Control Structures In structured programming, we use three basic control structures: –Sequence –Selection –Repetition So far, we have worked with sequential.

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Presentation transcript:

Control Structures In structured programming, we use three basic control structures: –Sequence –Selection –Repetition So far, we have worked with sequential algorithms, where each instruction is executed once, immediately after the one above it

Selection We use selection to handle making choices A condition that is evaluated to be either true or false determines the next action Is a > b? Print “Yes” Print “No” TrueFalse IF 1 is greater than 2 THEN Print “Yes” ELSE Print “No” ENDIF

Relational Operators Used to test the relationship between two expressions or two variables Condition evaluates as.FALSE..TRUE.

FORTRAN’s Relational Operators = =Equal uses two equal signs >Greater than <Less than >=Greater than or equal to <=Less than or equal to /=Not equal

FORTRAN’s Relational Operators = = or.EQ.Equal uses two equal signs >or.GT.Greater than <or.LT.Less than >= or.GE.Greater than or equal to <= or.LE.Less than or equal to /= or.NE.Not equal

Examples x = 500, y = 300 x= =y x > 100 y >=250 x /= 500 ((x + y) > 700) ((x / 2).LT. 99)

Compound Logical Expressions Used to combine or negate expressions containing logical operators. –used with compound conditions.AND. And.OR.Or.NOT.Not.EQV.Equivalence.NEQV.Nonequivalence

Logical Operators ((x == 5).AND. (y != 10))both conditions must be true for statement to evaluate as true ((x == 7).OR. (y > 10))either condition can be true for statement to evaluate is true. Only evaluates as false if both false (.NOT. (x >= 7))reverses the value of expression to its immediate right. same as if (x < 7) then

Order of Precedence ( ) Parentheseshighest **Exponentiation *, /Multiplication,Division +, -Addition, Subtraction >= Greater than, less than, …or equal = = /=Equal, not equal.AND.And.OR.Or =Assignment statementlowest Rule: Use parentheses to make evaluation order clear

Conditions Practice 1.“A” < “F” “cat” < “can” “June” > “July” “cat_ _ _” < “cattle” “c” = = “C”

Conditions Practice 1. Given a = 12, b = 6, c = 8 true or false? ((a < b).AND. (b /= c) (.NOT.(a c)) ((a > b).AND.(a > c).OR.(b > c)) 2. What would the values of a, b, and c need to be for this statement to be true? (use 1,2,3) ((a >c.OR. b > c).AND. (a < b))