The SUBSTR function can be used on the left and right sides of an ASSIGNMENT statement. The SUBSTRN function can be used on the right side only.

 
   x=substrn(y,3,2);

If the SUBSTRN function is used on the left side, an error results since SAS assumes SUBSTRN is an array reference.

     
  data _null_;
    y='test string';
    substrn(y,3,2)='CAT';
  run;
    ERROR: Undeclared array referenced: substrn.
    ERROR: Variable substrn has not been declared as an array.

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