Pick the right one: int vs. int64

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Use int for index

An index, length or capacity should normally be an int. The int type is either 32 or 64 bits, and always big enough to hold the maximum possible length of an array.

See Maximum value of an int for code to compute the maximum value of an int.

Use int64 and friends for data

The types int8, int16, int32, and int64 (and their unsigned counterparts) are best suited for data. An int64 is the typical choice when memory isn’t an issue.

In particular, you can use a byte, which is an alias for uint8, to be extra clear about your intent. Similary, you can use a rune, which is an alias for int32, to emphasize than an integer represents a code point.

Sometimes it makes little or no difference if you use 32 or 64 bits for data, and then it’s quite common to simply use an int. However, I prefer to be explicit. It forces you to spend a moment thinking about the choice and also makes the code a bit clearer.

Examples

In this code, the slice elements and the max variable have type int64, while the index and the length of the slice have type int.

func Max(a []int64) int64 {
	max := a[0]
	for i := 1; i < len(a); i++ {
		if max < a[i] {
			max = a[i]
		}
	}
	return max
}

The implementation of time.Duration is a typical example from the standard library where an int64 is used to store data:

type Duration int64

A Duration represents the time between two instants as a nanosecond count. This limits the largest possible duration to about 290 years.

Further reading

The Maximum value of an int article shows how to compute the size and limit values of an int as untyped constants.

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