Using Logic, IF Statements


Assignment :

Study the contents of the sample program iftests.f and compile it with the -g option. Step through the program under xldb or dbx using the "step" command to follow the flow of the program.

New Fortran :

IF, IF-THEN-ELSE-ELSE IF, GO TO, CONTINUE, statement labels, Arithmetic IF

Now that we can make some fairly complicated comparisons, we need some statements to put them to use in a Fortran program. The most important of these is the IF statement. In its simplest form a logical IF simply executes some other single statement if some logical expression evaluates to ".true.".

      if (iprint.eq.1) print *, 'The value of x is ', x
      if ( x.le.5) y=0
      if( x.gt. 5) y = (x-5.)**2
The last two of these if statements can be combined into a more powerful block IF ,THEN, ELSE construct.

      if ( x.le.5) then
         y=0
      else
         y=(x=5.)**2
      endif
Note the use of indentation in this string of statements to add clarity. Also note that the THEN never exists in isolation. It always exists at the end of a statement beginning with IF, and tells the computer to execute the following block of statements (block ended with an ELSE or ENDIF) if the logical expression is true.

The ELSE portion is not mandatory, and more than one statement can be included in the conditional block:

      if (iprint.gt.1)   then
         print *, 'The value of x is ', x
         iprint=iprint+1
      endif
The IF,THEN,ELSE block structure was introduced in Fortran 77. Before that the single statement form of the logical IF was used. How did we deal with the need for the more complicated conditional code structures? The secret was a combination of statement labels, and the Fortran GO TO statement. The above example might have been programmed as:

      if (iprint.le.1)   go to 100
         print *, 'The value of x is ', x
         iprint=iprint+1
  100 z=x+3
where the final labeled line is just the next statement in the program after the "endif" in the previous example. The label can be placed anywhere in columns 1 through 5, enough space to contain numbers through 99999. As you would expect a given label number can be assigned to only one Fortran statement in any given program unit. Several subroutines in the same program can all contain a label 100, because they are ignorant of the details of each other's internal contents. Please note the change in the conditional test so that execution branches to the statement at label 100 when iprint is less than or equal to one. This older Fortran construct is very close to what happens in the machine CPU conditional Branch instructions. In archaic Fortran, the earlier, more complex IF, THEN, ELSE example would be:

      if ( x.gt.5) go to 110
         y=0
      go to 120
 110     y=(x=5.)**2
 120  continue
The last line introduces another Fortran Statement, CONTINUE. It is a place marker statement, used as a target of a branch, or, as we will see later, the end of a loop. However, CONTINUE does not do anything itself. I admit this is a bit of a paradox in our grand scheme of things, an "executable statement" that actually doesn't represent the execution of any machine instruction.

If you think about the last example above, you should realize that "GO TO" can result in very complex, unreadable code, often producing surprising results. This problem led directly to the IF, THEN, ELSE, and some later Fortran 90 organized structures. It has also led to other languages that don't even contain the direct equivalent of a "GO TO", and teachers who don't admit its existence.

IF, THEN, ELSE's can be nested (one IF, THEN, ELSE contained in another.

      if ( x.le.5) then
         y=0
         if( x.le.2) then
            z=0.
         else
            z=2.
         endif
      else
         y=(x=5.)**2
         z=3.
      endif
Note the importance of proper indentation in the above structure, to make it readable. This is a point where I have problems with modern structured programming emphasis on using these structures to the exclusion of "GO TO" statements. There comes a point where so many lines of code exist between each IF and ELSE, and so much nesting is going on, that the code again becomes unreadable. There are instances where it is easier to decipher a program by doing a pattern search on specific label numbers, than by tracking indentation. Good programming like life is a balancing act.

IF structures have one more level of complexity, the ELSE IF statement.

      if ( x.gt.5) then
         y=100.
      else if ( x.gt.4.) then
         y=50
      else if ( x.gt.2) then
         y=25.
      else
         y=0.
      endif
It is important to know that when the first IF is found to be true in this structure, no other IF's are tested. Flow branches to the ENDIF statement after completion of statements associated with the first true IF test. In the above example, if x is 5.5, then the resulting value of y is 100.

While we are on the subject of IF's, you should be aware of one archaic form called the arithmetic IF. I introduce this only because some of you will be asked to improve an existing program. You should not use this in any new programming. There is a good chance that it will be dropped from some future Fortran standard. The arithmetic IF can be recognized by the fact that the expression within the parentheses results in a numerical result, not a logical (true, false) result. The other give-away is the string of three integers (statement label references) after the right parenthesis.

      if ( b**2-4*a*c) 100,200,300
  100    print *, 'Roots are Complex'
      go to 400
  200    print *, 'Single Real Root' 
      go to 400
  300    print *, 'Two Real Roots'
  400 continue
If the arithmetic expression produces a value less than zero, execution branches to the first label in the list. If it is zero, control branches to the second label, and if the value is greater than zero the program branches to the third label. Repetition of label numbers is permitted in the IF's label list. Note that I must include extra "GO TO" statements to prevent execution of some of the PRINT statements after the initial branch to another PRINT.


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Written and Maintained by John Mahaffy : jhm@psu.edu