public/subjects/listforeachif.en.md

## listforeachif

### Instructions

Write a function `ListForEachIf` that applies a function given as argument to the data within some of the nodes of the list `l`.

- This function receives two functions:

  - `f` is a function that is applied to the node.

  - `cond` is a function that returns a `boolean` and it will be used to determine if the function `f` should be applied to the node.

- The function given as argument must have a pointer `*NodeL` as argument.

### Expected function and structure

```go
type NodeL struct {
	Data interface{}
	Next *NodeL
}

type List struct {
	Head *NodeL
	Tail *NodeL
}

func IsPositive_node(node *NodeL) bool {
	switch node.Data.(type) {
	case int, float32, float64, byte:
		return node.Data.(int) > 0
	case string, rune:
		return false
	}
	return false
}

func IsNegative_node(node *NodeL) bool {
	switch node.Data.(type) {
	case int, float32, float64, byte:
		return node.Data.(int) > 0
	case string, rune:
		return false
	}
	return false
}

func IsNotNumeric_node(node *NodeL) bool {
	switch node.Data.(type) {
	case int, float32, float64, byte:
		return false
	case string, rune:
		return true
	}
	return true
}

func ListForEachIf(l *List, f func(*NodeL), cond func(*NodeL) bool) {

}
```

### Usage

Here is a possible [program](TODO-LINK) to test your function :

```go
package main

import (
	piscine ".."
	"fmt"
)

func PrintElem(node *piscine.NodeL) {
	fmt.Println(node.Data)
}

func StringToInt(node *piscine.NodeL) {
	node.Data = 2
}

func PrintList(l *piscine.List) {
	it := l.Head
	for it != nil {
		fmt.Print(it.Data, "->")
		it = it.Next
	}
	fmt.Println()
}

func main() {
	link := &piscine.List{}

	piscine.ListPushBack(link, 1)
	piscine.ListPushBack(link, "hello")
	piscine.ListPushBack(link, 3)
	piscine.ListPushBack(link, "there")
	piscine.ListPushBack(link, 23)
	piscine.ListPushBack(link, "!")
	piscine.ListPushBack(link, 54)

	PrintList(link)

	fmt.Println()
	fmt.Println("--------function applied--------")
	piscine.ListForEachIf(link, PrintElem, piscine.IsPositive_node)

	piscine.ListForEachIf(link, StringToInt, piscine.IsNotNumeric_node)

	fmt.Println("--------function applied--------")
	PrintList(link)

	fmt.Println()
}
```

And its output :

```console
student@ubuntu:~/piscine/test$ go build
student@ubuntu:~/piscine/test$ ./test
1 -> hello -> 3 -> there -> 23 -> ! -> 54 -> <nil>
--------function applied--------
hello
there
!
--------function applied--------
1 -> 1 -> 3 -> 1 -> 23 -> 1 -> 54 -> <nil>
student@ubuntu:~/piscine/test$
```