Check if value exists in nested array with template package

Using the text/template, I need to find out if any object in an array has a certain property value.

Let's say I have a list of people (json):

[
	{"name": "ANisus", "sex":"M"},
	{"name": "Sofia", "sex":"F"},
	{"name": "Anna", "sex":"F"}
]

Using the template, I want to have this output:

Females:
Sofia
Anna

But the header, Females:, should only show in case there are actually any person with sex set to F. How can I perform this within the template system? My first attempt was by using a variable:

{{$hasFemale := 0}}
{{range .}}{{if eq .sex "F"}}{{$hasFemale := 1}}{{end}}{{end}}
{{if $hasFemale}}Female:{{end}}

I didn't get this to work because the $hasFemale within the range is in a different scope and not the same as the one initiated with 0. I can't seem to find a way to change a variable once it is initiated.

"Working" example: http://play.golang.org/p/T-Ekx7n9YQ

And I can not move this logic to the application; it must be done within the template.

I would solve this differently. You generally don't want state in templates, they ought to be simple.

As you only want to know whether or not there are females in your data set, define a method HasFemales on your data and call it from the template. You don't necessarily need to define a struct, you can use a custom type and a definition like this (your modified example on play):

type People []interface{}

func (p People) HasFemale() bool {
	for _, v := range p {
		if m, ok := v.(map[string]interface{}); !ok {
			return false
		} else if _, ok := m["sex"]; ok && m["sex"] == "F" {
			return true
		}
	}
	return false
}

Your template would then look like this:

{{if .HasFemale}}Female:
	{{range .}}{{if eq .sex "F"}}{{.name}}{{end}}{{end}}
{{end}}

This is, of course, not as type safe as a struct and not as nice as a struct so I recommend
using the encoding/json reflection feature to map values to a struct and define the method on that. This also has the benefit of being able to cache the result of .HasFemale internally so you don't need to iterate every time.

Update regarding your comment:

> I have my Go application that takes two arguments: 1) a template file and 2) a json file. It executes the template with the data and outputs the file. The output file is then passed on to wkhtmltopdf to generade a pdf. Each template/data pair have arbitrary data structures beyond my control, so the Go application must be generic

In this case, make HasFemale generic. This is similar to what you did in your question but has the benefit to abstract the nesting of the range, making it possible for you to store the result in a variable and have the state throughout your template. Example:

type Data []interface{}

func (p Data) HasField(name string, value interface{}) bool {
	for _, v := range p {
		if m, ok := v.(map[string]interface{}); !ok {
			return false
		} else if _, ok := m[name]; ok && reflect.DeepEqual(m[name], value) {
			return true
		}
	}
	return false
}

Example usage:

{{$hasFemale := .HasField "sex" "F"}}
{{if $hasFemale}}Female:
	{{range .}}{{if eq .sex "F"}}{{.name}}{{end}}{{end}}
{{end}}`

I know this is an old question, however I just had a similar use case where I needed to make a column in an html table optional dependant on if a field exists anywhere within the range.

The original poster was almost there, they just had a frequent gotcha with go and was setting the variable in the wrong scope.

So instead of {{$hasFemale := 1}} which set the variable within the if, they should have written it as {{$hasFemale = 1}} then it would have worked as they originally written it.

{{$hasFemale := 0}}
{{range .}}{{if eq .sex "F"}}{{$hasFemale = 1}}{{end}}{{end}}
{{if $hasFemale}}Female:{{end}}

Here's their original example with the fix https://play.golang.org/p/m_RPJmqsuet