FORTRAN Lesson 4. Reese Haywood Ayan Paul

Transcription

1 FORTRAN Lesson 4 Reese Haywood Ayan Paul In Today s lesson we wll learn how to OPEN, READ and WRITE data fles. I wll then gve you the tools necessary for fttng a least squares lne to set of data ponts. Fnally, we wll wrte a program that reads n a three column data fle, fnds the least squares starght-lne ft to the data, prnts the results to the screen and wrtes the data to a fle. We wll then plot the results usng gnuplot. 1 Openng an external data fle All of the programs we have wrtten thus far have taken user nput drectly from the keyboard. Ths method s fne for small data sets, but s annoyng when the data sets become large compared, 1000 sets of x, y, and error, for example, the program runnng your detector spts out, energy, counts and error n the counts durng a data run. Gettng the data nto the Fortran program means we have to open the fle. The OPEN statement has the general form, bbbbbbopen (UNIT=nteger expresson, FILE=flename, STATUS=lteral) The nteger expresson desgnates the unt number assgned to the fle, the flename refers to the name gven to the fle when t was created, and the status lteral tells the computer whether the fle s an nput fle or an output fle. If the fle s an nput fle, t s specfed wth STATUS= OLD ; f the fle s an ouptut fle, t s specfed wth STATUS= NEW. If the fle exsts, and we want to add more data to t, we can use the STATUS= APPEND command. Ths wll append the data to the end of the fle; n ths case we 1

2 would not use a rewnd command, otherwse we wll lose the data we are tryng to save. The OPEN must be called before the READ or WRITE statments n the program. The OPEN should be executed only once; don t put t n a loop. Some systems requre a REWIND statement after openng an nput fle to poston the fle at ts begnnng. 2 Readng data from a fle To read from a data fle, we use an extenson of the lst-drected READ statement that has the followng form: bbbbbbread (unt number,*) varable lst The unt number s the same number entered n the OPEN(UNIT=nteger number) statment. Ths allows us to open multple fles for readng, wrtng, or readng and wrtng at the same tme,.e. use a dfferent nteger number for each fle to be opened. 3 Wrtng to a data fle To wrte nformaton to a data fle, we use a new statement a WRITE statement. Just lke the PRINT statement, the WRITE statement can be used wth lst-drected output or formatted output. The lst-drected WRITE statement has the followng form: bbbbbbwrite (unt number,*) expresson lst The formatted WRITE statement has the followng general form: bbbbbbwrite (unt number,k) expresson lst 2

3 where k s the statement number of the correspondng FORMAT statement. In all of these general forms, the unt number corresponds to the unt number assgned n the OPEN statement. The asterck followng the unt number specfes that we are usng lst-drected nput or output. Many systems assgn the standard nput devce (typcally the termnal keyboard) to unt number 5 and the standard output devce (typcally the termnal screen) to unt number 6; these devces are used when your program executes READ* or PRINT* statements. You should avod usng these numbers as unt numbers n yout OPEN statements etc. 4 Closng a fle When we are fnshed executng a program that has used the fles, the fles are automatcaly closed before the program termnates. Occasonlly there are stuatons n whch you would lke to specfcally close, or dsconnect a fle from your program. Ths s done wth the CLOSE command. It has the general form lsted below: bbbbbbclose (UNIT=nteger expresson) 5 Rules to remember We now summarze some mportant rules to remember when readng data from data fles. 1. Each READ statement wll start wth a new lne of data. If there are values left on the prevous lne that were not read, they wll be skpped. 2. If a lne does not contan enough values to match to the lst of varables on the READ statement, another data lne wll automatcally be read to acqure more values. Addtonal data lnes wll be read untl values have been acqured for all the varables lsted on the READ statement. 3. A READ statement does not have to read all the values on the data lne: however, t does have to read all the values on the lne pror to the values that you want. For example, f a fle has 5 values per lne and you are nterested n the thrd and fourth values, you must read 3

4 the frst and second values to get to the thrd and fourth values, but you do not need to read the ffth value. If you have a data fle that looks lke ths: The followng statement wll correctly read a tme and temperature value from the data fle: bbbbbbread(10,*) tme, temp If we had used the followng command, bbbbbbread(10,*) tme bbbbbbread(10,*) temp then the program would have read two lnes and the values stored n tme and temp wll be 0.0 and 0.1 respectvely. 6 Technques for Readng Data Fles To read nformaton from a data fle, we must know, the name of the fle and what nformaton s stored n the fle. The nformaton must be very specfc, such as the number of values entered per lne and the unts of the values.e. (sec, Kelvn, etc.). In addton, we need to know f there s any specal nformaton n the fle that wll be useful n decdng how many records are n the fle, or how to determne when we have read the last record. If we execute the READ statement after we have reached the end of a fle, we wll get an error message and the program wll qut. 4

5 6.1 If we know the number.. If we know the number of records, or data ponts, n the fle we can use a DO loop to process the data. If the number of lnes s gven to us n the header of the fle we can use that to read n the data. For example, we can read n the followng data wth the correspondng code below: The data are saved n a fle named RESULTS1. The followng code would read the data set: bbbbbbopen (UNIT=10, FILE= RESULTS1, STATUS= OLD ) bbbbbbread (10,*) COUNT bbbbbbif (COUNT.LT.1) THEN bbbbbbbbbbprint*, NO DATA ACCORDING TO RECORD COUNT bbbbbbelse bbbbbbbbbbsum=0.0 bbbbbbbbbbdo 20 J=1,COUNT bbbbbbbbbbbbbbread(10,*) TIME,TEMP bbbbbbbbbbbbbbsum=sum+temp bbb20 bbbbb CONTINUE 6.2 If we know there s a traler.. As the name suggest, the traler s the last record n the fle. The traler s usually a specfc number that represents the end of the data. We could read the data below nto the program wth the correspondng code:

