1 Algorithms and Structured Program Design and Development

1 Algorithms and Structured Program Design and Development Outline 0 Introduction 1 2 3 4 5 6 7 1 Top-down design technique Algorithms Control Structures - Flowcharts Control Structures - Pseudocode The If Selection Structure The If/Else Selection Structure The While Repetition Structure Formulating Algorithms: Case Study 1 (Counter-Controlled Repetition) 9 Formulating Algorithms with Top-down, Stepwise Refinement: Case Study 2 (Sentinel-Controlled Repetition) 10 Formulating Algorithms with Top-down, Stepwise Refinement: Case Study 3 (Nested Control Structures) 11 Assignment Operators 12 Increment and Decrement Operators

Introduction 2 • What is a program? – A set of step-by-step instructions that directs the computer to perform tasks and produce results. • Programming Languages – A programming language is a set of rules that instructs a computer what operations to perform.

Introduction • Before writing a program: – Have a thorough understanding of the problem – Carefully plan an approach for solving it “Top-Down design technique” • While writing a program: – Know what “building blocks” are available – Use good programming principles “Structured programs” 3

Introduction 4 • What Can a Program Do? • A program can only instruct a computer to: – – – – Read Input Sequence Calculate Store data Compare and branch Iterate or Loop Write Output

5 1 Top-Down Design • If we look at a problem as a whole, it may seem impossible to solve because it is so complex. Examples: – writing a game – writing a word processor • Complex problems can be solved using top-down design, also known as stepwise refinement, where – We break the problem into parts – Then break the parts into parts – Soon, each of the parts will be easy to do

6 1 Top-Down Design 1. Determine overall goal. 2. Break that goal into two, three or more detailed parts. 3. Put all details. Keep repeating steps 1 and 2 until you cannot reasonably break down the problem any further.

7 1 Advantages of Top-Down Design • Breaking the problem into parts helps us to clarify what needs to be done. • At each step of refinement, the new parts become less complicated and, therefore, easier to figure out. • Parts of the solution may turn out to be reusable. • Breaking the problem into parts allows more than one person to work on the

8 Simplified Development Cycle Analyze the problem Design the solution algorithm Design the user interface Write the code Test and debug the program Complete the documentation

9 1 Example_1 : Top-Down Design • Problem: – We own a home improvement company. – We do painting, roofing, and basement waterproofing. – A section of town has recently flooded (zip code 21222). – We want to send out pamphlets to our customers in that area.

10 1 Example_1 : The Top Level • Get the customer list from a file. • Sort the list according to zip code. • Make a new file of only the customers with the zip code 21222 from the sorted customer list. • Print an envelope for each of these customers. Main Read Sort Select Print

11 1 Example_2 : Another Example • Problem: Write a program that draws this picture of a house.

12 1 Example_2 : The Top Level • • Draw the the outline of the house chimney door windows Main Draw Outline Draw Chimney Draw Door Draw Windows

13 1 Example_2 : Pseudocode for Main Call Draw Outline Call Draw Chimney Call Draw Door Call Draw Windows

14 1 Example_2 : Observation • The door has both a frame and knob. break this into two steps. We could Main Draw Outline Draw Chimney Draw Door Frame Draw Door Draw Windows Draw Knob

15 1 Example_2 : Pseudocode for Draw Door Call Draw Door Frame Call Draw Knob

16 1 Example_2 : Another Observation • There are three windows to be drawn. Main Draw Outline Draw Windows . . . Draw Window 1 Draw Window 2 Draw Window 3

17 1 Example_2 : One Last Observation • But don’t the windows look the same? They just have different locations. • So, we can reuse the code that draws a window. – Simply copy the code three times and edit it to place the window in the correct location, or – Use the code three times, “sending it” the correct location each time (we will see how to do this later). • This is an example of code reuse.

18 1 Example_2 : Reusing the Window Code Main Draw Outline . . . Draw Windows Draw a Window

19 1 Example_2 : Pseudocode for Draw Windows Call Draw a Window, sending in Location 1 Call Draw a Window, sending in Location 2 Call Draw a Window, sending in Location 3

2 Algorithms 20 • Computing problems – All can be solved by executing a series of actions in a specific order • Algorithm: procedure in terms of – Actions to be executed – The order in which these actions are to be executed • Program control – Specify order in which statements are to executed

3 Control Structures 21 • Sequential execution – Statements executed one after the other in the order written • Transfer of control – When the next statement executed is not the next one in sequence – Overuse of goto statements led to many problems • All programs written in terms of 3 control structures – Sequence structures : Programs executed sequentially by default – Selection structures : if, if/else, and switch – Repetition structures: while, do/while and for

3 Control Structures 22 • Flowchart – Graphical representation of an algorithm – Drawn using certain special-purpose symbols connected by arrows called flowlines – Rectangle symbol (action symbol): • Indicates any type of action – Oval symbol: • Indicates the beginning or end of a program or a section of code • Single-entry/single-exit control structures – Connect exit point of one control structure to entry point of the next (control-structure stacking) – Makes programs easy to build

3 Flowcharts Process I/O Decision Flow Connector Terminal 23

3 Flowchart Rules 24 1. Use standard symbols. 2. The flowchart’s logic should generally flow from top of the page to the bottom and from left to right. 3. The decision symbol always has two exit point and should always ask a yes or no question. 4. Instruction inside the symbols should be clear English description.

