I have the following example script. ; reads data columns from file and plots it. ; columns are: year, month, day, hour, minute, second, msec, uvel, vvel, wvel, temp, co2, h2o, press files = ['wc151_1804_new.txt', 'wc151_18010_new.txt', 'wc151_18016_new.txt', $ 'wc151_18112_new.txt'] n = SIZE(files, /N_ELEMENTS) FOR i = 0, n-1 DO BEGIN f = files[i] l = FILE_LINES(f) cols = 14 data = FLTARR(cols, l) OPENR, iunit, f, /GET_LUN FOR j = 0, l-1 DO BEGIN READF, iunit, year, month, day, hour, minute, second, msec, uvel, $ vvel, wvel, temp, co2, h2o, press temp = [year, month, day, hour, minute, second, msec, uvel, $ vvel, wvel, temp, co2, h2o, press] FOR k = 0, cols-1 DO BEGIN data[k, j] = temp[k] ENDFOR ENDFOR FREE_LUN, iunit PLOT, data[10,*], data[13,*], $ TITLE = 'temperature vs pressure', $ XTITLE = 'degrees celsius', $ YTITLE = 'pascal' ENDFOR I am new to IDL and would appreciate if someone can inform me why I get these errors. % Attempt to subscript FILES with I is out of range. % Execution halted at: $MAIN$ % FILE_LINES: String expression required in this context: F. % Execution halted at: $MAIN$ % FLTARR: Variable is undefined: L. % Execution halted at: $MAIN$ % OPENR: Filename argument must be a scalar string: F. % Execution halted at: $MAIN$ % Variable is undefined: L. % Execution halted at: $MAIN$ % READF: Variable is undefined: IUNIT. % Execution halted at: $MAIN$ % Variable is undefined: YEAR. % Execution halted at: $MAIN$ % Variable is undefined: TEMP. % Execution halted at: $MAIN$ ENDFOR ^ % Syntax error. At: /home/malapradej/Documents/Programming/IDL/exercises/bowman/ch11/read_file_plot.pro, Line 21 ENDFOR ^ % Syntax error. At: /home/malapradej/Documents/Programming/IDL/exercises/bowman/ch11/read_file_plot.pro, Line 22 % FREE_LUN: Variable is undefined: IUNIT. % Execution halted at: $MAIN$ % Variable is undefined: DATA. % Execution halted at: $MAIN$ ENDFOR ^ % Syntax error. At: /home/malapradej/Documents/Programming/IDL/exercises/bowman/ch11/read_file_plot.pro, Line 30