fscanf problem and get invalid file identifier

base on handbook i write the fucntion as below.

but i am confuse why i got error

Error using fscanf

Invalid file identifier. Use fopen to generate a valid file identifier.

function [npoin,nelem,nvfix,ntype,nnode,ndofn,ndime,ngaus, ...
 nstre,nmats,nprop,lnods, matno,coord,props,nofix, ...
 iffix,fixed]=input_fem_elast();

 format long;

 global in;
 global out;

 %--- read the Control data:

 npoin=fscanf(in,'%d',1);
 

Answer 1

[Walter Roberson]

• possibly the fopen() failed
• possibly the fopen worked but you did not assign it to a variable named in
• possibly the fopen worked and you assigned to in but in was not declared global in the place it was assigned to