6 The code to read ths n would be, assumng the fle s named RESULTS2: bbbbbbopen (UNIT=10,FILE= RESULTS2, STATUS= OLD ) bbbbbbsum=0.0 bbbbbbcount=0 bbbb5 b READ (10,*) TIME, TEMP bbbbbbbbbbif (TIME.NE ) THEN bbbbbbbbbbsum=sum+temp bbbbbbbbbbcount=count+1 bbbbbbbbbbread(10,*) TIME, TEMP bbbbbbbbbbgo TO 5 bbbbbbend IF Here we have used the smple whle loop we used n the Average.f program. 6.3 The END opton The READ command has one other opton, END. In general, the READ statement would look lke ths: bbbbbbread (unt number,*,end=n) varable lst A READ statement usng the END opton s used when the number of data ponts s unknown. The END opton tells the program that the end of the fle has been read, and nstead of crashng, sends executon to the statement labeled by n. An example code s gven below: bbbb5 b READ(10,*,END=15) TIME, TEMP bbbbbbbbbbsum=sum+temp bbbbbbbbbbcount=count+1 bbbbbbbbbbgo TO 5 bbb15 b PRINT*,SUM 6

7 Ths specal mplementaton fo the whle loop should only be used when you do not know the number of data lnes to be read and there s no traler sgnal at the end of the fle. The choce of the correct technque for readng data from a data fle depends on the nformaton n the data fle. If you know each lne represents the data you need, you can use the Lnux command wc -l flename to get the number of lnes n the fle. Ths can then be nput nto the program, ether from the user, or hardcoded n. 7 Wrtng data fles Wrtng data s essentally the exact opposte of readng. If we wanted to wrte out the values of the tme and the values of a snusod to a data fle we would use the followng code: bbbbbbopen (UNIT=10,FILE= SONAR,STATUS= NEW ) bbbbbbdo K=0,NUMBER-1 bbbbbbbbbbtime=real(k)*t bbbbbbbbbbsignal=a*cos(2.0*pi*freq*time) bbbbbbbbbbwrite (10,*) TIME,SIGNAL bbbbbbend DO For now we wll use the unformatted wrte statement, f we needed a specfc format we could use the formatted wrte statement, whch we wll return to later. 8 Vectors Before we can wrte our lne fttng routne, we need to learn how to defne vectors n Fortran. There are two dfferent ways to defne a vector, we can use the DIMENSION statement, or the REAL/INTEGER statements mentoned earler. The general form of each s below: bbbbbbdimension array1(sze),array2(sze), 7

8 bbbbbbreal array1(sze) bbbbbbinteger array2(sze).. When we use the DIMENSION statement the data type s defned by the name we gve the array. If the array name begns wth (A-H) or (O-Z) the data type s assumed REAL. If the name begns wth (I-N) the data type s assumed INTEGER. Usng the REAL and INTEGER declaraton avods the mplct namng problems that can occur, I prefer usng them. Notce that the data type for all values n the array are the same, all elements are ether REAL or INTEGER, they don t mx and match. As an example, f we wanted to defne an nteger array, J, and a real array, DIST, each wth 50 ponts, we would use the followng equvalent commands: bbbbbbdimension J(50), DIST(50) or bbbbbbreal DIST(50) bbbbbbinteger J(50) Once the vectors are defned we stll have to put data values n them. Some complers wll automatcally set all elements equal to 0. Do not assume your compler wll. If we wanted to set the values of DIST to zero before we begn any operatons we can use the followng code: bbbbbbdo I=1,50 bbbbbbbbbbdist(i)=0. bbbbbbend DO Here the ndex, I, represents the elements subscrpt,.e. a. To pck off any gven element, we use parantheses besde the name of the varable. If we wanted to defne a sngle element to be 0, we would use the followng command, J(5)=0. Ths would let element number 5 n the vector J be 0. Note Arrays start wth ndex 1 and not wth ndex 0. 8

9 9 Your next task You wll wrte a program to read n a set of data, x,y, and the error n y. The data wll be used to fnd the lne that mnmzes χ 2. Fnally, wrte a new fle that contans the data values, and the ft to the lne. We wll plot ths usng gnuplot. 9.1 Before you begn You need to know the equatons needed for fndng the slope and the ntercept these are gven below. The slope, a, and the ntercept, b, that mnmze χ 2 are: where, A = D = =1 =1 X X 2 The errors n a and b are: a = b = B = E = δa = δb = E B C A D B A 2 D C E A D B A 2 =1 =1 1 X Y B D B A 2 D D B A 2 Wth these quanttes we can now calculate χ 2 by: χ 2 = C = F = (Y (a X + b )) 2 =1 =1 =1 Y Y 2 9

10 9.2 The data fle You wll fnd a data fle you can use to check your program on my webpage. You should remember the 5 step problem solvng process whle wrtng you program. You can verfy your program s gvng you the correct answers by creatng you own set of straght lne data,.e., use a data fle lke the followng as your nput: Usng the above data, your program should gve you slope=2 and ntercept=0 (You can fnd a fle contanng the lnear test data set on my webpage). The errors n each should be rather small also. You wll fnd my verson of the program and the output fles (lne.dat and stat.dat) on my webpage. 9.3 Usng gnuplot To use gnuplot type gnuplot <ENTER> n the termnal. once the program opens, type: prompt% plot nputflename usng 1:2:3 wth errorbars, outputflename wth lne <ENTER> Ths should gve you a plot of the ponts, and the straght lne ft you found wth your program. You can use any other plottng program you lke, gnuplot s the one I am most famlar wth. 10