Hello I have problems with strings in my sketch. I assingn values to my string (for example tempStrReturn) in the functions setNr1, setNr2. If i pass the value 1 to the function i get "" or "application/x-www-form-urlencoded" when there should be 1. I don't understand why this is happening.

If you see the output printout from the function setNr1 i get "" then after assigning the value (nr1+"") to the tempStrReturn i still get "". After i use the tempStrReturn in the setNr2 function it asigns a mishmash value to it.

I dont use the tempStrReturn, she is just there for showing the error. When i change the int values of nr1 or nr2 that i want to assign to the string, the value mishmash in the strings changes. With the same numbers the string value has always the same mishmash.

Function setNR1............
Got: 0
NR1 int: 0
tempStrReturn before:
tempStrReturn after:
Function setNR2............
Got: 1
NR2 int: 1
tempStrReturn before:
tempStrReturn after: application/x-www-form-urlencoded
Function setID...........
String id: ID
Set msgId to: 0
Set msgPos to: 1
Function setNR1............
Got: 1
NR1 int: 1
tempStrReturn before: ID
tempStrReturn after: application/x-www-form-urlencoded
Function setNR2............
Got: 1
NR2 int: 1
tempStrReturn before: application/x-www-form-urlencoded
tempStrReturn after: application/x-www-form-urlencoded
Function setH...........
nr1: 1
nr2: 1
Created value: 11
Original value in string: 0
nrX: 0 nrPos: 1 Value i want to enter: n/x-www-form-urlencoded
Value afther assignment: n/x-www-form-urlencoded


#include <Arduino.h>
#include <ESP8266WiFi.h>
#include <ESPAsyncTCP.h>
#include <ESPAsyncWebServer.h>
#include <FS.h>
#include <Wire.h>

const char* ssid = "Ti";
const char* password = "1234567891234";
// Stores LED state
String ledState;
// Create AsyncWebServer object on port 80
AsyncWebServer server(80);

#define ledPin 2  //BuiltInLed is inverted
#define countMax 2500
#define multiplyMax 100 
int count = 0, multiply = 0, msgDel = 0;
int nr1=0, nr2=0,msgPos, msgId, msgVar;
String inOut[10][10];
String tempStr1, tempStr2, tempStrReturn;
bool oneShot = false;


/*Set value of String[] on the nrPos to val*/
void inOutSet(int nrX, int nrPos, String val)
  Serial.print("Original value in string: "); Serial.println(inOut[nrX][nrPos]);
  Serial.print("nrX: ");Serial.print(nrX);Serial.print(" nrPos: ");Serial.print(nrPos);
  Serial.print(" Value i want to enter: "); Serial.println(val);
  inOut[nrX][nrPos] = val;
  Serial.print("Value afther assignment: ");Serial.println(inOut[nrX][nrPos]); 

String inOutGet(int nrX, int nrPos)
  return inOut[nrX][nrPos];

String dataRequest(int roomNr) 
  tempStrReturn = "";
  Serial.println("Preparing to send data as word...");
  Serial.print("Value in TempStringReturn before: "); Serial.println(tempStrReturn);
  tempStrReturn = "g" + roomNr;
  tempStrReturn = tempStrReturn+".";
  Serial.print("Adding g and . : "); Serial.println(tempStrReturn);
  nr1 = 0; nr2 = 0;

  for (int i = 0; i < 9; i++)
    Serial.print("Data Request: "); Serial.println(msgId);
    tempStrReturn = tempStrReturn+inOut[roomNr][i];
    Serial.print("Ading value from inOut: "); Serial.println(tempStrReturn);
  Serial.print("Returning value: ");

  return tempStrReturn;

//saves value to nr1 as int
String setNr1(String var)
  Serial.println("Function setNR1............");
  Serial.print("Got: "); Serial.println(var);
  nr1 = var.toInt();
  Serial.print("NR1 int: "); Serial.println(nr1);
  Serial.print("tempStrReturn before: "); Serial.println(tempStrReturn);
  tempStrReturn = nr1+"";
  Serial.print("tempStrReturn after: "); Serial.println(tempStrReturn);
  return nr1+"";

//saves value to nr2 as int
String setNr2(String var)
  Serial.println("Function setNR2............");
  Serial.print("Got: "); Serial.println(var);
  nr2 = var.toInt();
  Serial.print("NR2 int: "); Serial.println(nr2);
  Serial.print("tempStrReturn before: "); Serial.println(tempStrReturn);
  tempStrReturn = nr2+"";
  Serial.print("tempStrReturn after: "); Serial.println(tempStrReturn);
  return nr2+"";

//Creates values for the array
String setID()
  Serial.println("Function setID...........");
  msgId = nr1;
  msgPos = nr2;
  nr1 = 0; nr2 = 0;
  tempStrReturn = "ID";
  Serial.print("String id: "); Serial.println(tempStrReturn);
  Serial.print("Set msgId to: "); Serial.println(msgId);
  Serial.print("Set msgPos to: "); Serial.println(msgPos);
  return "ID";

