Convert Index to Column A1 Notation and vice-versa

Solution 1:

This was an interesting problem to tackle. The solution involves 2 functions:

indexToColumn(int) (string, error) will convert an index to A1 Notation. e.g. 703 to AAA

columnToIndex(string) (int, error) will convert A1 Notation to an index. e.g. BA to 53

Here is the code:

// indexToColumn takes in an index value & converts it to A1 Notation
// Index 1 is Column A
// E.g. 3 == C, 29 == AC, 731 == ABC
func indexToColumn(index int) (string, error) {

    // Validate index size
    maxIndex := 18278
    if index > maxIndex {
        return "", web.Errorf("index cannot be greater than %v (column ZZZ)", maxIndex)
    }

    // Get column from index
    l := "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    if index > 26 {
        letterA, _ := indexToColumn(int(math.Floor(float64(index-1)/26)))
        letterB, _ := indexToColumn(index%26)
        return letterA + letterB, nil
    } else {
        if index == 0 {
            index = 26
        }
        return string(l[index-1]), nil
    }

}

// columnToIndex takes in A1 Notation & converts it to an index value
// Column A is index 1
// E.g. C == 3, AC == 29, ABC == 731
func columnToIndex(column string) (int, error) {

    // Calculate index from column string
    var index int
    var a uint8 = "A"[0]
    var z uint8 = "Z"[0]
    var alphabet = z - a + 1
    i := 1
    for n := len(column) - 1; n >= 0; n-- {
        r := column[n]
        if r < a || r > z {
            return 0, web.Errorf("invalid character in column, expected A-Z but got [%c]", r)
        }
        runePos := int(r-a) + 1
        index += runePos * int(math.Pow(float64(alphabet), float64(i-1)))
        i++
    }

    // Return column index & success
    return index, nil

}