fortran에서 특정 확장자 파일만 불러오는거는 어떻게 해야하죠?

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읽고 쓰는 예제
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program TEST
	integer ans
		do
			read (*,*) ans
			if (ans.eq.42) stop
			write (*,*) ans
		enddo
	stop
end
 
 
---------
입력 내용
---------
1
2
10
42
11
 
 
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출력 내용
---------
           1
           2
          10

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5자리 우측에 값을 출력하기
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program TEST
	character(len=20) :: str = '   1234567890.txt'
	str = adjustl(str)
	str = str(5:);
	print *, str
	stop
end
 
 
---------
출력 내용
---------
567890.txt          

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한문자씩 출력하기
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program TEST
	! your code goes here
	integer i
	character(len=20) :: str = '   1234567890.txt'
	do i=1,100
		print *, str(i:i)
	end do
	stop
end
 
 
 
---------
출력 내용
---------
 1
 2
 3
 4
 5
 6
 7
 8
 9
 0
 .
 t
 x
 t

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한문자씩 늘리며 출력하기
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program TEST
	! your code goes here
	integer i
	character(len=20) :: str = '   1234567890.txt'
	do i=1,100
		print *, str(1:i)
	end do
	stop
end
 
 
---------
출력 내용
---------
    1
    12
    123
    1234
    12345
    123456
    1234567
    12345678
    123456789
    1234567890
    1234567890.
    1234567890.t
    1234567890.tx
    1234567890.txt
    1234567890.txt 
    1234567890.txt
    1234567890.txt 
    1234567890.txt
    1234567890.txt 

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.txt 를 찾아서 출력하기
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program TEST
	! your code goes here
	integer i
	character ch
	character(len=21) :: str = '   1234567890.txt'
	integer find
	find = 0
	do i=1,20
		ch = str(i:i)
 
		if (ch .EQ. '.') then
			print *, str(i:i)
			find = find + 1
			continue
		endif
 
		if ((ch .EQ. 't') .AND. (find .EQ. 1)) then
			print *, str(i:i)
			find = find + 1
		endif
 
		if ((ch .EQ. 'x') .AND. (find .EQ. 2)) then
			print *, str(i:i)
			find = find + 1
		endif
 
		if ((ch .EQ. 't') .AND. (find .EQ. 3)) then
			print *, str(i:i)
			find = find + 1
		endif
	end do
 
	if(find .EQ. 4) then
		print *, ".txt를 찾았다."
	endif
 
	stop
end
 
 
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출력 내용
---------
 .
 t
 x
 t
 .txt를 찾았다.