How to fix Error: Cannot access value of type list without an index in Terraform

# ❌ Wrong - accessing list without index
variable "servers" {
  type    = list(string)
  default = ["web-1", "web-2", "web-3"]
}

output "server_name" {
  value = var.servers  # Error: Cannot access without index
}

# ✓ Correct - specify which element to access
output "server_name" {
  value = var.servers[0]  # Returns "web-1"
}

# ❌ Wrong - trying to index a set
resource "aws_security_group" "example" {
  # ...
}

output "first_tag" {
  value = aws_security_group.example.tags[0]  # Error: sets can't be indexed
}

# ✓ Correct - convert set to list then index
output "first_tag" {
  value = tolist(aws_security_group.example.tags)[0]
}

# ❌ Wrong - trying to access list without index
resource "aws_instance" "servers" {
  count = 3
  # ...
}

output "all_ips" {
  value = aws_instance.servers.private_ip  # Error
}

# ✓ Correct - use splat syntax to get all values
output "all_ips" {
  value = aws_instance.servers[*].private_ip  # Returns ["10.0.1.1", "10.0.1.2", "10.0.1.3"]
}

# Or access a specific instance
output "first_ip" {
  value = aws_instance.servers[0].private_ip
}

# ❌ Wrong - assuming list has elements
data "aws_subnets" "available" {
  filter {
    name   = "vpc-id"
    values = [aws_vpc.example.id]
  }
}

output "subnet_id" {
  value = data.aws_subnets.available.ids[0]  # Fails if no subnets found
}

# ✓ Correct - verify with length() or use try()
output "subnet_id" {
  value = length(data.aws_subnets.available.ids) > 0 ? data.aws_subnets.available.ids[0] : null
}

# Or use try() for graceful fallback
output "subnet_id" {
  value = try(data.aws_subnets.available.ids[0], null)
}

variable "availability_zones" {
  type    = list(string)
  default = ["us-east-1a", "us-east-1b", "us-east-1c"]
}

# ❌ Wrong - resource expects single AZ, not list
resource "aws_subnet" "example" {
  availability_zone = var.availability_zones  # Error
}

# ✓ Correct - use for_each to create one subnet per AZ
resource "aws_subnet" "example" {
  for_each          = toset(var.availability_zones)
  availability_zone = each.value  # References current AZ
  # ...
}

variable "instance_names" {
  type    = list(string)
  default = ["web-1", "web-2", "web-3"]
}

# ❌ Wrong - passing entire list
resource "aws_instance" "servers" {
  count = length(var.instance_names)
  tags = {
    Name = var.instance_names  # Error: expects string, not list
  }
}

# ✓ Correct - use count.index to access specific element
resource "aws_instance" "servers" {
  count = length(var.instance_names)
  tags = {
    Name = var.instance_names[count.index]  # Returns one name per instance
  }
}