can not getting the data from if condtion ROS ardunio

Question

I have a node running on pc which published [std_msgs/String] in string It published M, R, S, I have the following Arduino code.

#include <ros.h>
#include <std_msgs/Empty.h>
#include <std_msgs/String.h>
ros::NodeHandle  nh;

void messageCb( const std_msgs::String & toggle_msg)
{
nh.loginfo("recived new message ");
nh.loginfo(toggle_msg.data);

if(toggle_msg.data == "M")
{  
nh.loginfo("Recived M if-statment ");  
}

else if(toggle_msg.data == "R")
{  
nh.loginfo("Recived R if-statment ");  
}

else if(toggle_msg.data == "S")
{  
nh.loginfo("Recived S if-statment ");  
}
}

ros::Subscriber<std_msgs::String> sub("talker_vision", &messageCb );

void setup()
{ 
  pinMode(13, OUTPUT);
  nh.initNode();
  nh.subscribe(sub);
}

void loop()
{  
  nh.spinOnce();
  delay(1);
}

I received M , R , S from nh.loginfo(toggle_msg.data); but when I applied If condition

if(toggle_msg.data == "M")
{  
nh.loginfo("Recived M if-statment ");  
}

there is no output.

Answer 1

The problem you are facing is that you are comparing two strings here which are defined as char*, not the Arduino classic String type is used. You can see the type of the string inside the message which was generated for example here.

Comparing

const char* str1 = "abc";
const char* str2 = "abc";
bool equal = str1 == str2;

will be always false since you are comparing memory positions and not the string itself.

So you have two options:

Create two Arduino string objects and compare them

String arduinoString1 = String(str1);
String arduinoString2 = String(str2);
bool equal = arduinoString1 == arduinoString2;

Or even better is to use the standard string comparision strcmp (documentation) function as described here for example:

bool equal = strcmp(str1, str2) == 0;