How to calculate a total for a column inside a golang template?

Question

I have this code inside a html/template:

{{ $TotalPrice := 0.0 }}
{{ range $i, $tx := .Transactions }}
{{ $TotalPrice := FloatInc $TotalPrice (StrToFloat .TotalPrice) }}
  <tr>
    <td>{{ inc $i 1 }}</td> 
    <td>{{ .Description.String }}</td>
    <td>{{ .Type }}</td>
    <td>{{ .TotalPrice }}</td>
    <td>{{ .Note }}</td>  
  </tr>  
{{ end }}
<tr>
  <td></td> 
  <td></td>
  <td></td>
  <td>{{ $TotalPrice }}</td>
  <td></td>
  <td></td>
</tr> 

Transactions are Money Transaction with TotalPrice DB Fields and I have 4 functions according Iris framework spec.

tmpl.AddFunc("dec", func(num int, step int) int {
    return num - step
})

tmpl.AddFunc("inc", func(num int, step int) int {
    return num + step
})

tmpl.AddFunc("FloatDec", func(num float64, step float64) float64 {
    return num - step
})

tmpl.AddFunc("FloatInc", func(num float64, step float64) float64 {
    return num + step
})

tmpl.AddFunc("StrToFloat", func(s string) (float64, error) {
    return strconv.ParseFloat(s, 64)
}) 

I've note the $TotalPrice keeps the initial value (0.0) for every iteration so {{ $TotalPrice }} inside the range will print the .TotalPrice value and the value of $TotalPrice at the last row will be 0.0 too then what is the equivalent, inside go template, of:

nums := []int{2, 3, 4}
sum := 0
for _, num := range nums {
    sum += num
}
fmt.Println("sum:", sum)

Answer 1

In Go's template, once you declare a variable and assign value to it, you can't change its value. What happen in your code are:

1. The value of outer total price which is declared as $TotalPrice := 0.0 is always 0.0, and the variable scope is extended to the end of the template.
2. When you defined a variable named $TotalPrice inside range, although the variable name is the same, a completely new variable will be allocated. The value assigned to this variable is given by FloatInc($TotalPrice, .TotalPrice). Note that argument $TotalPrice refers to outer total price which is 0.0, so the statement will be equal to $TotalPrice := 0.0 + .TotalPrice. Thus, when you print the $TotalPrice in each iteration, you got current .TotalPrice instead of accumulated total price.
3. The scope of variable declared in (2) is between range and end. Thus when you print $TotalPrice at the last row, you got the value of outer total price declared in (1), i.e. 0.0.

In your case, you need to declare a function which takes Transactions as its argument then calculate the total inside the function, e.g.

tmpl.AddFunc("sum", func(transactions []Transaction) float64 {
    sum := 0.0
    for _, t := range transactions {
        if v, err := strconv.ParseFloat(t.TotalPrice, 64); err == nil {
            sum += v
        }
    }
    return sum
})

then use it in the template as:

{{ $TotalPrice := sum .Transactions }}

Answer 2

There is a hack.

https://masterminds.github.io/sprig/

You can add sprig template helpers (or something similar) and accumulate the sum as an entry in the dictionary. This lib (and its alternatives) provides basic math operations, string to numbers conversions and so on.

It will require fewer code changes; you will just have to pass these helper functions to the template init.