25 3 Flowchart Structures - IF-THEN Entry false Test condition true True statement Exit

26 3 Flowchart Structures - IF-THEN-ELSE Entry false Test condition False statement true True statement Exit

27 3 Flowchart Structures - Iterate • A program loop is a form of iteration. • A computer can be instructed to repeat instructions under certain conditions. No

3 Flowchart Structures – DO WHILE Loop 28 Entry Exit Test condition Yes True statement No

3 Flowchart Structures – DO UNTIL Loop 29 Entry True statement Exit No Test condition Yes

3 Flow Chart - Example 30 • “find the greatest number in 3 numbers”. . – Output = ? • Input = ? An integer value which is the greatest one. Read three integer values (a, b and c) from screen. • The Logic steps = 1. Compare a and b and find the big one. ? 2. Compare the big value and c and decide the biggest value. 3. Show the user this biggest value.

3 Flow Chart - Example 31 “find the greatest number in 3 numbers”. . An integer value which is the greatest one. Read a, b, c a>b Read three integer values (a, b and c) from screen. Y a>c Y The biggest value is a. N b>c Y The biggest value is b. N The biggest value is c. Show the user this biggest value. 1. Compare a and b and find the big one. 2. Compare the big value and c and decide the biggest value.

4 Pseudocode 32 • Pseudocode – Artificial, informal language that helps us develop algorithms – Similar to everyday English – Not actually executed on computers – Helps us “think out” a program before writing it • Easy to convert into a corresponding C program • Consists only of executable statements

33 5 The if Selection Structure • Selection structure: – Used to choose among alternative courses of action – Pseudocode: If student’s grade is greater than or equal to 60 Print “Passed” • If condition true – Print statement executed and program goes on to next statement – If false, print statement is ignored and the program goes onto the next statement – Indenting makes programs easier to read • C ignores whitespace characters

5 34 The if Selection Structure • Same statement in C: if ( grade >= 60 ) printf( "Passedn" ); – C code corresponds closely to the pseudocode • Diamond symbol (decision symbol) – Indicates decision is to be made – Contains an expression that can be true or false – Test the condition, follow appropriate path

5 35 The if Selection Structure • if structure is a single-entry/single-exit structure grade >= 60 false true A decision can be made on any expression. print “Passed” zero - false nonzero - true Example: 3 - 4 is true

36 6 The if/else Selection Structure • if – Only performs an action if the condition is true • if/else – Specifies an action to be performed both when the condition is true and when it is false • Psuedocode: If student’s grade is greater than or equal to 60 Print “Passed” else Print “Failed” – Note spacing/indentation conventions

37 6 The if/else Selection Structure • C code: if ( grade >= 60 ) printf( "Passedn"); else printf( "Failedn");

38 6 The if/else Selection Structure • Flow chart of the if/else selection structure false print “Failed” grade >= 60 true print “Passed” • Nested if/else structures – Test for multiple cases by placing if/else selection structures inside if/else selection structures – Once condition is met, rest of statements skipped – Deep indentation usually not used in practice

39 6 The if/else Selection Structure – Pseudocode for a nested if/else structure If student’s grade is greater than or equal to 90 Print “A” else If student’s grade is greater than or equal to 80 Print “B” else If student’s grade is greater than or equal to 70 Print “C” else If student’s grade is greater than or equal to 60 Print “D” else

6 40 The if/else Selection Structure • Compound statement: – Set of statements within a pair of braces – Example: if ( grade >= 60 ) printf( "Passed. n" ); else { printf( "Failed. n" ); printf( "You must take this course again. n" ); } – Without the braces, the statement printf( "You must take this course again. n" ); would be executed automatically

41 6 The if/else Selection Structure • Block: – Compound statements with declarations • Syntax errors – Caught by compiler • Logic errors: – Have their effect at execution time – Non-fatal: program runs, but has incorrect output – Fatal: program exits prematurely

42 7 The while Repetition Structure • Repetition structure – Programmer specifies an action to be repeated while some condition remains true – Psuedocode: While there are more items on my shopping list Purchase next item and cross it off my list – while loop repeated until condition becomes false

