Any way to include the text of a file inside a sketch, to make it shorter and neater?
As suggested in comments, you can use #include statements to include texts of files into a sketch.  For example, suppose files c1.h,  c2.h,  c3.h,  c4.h, and  c5.h are in directory ~/sketchbook/tincludes, along with the file tincludes.ino, which is as follows:

#include &lt;Streaming.h&gt;
void setup() {
  const long int
#include "c1.h"
#include "c2.h"
#include "c3.h"
#include "c4.h"
#include "c5.h"
 etc=5   ;
  Serial.begin(115200);
  Serial &lt;&lt; z1o &lt;&lt;  z1c &lt;&lt;  z2o &lt;&lt;  z2c &lt;&lt;  z3o &lt;&lt;  z3c &lt;&lt;  z4o &lt;&lt;  z4c &lt;&lt;  z5o &lt;&lt;  z5c;
}


Further, suppose c1.h ... c5.h each contain one line of code, as follows:

==&gt; c1.h &lt;==
  z1o=25829, z1c=9774,

==&gt; c2.h &lt;==
  z2o=7850, z2c=25585,

==&gt; c3.h &lt;==
  z3o=7103, z3c=17981,

==&gt; c4.h &lt;==
  z4o=30827, z4c=24511,

==&gt; c5.h &lt;==
  z5o=4167, z5c=10036,


Then the sketch will compile ok, and the sketch-lines with const long int through etc=5   ; will compile the same as if they were replaced by 

const long int z1o=25829, z1c=9774, z2o=7850, z2c=25585, z3o=7103,
     z3c=17981, z4o=30827, z4c=24511, z5o=4167, z5c=10036, etc=5;