Problems assigning string into another string inside IF statement

Question

I'm trying to assign one string to another inside a if statement, basically how it works is, when the program starts the String is NULL, when I assign a new value to it on the console, the main string receives the value, and I created a temporary string to receive the value too. If I insert the temporary_string = main_string outside the if statement it will receive the main_string, but if I do so inside the if statement it won't work.

String wifi_name;

void setup() {
  Serial.begin(115200);
}

void loop() {
  String last_wifi_name;

  if (Serial.available()){
    wifi_name = Serial.readString();
  }

  wifi_name.trim();

  Serial.print("wifi_name: ");
  Serial.println(wifi_name);

  Serial.print("last_wifi_name: ");
  Serial.println(last_wifi_name);

  if (wifi_name == ""){
    Serial.println("No value");
  }else{
    Serial.println("It enters here");
    last_wifi_name = wifi_name; //This is not working
  }

  delay(2000);
 
}

Output:

wifi_name: 
last_wifi_name: 
NULL

wifi_name: Trying to assign this
last_wifi_name: 
It enters here

The problem is, the last_wifi_name is not being received from the =

Answer 1

The problem is that last_wifi_name is a local variable. You are setting it with

last_wifi_name = wifi_name;

but at the end of the loop iteration it's destroyed and at the beginning of the next loop iteration a new empty variable is created.