Functions and Decision Structures: CSSE 120—Rose Hulman Institute of Technology, Study notes of Software Engineering

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FUNCTIONS THAT RETURN

RESULTS,

DECISION STRUCTURES

CSSE 120—Rose Hulman Institute of Technology

Functions Can Return Values

 We've written functions that just do things  printFactorial(5)  printDistance(p1,p2)

 We've used functions that return values  abs(-1)  fn_root_1 = math.sqrt(bb – 4a*c)

 Define a function that returns a value  # _____________________ def square(x): return x * x

Returns the square of x

return statement

Pair Programming: Three Squares

 Download threeSquares.py from Angel

 Run the program to be sure it works

 Add a function, stats that takes a Rectangle, r , as a parameter and returns the area of r

 Change the program to display the area of each rectangle inside the shape

 Finally, change stats to return the area and perimeter (see figure at right)

 Upload to threeSquares dropbox on Angel

Example Display

Modifying Parameters

 How do functions send information back?

 Return statements  Mutating parameters

 Consider: def awardEC(score, extra): newScore = score + extra score = newScore

earned = 87 bonus = 4 awardEC(earned, bonus) print earned

Full Credit for Getting It Right

 def awardEC(score, extra): newScore = score + extra return newScore

earned = 87 bonus = 4 earned = awardEC(earned, bonus) print earned

Extra-Credit for Everyone!

 def awardEC(scores, extras): for i in range(len(scores)): scores[i] = scores[i] + extras[i] earned = [87, 63, 94] bonuses = [3, 5, 0] awardEC(earned, bonuses) print earned

 Did the value of amounts change?

 No, it refers to the same list  The list's contents changed  Functions can change state of mutable objects

Simple Decisions

 The if statement

 if :

 Semantics: "if the condition is true, run the body, otherwise skip it"

 Simple conditions

  Some relational operators: Math < ≤ = ≥ > ≠ Python < <= == >= > !=

Class Exercise

 Define a function grade(score)

 Where score is an exam score  and result is "perfect", "passing", or "failing" based on the score

Having It Both Ways: if-else

 Syntax: if :

else:

 Semantics: "If the condition is true, execute the statements for true, otherwise execute the statements for false"

Individual Exercise on Using if-else

 Define a function countFailPass(scores) that

 takes a list of exam scores  returns two values:  the count of failing scores in the list (those less than 60), and  the count of passing scores in the list

 Examples:

 countFailPass([57, 100, 34, 87, 74]) returns (2,3)  countFailPass([59]) returns (1,0)  countFailPass([]) returns (0,0)

 Upload to countFailPass dropbox on Angel

Multi-way Decisions

 Syntax: if :

elif :

elif :

… else:

reach here if condition1 is false reach here if condition1 is false AND condition2 is true reach here if BOTH condition1 AND condition2 are false

Cleaning the Bird Cage

 Advantages of if-elif-else vs. nesting

 Number of cases is clear  Each parallel case is at same level in code  Less error-prone

 Fix grade function to use if-elif-else statement instead of nesting