Chapter 4. More Fortran Elements: DO Loops and Input/Output 4.1 ...

Chapter 4. More Fortran Elements: DO Loops and Input/Output18184.1 Iteration: Fortran DO LoopsOne of the easiest ways to repeat a calculation or series of steps multiple times in a Fortranprogram is to use a construction called a DO loop. The basic structure of a DO loop is as follows:DO xxx I = START, STOP, STEP::calculations to be iterated::xxxCONTINUEEach DO loop must begin with a DO statement and end with a CONTINUE statement. Anynumber of lines of calculations and I/O may be included in between. The continue statement musthave a statement number label (xxx in the example above). This statement number must be indexedin the initial DO statement.The "I" in the DO statement is the counter, which gets incremented on each pass throughthe loop. It can be any integer variable. START and STOP in the example above are also integers(either variables or explicit numbers), and give the range over which the DO loop runs. The counterI starts with the value START and continues until the value STOP. Once the loop counter hasexceeded the value STOP, execution of the program will proceed beyond the CONTINUEstatement. At that point the loop counter will have a value of STOP+STEP.Each time through the loop, the counter I is incremented by an amount STEP (STEP is alsoan integer constant or variable). The value of STEP is usually 1, and STEP must be given on theDO line only if it is a value other than 1. The increment STEP may be any positive or negativeinteger.The following segment of Fortran code gives an example of a typical DO loop construction.SUM = 0.0DO 100 J = 1, 5SUM = SUM + JWRITE(*,*) SUM100 CONTINUENote that no STEP variable is given in the DO loop, so the counter J is incremented by one eachtime through the loop. Though not required, it is common practice to indent the lines inside the DOloop. Table 4.1 shows the outcome of each time through the DO loop.

4.2 Use of Files for Input and OutputIn addition to reading and writing on the screen in Fortran, data can be read and written toand from files. This is particularly useful in order to save the results for later plotting or analysis.To read and write data to and from files, each file must be assigned a "unit number" that identifiesthe file.To write the results of a calculation to a file instead of to the screen, a WRITE statement isused. In addition, for writing results to a file, the file must be opened before data can be written andclosed after the writing is complete. Before writing to a file, an OPEN statement is used,2020OPEN(UNIT=xx, FILE='filename', STATUS='unknown')The unit number xx in the example above is associated with the file called "filename". The unitnumber may be any integer value or integer variable (with the exception of 5 and 6), but typicallynumbers between 1 and 100 are used. The values 5 and 6 are reserved for reading from the screenand writing to the screen. More than one file may be opened at any one time, if data is to be writtento more than one file.The STATUS indicator in the OPEN statement may take three possible values: 'new', 'old',and 'unknown'. If the STATUS is 'new', a new file with the name 'filename' is created, and if thefile already exists, an error message is generated and the program exits. If the STATUS is 'old', it isassumed that the file with the name 'filename' already exists. If it does not exist, an error message isgenerated and the program exits. If the STATUS is 'unknown', the program proceeds regardless ofwhether the file 'filename' exists or not. If it does not exist, a new file will be created. If it doesexist, the old file will be overwritten. The use of STATUS= 'unknown' is generally recommendedin most programming situations.To write data to a file that has been OPENed, the WRITE statement has the form:WRITE(xx,*) item1, item2, …where xx denotes the same unit number chosen in the OPEN statement above. This will placewhatever variables are designated in the WRITE statement in the file designated by UNIT xx. Eachseparate WRITE statement will place output on a new line in the file. A blank line can be writtenusing a WRITE statement of the form:WRITE(xx,*)Once all the results have been written to a file, the file must be closed using the statement:CLOSE(UNIT=xx)To read data from a file, the OPEN and CLOSE statements also must be used. Data may beread from a file using a READ statement:READ(xx,*) item1, item2, …Each READ statement reads data from a single line of the file. In the case of a READ statement, itis assumed that the file named by the OPEN statement already exists and contains some input datato the program. Otherwise, if the file does not exist, the program will attempt to read data from it,

and an error message will result. A blank line can be read in a file using a READ statement of theform:2121READ(xx,*)The following example program adds the first 100 integers and saves the running total to a file.PROGRAM SUM100IMPLICIT NONEINTEGER SUM, IC Open a file (unit 25) for writing.OPEN(UNIT=25, FILE='sum.dat', STATUS='unknown')C Put a heading on the output.WRITE(25,*) 'Integer', 'Cumulative Sum'C Initialize the sum to zero.SUM = 0C Use a DO loop from 1 to 100 to add the first 100C integers. Write out a running total to unit 25.DO 100 I = 1, 100SUM = SUM + IWRITE(25,*) I, SUM100 CONTINUEC Close the data file.CLOSE(UNIT=25)STOPEND

Chapter 4 Review Problems1. For each example, how many times will the loop be executed?a. DO 10 I = 10, 10b. DO 10 I = 1, 10, 3c. DO 10 I = –2, 2d. DO 10 I = 10, 5, –12. After executing the following segment of code, what values will be displayed?INTEGER I, MM = 0DO 100 I = 1, 5M = M + 1WRITE(*,*) I, M100 CONTINUE3. Construct a DO loop to calculate the value of n!, the factorial. Assume that you are given thevalue of n (a positive integer).4. Consider a file containing the following data.1.0 2.22.0 3.93.0 6.14.0 7.85.0 9.56.0 12.1The data consists of six X, Y pairs. Construct a segment of Fortran code that employs a DOloop to read in the pairs from a file assigned to unit 30.2222