//Creates value from nr1 and nr2
String setH()
  Serial.println("Function setH...........");
  msgVar = (nr1*10)+nr2;
  Serial.print("nr1: "); Serial.println(nr1);
  Serial.print("nr2: "); Serial.println(nr2);
  nr1 = 0; nr2 = 0;
  if (msgVar >= 99)  {  msgVar = 100;  }
  Serial.print("Created value: "); Serial.println(msgVar);
  inOutSet(msgId, msgPos, msgVar+"");
  return "H";  

String getIO()
  nr1 = nr1/10;
  return dataRequest(nr1);

String processor(const String& var){
  Serial.print("Processor in: ");  Serial.println(var);
  return var;

void setup() {
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LOW);

  //Filling the inOut array
  for (int i = 0; i < 10; i++)
    for (int x = 0; x < 10; x++)
      inOut[i][x]= "0";


  if(!SPIFFS.begin()){Serial.println("\nSPIFFS chyba!"); return;}

  // Connect to Wi-Fi
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    Serial.println("Connecting to WiFi..");

  // Print ESP32 Local IP Address

  // Route for root / web page
  server.on("/", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send(SPIFFS, "/index.html", String(), false, processor);//change

  //route to favicon
  server.on("favicon.ico", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send(SPIFFS, "/favicon.png", "image/png");

  // Route to load style.css file
  server.on("/style.css", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send(SPIFFS, "/style.css", "text/css");

  // Route to javascript
  server.on("/main.js", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send(SPIFFS, "/main.js", "text/javascript");

// Nr1 of the msg
  server.on("/0", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("0").c_str());
  server.on("/1", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("1").c_str());
  server.on("/2", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("2").c_str());
  server.on("/3", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("3").c_str());
  server.on("/4", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("4").c_str());
  server.on("/5", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("5").c_str());
  server.on("/6", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("6").c_str());
  server.on("/7", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("7").c_str());
  server.on("/8", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("8").c_str());
  server.on("/9", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr1("9").c_str());
// Nr2 of the msg
  server.on("/X0", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X0").c_str());
  server.on("/X1", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X1").c_str());
  server.on("/X2", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X2").c_str());
  server.on("/X3", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X3").c_str());
  server.on("/X4", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X4").c_str());
  server.on("/X5", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X5").c_str());
  server.on("/X6", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X6").c_str());
  server.on("/X7", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X7").c_str());
  server.on("/X8", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X8").c_str());
  server.on("/X9", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setNr2("X9").c_str());
//end commands for use the nr values
  server.on("/ID", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setID().c_str());//Set ID nr
  server.on("/H", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", setH().c_str());//Set value to id
  server.on("/G", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send_P(200, "text/plain", getIO().c_str());//Get value

  // Start server

void loop() {
  if (count > countMax)  {    multiply++; count = 0;  }
  if (multiply > multiplyMax)
    if (!oneShot)
      oneShot = true;

    if (nr1 != 0 || nr2 != 0)
      if (msgDel > 5)
        nr1 = 0; nr2 = 0;
        msgDel = 0;


    multiply = 0;

1 Answer 1


nr1+"" is not how you convert an integer to a String. All you are doing there is taking the value of nr1 and adding it to the address of a string constant that consists of just a \0. Then assigning that to a String.

The results of such an operation are undefined.

Instead you should go:

tempStrReturn = String(nr1);

which will create a new String object, populate it with the text representation of the value of nr1 then copy that content into the tempStrReturn String.

However I would recommend avoiding using String, even on a chip as capable as the ESP32. Instead use C strings (NULL terminated character arrays) which are far less wasteful of resources.

  • My point is to add it to a String array. In the function setH i add it via inOutSet() to the two dimensional array inOut[][]. I then send a string message to my website that is completed in dataRequest(). It adds part of the array to one string that i send back to my website. I know that String is a little bit overkill for the poor processor. Any better idea?
    – Tomas
    Jun 5, 2020 at 10:20
  • Why use strings at all? You have numbers. Store them as such. You only care about them being stringified when you construct your HTTP request.
    – Majenko
    Jun 5, 2020 at 10:27
  • 80%of the values are numbers but not all. I wanted to have one inOut array with all my values. I try to make a web gui. This inOut array will store the values of Analog values, digital values or messages like "Something overfilled". I created the Website and the javascript so that i doesnt need to change the html, css or javascript. All i need to do is to program the sketch.
    – Tomas
    Jun 5, 2020 at 10:38
  • Do you really need to store the text "Something overfilled" in your array? Why not just a simple flag that says something overfilled, then when you construct your request you say "Is the something overfilled flag 1? Yes? Ok, send 'something overfilled'".
    – Majenko
    Jun 5, 2020 at 10:39
  • I use it as addresses. inOut[0][1] is the first item on the first page. It should be modular. I don't know in advance which address will be int and which string or what will be the text in it
    – Tomas
    Jun 5, 2020 at 10:40

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