)

	for i := 0; i < m1.Rows(); i++ {
		for j := 0; j < m2.Columns(); j++ {
			i, j := i, j // Capture the loop variable for the goroutine
			wg.Add(1)
			go func() {
				defer wg.Done()

View on GitHub (pinned to 5ba447ec5f)

Solutions

Ensure m2.Rows() equals m1.Columns(); transpose or reshape one operand if needed.
Verify dimensions with m1.Columns() == m2.Rows() before multiplying.
Check that prior pipeline steps (add/subtract/submatrix) did not change the expected shape.

Example fix

// before
// m1: 2x3, m2: 2x3
r, err := m1.Multiply(m2) // error

// after
t, _ := m2.Transpose() // 3x2
r, err := m1.Multiply(t) // 2x2

Defensive patterns

Strategy: validation

Validate before calling

if m1.Columns() != m2.Rows() {
    return fmt.Errorf("cannot multiply %dx%d by %dx%d",
        m1.Rows(), m1.Columns(), m2.Rows(), m2.Columns())
}
res, err := m1.Multiply(m2)

Type guard

func multipliable[T constraints.Integer](m1, m2 math.Matrix[T]) bool {
    return m1.Columns() == m2.Rows()
}

Try / catch

res, err := m1.Multiply(m2)
if err != nil {
    return Matrix{}, fmt.Errorf("multiply shapes %dx%d * %dx%d: %w",
        m1.Rows(), m1.Columns(), m2.Rows(), m2.Columns(), err)
}

Prevention

Record expected shapes in comments/types through a computation pipeline.
Transpose explicitly when multiplying by Aᵀ.
Add a table test with mismatched shapes asserting the error.

When it happens

Trigger: Calling m1.Multiply(m2) with shapes like (2x3)·(2x3) or (3x2)·(3x4); multiplying a matrix by its own transpose without transposing first.

Common situations: Stacking operations where an intermediate result changed shape; multiplying A·Aᵀ instead of A·Aᵀ with an explicit transpose; dimension mismatches from differently sized datasets.

Related errors

AI-assisted analysis of TheAlgorithms/Go@5ba447ec5f (2026-09-02). Data as JSON: /api/errors/9ad08fce916d66fa. Report an issue: GitHub.