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Arithmetic versus logical IF

Short answer

Arithmetic IF uses sign of an expression to branch to three labels (negative, zero, positive). Logical IF executes one statement when a logical condition (.GE., .LT., etc.) is true. Arithmetic IF is obsolescent, needs labels; logical IF is modeern, no label.

Compare arithmetic and logical if statements in FORTRAN with suitable example.

Arithmetic IF

It is a three-way branch controlled by the sign of an arithmetic expression.

Syntax: IF (expression) label1, label2, label3

Control goes to label1 if the value is negative, label2 if zero, label3 if positive.

      READ *, X
      IF (X) 10, 20, 30
   10 PRINT *, 'NEGATIVE'
      GO TO 40
   20 PRINT *, 'ZERO'
      GO TO 40
   30 PRINT *, 'POSITIVE'
   40 STOP
      END

Logical IF

It executes one statement only when a logical (true/false) condition holds.

Syntax: IF (logical expression) statement

The logical expression uses relational operators (.LT. .LE. .EQ. .NE. .GT. .GE.) and logical operators (.AND. .OR. .NOT.).

      READ *, X
      IF (X .LT. 0.0) PRINT *, 'NEGATIVE'
      IF (X .GE. 0.0) PRINT *, 'NOT NEGATIVE'

The statement cannot be another logical IF or a DO. For more than one statement the block form IF (cond) THEN ... ELSE ... END IF is used.

Comparison

BasisArithmetic IFLogical IF
ExpressionArithmetic (number)Logical (true/false)
BranchesThree (negative, zero, positive)One statement, executed if true
Needs labelsYes, threeNo
StyleOld, hard to read (GOTO-like)Clear and modern
Marked obsolescentYes (deleted in newer standards)No

✓ Answer checked by Claude Opus 5.5 (high effort)

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