The WHILE loop

If you need a loop for which you don't know in advance how many iterations there will be, you can use the `while' statement.

while condition do statement

Again, if you are using this in a program you can replace statement with a block of statements - the block must end with an endwhile . You can construct condition in the same way as for an if statement.

Here is an example (modtran_while.pro), but this time we don't know the number of lines in advance.

;Set up array with far too many rows to contain the  data 
modtrandata=fltarr(14,5000)

openr,1,'~lecjm/public_html/Teaching/IDL_course/Data/pldat.dat'

inputline = fltarr(14)          ; set up a variable to hold one row of the file
linecount=0                     ; variable to count lines

;;;; start while loop to read in the data
while not eof(1) do begin       ; eof becomes true when we reach the End Of
                                ; the File.
    readf,1,inputline
    modtrandata(*,linecount)=inputline
    linecount=linecount+1

endwhile
modtrandata=modtrandata(*,0:linecount-1) ; truncate array to actual length
                                         ;  of file.
close,1

OR (modtran_while2.pro)

pro modtran_while2,modtrandata

openr,1,'~lecjm/public_html/Teaching/IDL_course/Data/pldat.dat'

inputline = fltarr(14)          ; set up a variable to hold one row of the file
linecount=0                     ; variable to count lines

;;;; start while loop to read in the data
while not eof(1) do begin       ; eof becomes true when we reach the End Of
                                ; the File.
    readf,1,inputline
    if linecount eq 0 then modtrandata=inputline else modtrandata=[[modtrandata],[inputline]]
    linecount=linecount+1

endwhile
modtrandata=modtrandata(*,0:linecount-1) ; truncate array to actual length
                                         ;  of file.
close,1