How to Convert a Day of Week String to `time.Weekday` in Go?

Convert Day of Week String to time.Weekday

In Go, the time package doesn't offer a built-in function to convert a string representing a day of the week into a time.Weekday value. So, a common solution is to create a custom function to perform this conversion.

One approach is to use an array to store the weekday names and their corresponding time.Weekday values. However, a more efficient and idiomatic solution is to utilize a map.

Using a Map for Conversion:

A map provides faster and direct lookups compared to an array. Here's a snippet demonstrating this approach:

<code class="go">var daysOfWeek = map[string]time.Weekday{
    "Sunday":    time.Sunday,
    "Monday":    time.Monday,
    // ... and so on for all weekdays
}

func parseWeekday(v string) (time.Weekday, error) {
    if d, ok := daysOfWeek[v]; ok {
        return d, nil
    }

    return time.Sunday, fmt.Errorf("invalid weekday '%s'", v)
}</code>

Safely Initializing the Map:

To ensure correct initialization, you can use a for loop to populate the daysOfWeek map:

<code class="go">var daysOfWeek = map[string]time.Weekday{}

func init() {
    for d := time.Sunday; d <= time.Saturday; d++ {
        daysOfWeek[d.String()] = d
    }
}</code>

Extending the Conversion:

The map-based solution allows for easy extension. For instance, you can add 3-letter short weekday names as valid input:

<code class="go">func init() {
    // ... existing initialization code
    for d := time.Sunday; d <= time.Saturday; d++ {
        name := d.String()
        daysOfWeek[name] = d
        daysOfWeek[name[:3]] = d
    }
}</code>

Conclusion:

Using a map for weekday string conversion in Go is both efficient and flexible. It allows for easy lookups, safe initialization, and straightforward extension to accommodate alternative input formats.

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