In C, when you define an array like:

int a[2][3];

DOES C consider that as * 2* arrays of

**ints, or is it**

*3**arrays of*

**3****ints?**

*2*I’ll tell you the answer soon .

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But you have to guess first.

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Ok.

IT IS TWO arrays of THREE ints. You read it from left to right.

So, if we define:

int a[2][3] = { { 3, 5, 7 }, { 9, 1, 0 } } ;

We know that’s right because if we tried to do it any other way, (like

int a[2][3] = { { 9, 9 }, { 9, 0 }, { 2, 2 } } ;

) we’d get something like “error: too many initializers” from the C compiler. That’s because this HAS TO BE an array of TWO arrays of THREE ints each.

## ok ok but what about row major vs column major?

Row major vs column major really has to do with whether you think of a matrix ** as a collection of row vectors** (“row major”), or if you prefer to think of a matrix

**(aka “column major”).**

*as a collection of COLUMN VECTORS*Let’s start with **column major**.

## interpreting the matrix as column major

So, don’t forget, we had:

int a[2][3] = { { 3, 5, 7 }, { 9, 1, 0 } } ;

If we’re thinking COLUMN MAJOR, then we will understand this to be **2** COLUMN VECTORS, WITH **3** ENTRIES EACH.

SO, we’d visualize the array as:

3 | 9 |

5 | 1 |

7 | 0 |

## interpreting the matrix as ROW MAJOR

If we were to think of that same matrix:

int a[2][3] = { { 3, 5, 7 }, { 9, 1, 0 } } ;

If we’re thinking ROW MAJOR, then we will understand this to be **2** __ROW__ VECTORS, WITH **3** ENTRIES EACH.

So we’d visualize:

3 | 5 | 7 |

9 | 1 | 0 |

## dealing with both types

Ok so notice the COLUMN MAJOR matrix was like:

3 | 9 |

5 | 1 |

7 | 0 |

and the ROW MAJOR was like:

3 | 5 | 7 |

9 | 1 | 0 |

So, let’s notice a few things that are interesting.

Notice that the original data was:

int a[2][3] = { { 3, 5, 7 }, { 9, 1, 0 } } ;

Now, notice this. The COLUMN MAJOR version has the values go inside it IN ORDER, DOWN THE COLUMNS.

3 | 9 |

5 | 1 |

7 | 0 |

The ROW MAJOR version has the values go inside it, IN ORDER, DOWN THE ROWS.

3 | 5 | 7 |

9 | 1 | 0 |

## accessing and modifying values of row major and column major mats

If you’re accessing the COLUMN MAJOR matrix, you’d write something like:

a[1][2] = 99; // set a[COL=1][ROW=2] TO 99; // In the COLUMN MAJOR MATRIX, // the specification of the COLUMN to hit // comes first.

3 | 9 |

5 | 1 |

7 | 99 |

If you’re accessing the ROW MAJOR matrix, you’d write something like:

a[1][2] = 2000; // set a[ROW=1][COL=2] TO 2000; // In the ROW MAJOR matrix, // specification of the ROW to hit // comes first.

3 | 5 | 7 |

9 | 1 | 2000 |

BUT NOTICE, in the two green examples, we are HITTING THE SAME ENTRY IN THE MATRIX.

# accessing a 1D array as row major, or column major

SAY you stored the matrix values in a 1D array.

int m[6] = {3,5,7,9,1,0};

Results in something like:

3 | 5 | 7 | 9 | 1 | 0 |

in C memory.

If you want to treat this 1D array as a COLUMN MAJOR matrix, you’d access it like this:

A[col][row] = A[row + col*NUMROWS];

If you want to treat this 1D array as a ROW MAJOR matrix, you’d access it like this:

A[row][col] = A[col + row*NUMCOLS];

## One Comment

Thanks!