For Loops Constant Value Role Accumulator Role Introduction

for loops The code inside the loop occurs once for each element in the

What Are Variable Roles? • A variable role is the reason we are using

What Are Variable Roles? • Certain reasons for using a variable come up over

Quick Summary v Fixed Assigned once v Accumulator Running total v Stepper Predetermined sequence

What Are Variable Roles? • Roles say why we are using the variable. •

Variable Role: Fixed Why use a fixed variable? 01 02 def calculate_total(bill): LOCAL_TIPPING_CUSTOM =

Variable Role: Fixed • Pattern: • Assign at top of block of code. •

Variable Role: Accumulator Why use an accumulator variable? • To keep a running total

Review: Accumulator Pattern: Initialize-Accumulate-Report 01 accumulator = 0 # Initialize 02 for element in

Variable Role: Accumulator Pattern: Initialize-Accumulate-Report 01 def add_up(prices): 02 total_price = 0 # Initialize

Variable Role: Accumulator Pattern: Initialize-Accumulate-Report • Problem: Some square rooms need carpet • room_sizes

Variable Role: Accumulator Pattern: Initialize-Accumulate-Report 01 def add_carpet_areas(room_sizes): 02 sq_ft = 0 # Initialize

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for loops The code inside the loop occurs once for each element in the iterable. 01 for element in iterable: 02 # This indented block of code 03 # is inside the loop. 04 # This indented block happens once 05 # for each element 06 # This code runs after the loop is done. 07 # End of indentation is like } in Robot. C Introduction to Computer Science © 2015 Project Lead The Way, Inc.

for loops The code inside the loop occurs once for each element in the iterable. 01 02 03 04 05 06 numbers = [2, 3, 5, 7, 11] for number in numbers: print number, 'is prime but ', print number*2, 'is not prime. ' print numbers, 'is a list. ' print number, 'is an element in the list. ' 2 is prime but 4 is not prime. 3 is prime, but 6 is not prime. 5 is prime, but 10 is not prime. 7 is prime, but 14 is not prime. 11 is prime, but 22 is not prime. [2, 3, 5, 7, 11] is a list. 11 is an element in the list. Introduction to Computer Science © 2015 Project Lead The Way, Inc.

What Are Variable Roles? • A variable role is the reason we are using the variable. • Variables always remember data for later use. But why are we trying to remember something? Introduction to Computer Science © 2015 Project Lead The Way, Inc.

What Are Variable Roles? • Certain reasons for using a variable come up over and over. • Eight roles cover 90% of the variable use by first-year programmers. v Fixed v Accumulator v Stepper v Best-so-far Introduction to Computer Science v Most recent v Aggregator v Walker v One-way flag © 2015 Project Lead The Way, Inc.

Quick Summary v Fixed Assigned once v Accumulator Running total v Stepper Predetermined sequence of values v Best-so-far Record holder Introduction to Computer Science v Most recent Assigned unpredictably v Aggregator Running list v Walker Elements of iterator v One-way flag Won’t reset flag until after iteration © 2015 Project Lead The Way, Inc.

Variable Role: Fixed Why use a fixed variable? 01 02 def calculate_total(bill): LOCAL_TIPPING_CUSTOM = 0. 15 • Single line of code to tweak a number used throughout a program. Easy maintenance! • Makes self-commenting code Introduction to Computer Science © 2015 Project Lead The Way, Inc.

Variable Role: Fixed • Pattern: • Assign at top of block of code. • Use, but never assign again • Convention: all capital letters 01 02 def calculate_total(bill): LOCAL_TIPPING_CUSTOM = 0. 15 Introduction to Computer Science © 2015 Project Lead The Way, Inc.

Variable Role: Accumulator Why use an accumulator variable? • To keep a running total • Pattern 1. Given an initial value before loop 2. Add to itself inside the loop 3. Result used after loop Introduction to Computer Science © 2015 Project Lead The Way, Inc.

Review: Accumulator Pattern: Initialize-Accumulate-Report 01 accumulator = 0 # Initialize 02 for element in iterable: 03 # Accumulate 04 accumulator = accumulator + worth(element) 05 print accumulator # Report/use Introduction to Computer Science © 2015 Project Lead The Way, Inc.

Variable Role: Accumulator Pattern: Initialize-Accumulate-Report 01 def add_up(prices): 02 total_price = 0 # Initialize 03 for price in prices: 04 # Accumulate 05 total_price = total_price + price 06 return total # Report/use Introduction to Computer Science © 2015 Project Lead The Way, Inc.

Variable Role: Accumulator Pattern: Initialize-Accumulate-Report • Problem: Some square rooms need carpet • room_sizes is a list of widths • Calculate total square feet 01 def add_carpet_areas(room_sizes): 02 sq_ft = 0 # Initialize 03 for room_size in room_sizes: 04 # Accumulate 05 sq_ft = sq_ft + room_size**2 06 return sq_ft # report/use Introduction to Computer Science © 2015 Project Lead The Way, Inc.

Variable Role: Accumulator Pattern: Initialize-Accumulate-Report 01 def add_carpet_areas(room_sizes): 02 sq_ft = 0 # Initialize 03 for room_size in room_sizes: 04 # Accumulate 05 sq_ft = sq_ft + room_size**2 06 return sq_ft # Report/use rooms = [10, 30, 20] add_carpet_areas(rooms) Introduction to Computer Science © 2015 Project Lead The Way, Inc.