map object

// Package strand is a small library for returning the RNA complement of a DNA strand.
package strand

var translate = map[rune]byte{
	'G': 'C',
	'C': 'G',
	'T': 'A',
	'A': 'U',
}

// ToRNA returns the RNA complement of the input DNA strand.
func ToRNA(dna string) string {
	rna := make([]byte, len(dna))
	for i, c := range dna {
		rna[i] = translate[c]
	}
	return string(rna)
}

This approach starts be defining a map to translate the DNA rune to an RNA byte. It does this outside of the ToRNA() function, so it is not created every time ToRNA() is called. A byte slice is defined using make, which is passed the len() of the input string in bytes. A string can be seen as bytes or runes. As of the time of this writing, all of the characters in the input string are single-byte ASCII characters, so they can be treated as bytes.

The ways to iterate characters are by Unicode runes, or by each letter being a string, or by each letter being a byte. The runes are from range on a string, the strings from Split(), and the bytes from indexing into the string. Another way to iterate runes is to convert the string to a rune slice and range on it. The difference between ranging on a rune slice vs ranging on a string is that the index returned from a string is the position of the next rune in bytes, not which rune it is. For example, if the first unicode character is two bytes, then the second unicode character index will be 2 when ranging on a string and 1 when ranging on a rune slice.

This iterates over the runes of the input string and passes them as the key to the map. For this exercise we are not concerned with validating the letter and handling an error for an illegal character. The map returns the matching RNA byte value for the DNA rune, which is placed at the same index in the byte slice.

After the iteration of the input string finishes, the function returns the byte slice converted to a string.