user50082: I'm beginning in Arduino I do not understand what the error means. Please tell me where I can find the resources to understand it and allow the code to compile.
#include <RCSwitch.h>
#include <Ultrasonic.h>

Ultrasonic ultrasonic(11,10);
RCSwitch mySwitch = RCSwitch();

int i;

void setup() {
    mySwitch.enableTransmit(4);  
}

void loop() {  
    i = ultrasonic.Ranging(CM);
    mySwitch.send(i, 24);
    delay(100); 
}


Arduino: 1.8.7 (Windows 7), Board: "Arduino/Genuino Uno"
433mhz-lcd-water-level-transmitter:2:24: error: Ultrasonic.h: No such
  file or directory
compilation terminated.
exit status 1 Ultrasonic.h: No such file or directory
This report would have more information with "Show verbose output
  during compilation" option enabled in File -> Preferences.


user4143: It means what it says:

Ultrasonic.h: No such file or directory

The file Ultrasonic.h can't be found.
Anything in your code that starts #include... is usually a library. These are third party pieces of software that you have to obtain and install into the relevant location (See Installing Additional Arduino Libraries).
Some libraries are bundled with the IDE, and some are available through the "library manager" in the IDE. However most aren't, and you will have to get used to finding those libraries and installing them.
The location you got your example code from will most likely link you to where to find the relevant libraries to get the code working.

user313: This kind of error means that the Arduino IDE (assuming that is what your are using) did not find 'Ultrasonic.h' in a folder named 'libraries' within your Arduino sketch folder. (Some libraries are contained in other places within the IDE's executable, but putting your additional libraries in '.../libraries' keeps everything easy to find and maintain).
Go to File | Preferences (in Windows or Linux) or Arduino | Preferences (Mac). The top box in that dialog is the path to your sketch folder. That folder should contain one called 'libraries'.
The libraries folder should have a folder for each library, so in this case, there should be a folder named 'Ultrasonic', containing files 'Ultrasonic.h' and 'Ultrasonic.cpp'. There may be other items in there as well, perhaps an Examples folder, a 'keywords.txt' file (helps the IDE with syntax coloring), a text file with release notes, etc., but the important structure is
[path/to/your/sketch/folder]
     libraries
         Ultrasonic
              Ultrasonic.h
              Ultrasonic.cpp
         someotherlibrary
              someotherlibrary.h

etc.

50082: <p>I'm beginning in Arduino I do not understand what the error means. Please tell me where I can find the resources to understand it and allow the code to compile.</p>

<pre><code>#include &lt;RCSwitch.h&gt;
#include &lt;Ultrasonic.h&gt;

Ultrasonic ultrasonic(11,10);
RCSwitch mySwitch = RCSwitch();

int i;

void setup() {
    mySwitch.enableTransmit(4);  
}

void loop() {  
    i = ultrasonic.Ranging(CM);
    mySwitch.send(i, 24);
    delay(100); 
}
</code></pre>

<blockquote>
  <p>Arduino: 1.8.7 (Windows 7), Board: "Arduino/Genuino Uno"</p>
  
  <p>433mhz-lcd-water-level-transmitter:2:24: error: Ultrasonic.h: No such
  file or directory</p>
  
  <p>compilation terminated.</p>
  
  <p>exit status 1 Ultrasonic.h: No such file or directory</p>
  
  <p>This report would have more information with "Show verbose output
  during compilation" option enabled in File -> Preferences.</p>
</blockquote>
 4143: <p>It means what it says:</p>

<blockquote>
  <p>Ultrasonic.h: No such file or directory</p>
</blockquote>

<p>The file <code>Ultrasonic.h</code> can't be found.</p>

<p>Anything in your code that starts <code>#include...</code> is usually a <em>library</em>. These are third party pieces of software that you have to obtain and install into the relevant location (See <a href="https://www.arduino.cc/en/Guide/Libraries" rel="nofollow noreferrer">Installing Additional Arduino Libraries</a>).</p>

<p>Some libraries are bundled with the IDE, and some are available through the "library manager" in the IDE. However most aren't, and you will have to get used to finding those libraries and installing them.</p>

<p>The location you got your example code from will most likely link you to where to find the relevant libraries to get the code working.</p>
 313: <p>This kind of error means that the Arduino IDE (assuming that is what your are using) did not find 'Ultrasonic.h' in a folder named 'libraries' within your Arduino sketch folder. (Some libraries are contained in other places within the IDE's executable, but putting your additional libraries in '.../libraries' keeps everything easy to find and maintain).</p>

<p>Go to <code>File | Preferences</code> (in Windows or Linux) or <code>Arduino | Preferences</code> (Mac). The top box in that dialog is the path to your sketch folder. That folder should contain one called 'libraries'.</p>

<p>The libraries folder should have a folder for each library, so in this case, there should be a folder named 'Ultrasonic', containing files 'Ultrasonic.h' and 'Ultrasonic.cpp'. There may be other items in there as well, perhaps an Examples folder, a 'keywords.txt' file (helps the IDE with syntax coloring), a text file with release notes, etc., but the important structure is</p>

<pre><code>[path/to/your/sketch/folder]
     libraries
         Ultrasonic
              Ultrasonic.h
              Ultrasonic.cpp
         someotherlibrary
              someotherlibrary.h
</code></pre>

<p>etc.</p>
