help with file handing and if-elseif-else

The 'else' portion of this code will not work. If I type in something not found in the file nothing will be returned but 'City not found' is supposed to be returned. Can anyone help?

This is the code:

fid=fopen('cities.txt','r'); % opens the file

city = input('Enter a city ','s'); % asks for input and converts it to string

while ~feof(fid) % tests for end of file
  l=fgets(fid);
  if ~contains(l,city), continue, end  % keep going until found
  a = str2double(l(1:2)); % defines variables for coordinates
  b = str2double(l(4:5));
  c = str2double(l(8:9));
  d = str2double(l(11:12));

    if b > 66 % if the coordinate in range print 
        fprintf(city)
        fprintf(' is in a polar region \n')
    elseif b > 35 && b < 66
        fprintf(city)
        fprintf(' is in a temperate region \n')
    elseif b < 35
        fprintf(city)
        fprintf(' is in a tropical region \n')
    else
        fprintf('City not found!')
    end
  end

fclose(fid); % closes file

Answer 1

[Walter Roberson]

You have

while ~feof(fid) % tests for end of file
  l=fgets(fid);
  if ~contains(l,city), continue, end  % keep going until found

Suppose you have not reached end of file, then you fgets() to read a line. Then you check to see if the line contains the target string. If it does not, you "continue", which means you loop back to the "while" . Still not end of file? Read a line, test, it still doesn't contain, loop back. Under the assumption that the target string is not in the file, then contains() is never true, so you just keep looping until the feof() becomes true and ~feof() becomes false and you exit the loop -- without ever once having tested the value of b .

You should probably be using something like

found = false;
while ~found & ~feof(fid)
  ...
          fprintf(' is in a polar region \n');
          found = true;
          break;
  ...
end

if ~found
  fprintf('City not found!');
end

By the way: consider using the 'IgnoreCase' option of contains(). But if you do, watch out for ẞ because the rules for ignoring case might possibly not be well defined.

[Edgar Gonzalez]

using

while ~found & ~feof(fid)

returned this error

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

Error in test (line 7)
while ~found & ~feof(fid) % tests for end of file

but the file has already been opened in the first line

[Walter Roberson]

filename = 'cities.txt';
[fid, msg] = fopen(filename, 'r');
if fid < 0
  error('Could not open file "%s" because "%s"', filename, msg);
end

city = input('Enter a city ','s'); % asks for input and converts it to string

found = false;
while ~found && ~feof(fid) % tests for end of file
  l=fgets(fid);
  if ~contains(l,city), continue, end  % keep going until found
  a = str2double(l(1:2)); % defines variables for coordinates
  b = str2double(l(4:5));
  c = str2double(l(8:9));
  d = str2double(l(11:12));

      if b > 66 % if the coordinate in range print 
          fprintf(city);
          fprintf(' is in a polar region \n');
          found = true;
          break;
      elseif b > 35 && b < 66
          fprintf(city);
          fprintf(' is in a temperate region \n');
          found = true;
          break;
      elseif b < 35
          fprintf(city);
          fprintf(' is in a tropical region \n');
          found = true;
          break;
      else
          fprintf('city found, but b was infinite or nan or exactly equal to 66 or exactly equal to 35!')
          found = true;
          break;
      end
    end

fclose(fid); % closes file

if ~found
  fprintf('City not found!');
end