7 43 The while Repetition Structure • Example: int product = 100; while ( product <= 1000 ) product = 2 * product; printf(“product= %d n”, product); product <= 1000 false true product = 2 * product

8 Formulating Algorithms 44 (Counter-Controlled Repetition) • Counter-controlled repetition – Loop repeated until counter reaches a certain value – Definite repetition: number of repetitions is known – Example: A class of ten students took a quiz. The grades (integers in the range 0 to 100) for this quiz are available to you. Determine the class average on the quiz – Pseudocode: Set total to zero Set grade counter to one While grade counter is less than or equal to ten Input the next grade Add the grade into the total Add one to the grade counter Set the class average to the total divided by ten

45 1 /* Outline 2 Class average program with 3 counter-controlled repetition */ 1. Initialize Variables 4 #include <stdio. h> 5 6 int main() 2. Execute Loop 7 { 8 int counter, grade, total, average; 3. Output results 9 10 /* initialization phase */ 11 total = 0; 12 counter = 1; 13 14 /* processing phase */ 15 while ( counter <= 10 ) { 16 printf( "Enter grade: " ); 17 scanf( "%d", &grade ); 18 total = total + grade; 19 counter = counter + 1; 20 } 21 22 /* termination phase */ 23 average = total / 10; 24 printf( "Class average is %dn", average ); 25 26 return 0; /* program ended successfully */ 27}

Outline Enter Enter Enter Class grade: 98 grade: 76 grade: 71 grade: 87 grade: 83 grade: 90 grade: 57 grade: 79 grade: 82 grade: 94 average is 81 Program Output 46

47 9 Formulating Algorithms with Top. Down, Stepwise Refinement • Problem becomes: Develop a class-averaging program that will process an arbitrary number of grades each time the program is run. – Unknown number of students – How will the program know to end? • Use sentinel value – – Also called signal value, dummy value, or flag value Indicates “end of data entry. ” Loop ends when user inputs the sentinel value Sentinel value chosen so it cannot be confused with a regular input (such as -1 in this case)

Formulating Algorithms with 48 Top. Down, Stepwise Refinement • Top-down, stepwise refinement 9 – Begin with a pseudocode representation of the top: Determine the class average for the quiz – Divide top into smaller tasks and list them in order: Initialize variables Input, sum and count the quiz grades Calculate and print the class average • Many programs have three phases: – Initialization: initializes the program variables – Processing: inputs data values and adjusts program variables accordingly – Termination: calculates and prints the final results

9 49 Formulating Algorithms with Top. Down, Stepwise Refinement • Refine the initialization phase from Initialize variables to: Initialize total to zero Initialize counter to zero • Refine Input, sum and count the quiz grades to Input the first grade (possibly the sentinel) While the user has not as yet entered the sentinel Add this grade into the running total

9 50 Formulating Algorithms with Top. Down, Stepwise Refinement • Refine Calculate and print the class average to If the counter is not equal to zero Set the average to the total divided by the counter Print the average else Print “No grades were entered”

1 /* Outline 2 Class average program with 3 sentinel-controlled repetition */ 4 #include <stdio. h> 5 6 int main() 7 { 8 float average; /* new data type */ 9 int counter, grade, total; 10 11 /* initialization phase */ 12 total = 0; 13 counter = 0; 14 15 /* processing phase */ 16 printf( "Enter grade, -1 to end: " ); 17 scanf( "%d", &grade ); 18 19 while ( grade != -1 ) { 20 total = total + grade; 21 counter = counter + 1; 22 printf( "Enter grade, -1 to end: " ); 23 scanf( "%d", &grade ); 24 } 51

25 26 27 28 29 30 31 32 33 34 35} /* termination phase */ if ( counter != 0 ) { average = total / counter; printf( "Class average is %. 2 f", average ); } else printf( "No grades were enteredn" ); return 0; Outline /* program ended successfully */ 3. Calculate Average Enter Enter Enter Class grade, -1 to end: grade, -1 to end: grade, -1 to end: average is 82. 50 75 94 97 88 70 64 83 89 -1 3. 1 Print Results Program Output 52

10 Nested control structures 53 • Problem – A college has a list of test results (1 = pass, 2 = fail) for 10 students – Write a program that analyzes the results • If more than 8 students pass, print "Raise Tuition" • Notice that – The program must process 10 test results • Counter-controlled loop will be used – Two counters can be used • One for number of passes, one for number of fails – Each test result is a number—either a 1 or a 2 • If the number is not a 1, we assume that it is a 2

54 10 Nested control structures • Top level outline Analyze exam results and decide if tuition should be raised • First Refinement Initialize variables Input the ten quiz grades and count passes and failures Print a summary of the exam results and decide if tuition should be raised • Refine Initialize variables to Initialize passes to zero Initialize failures to zero Initialize student counter to zero

55 10 Nested control structures • Refine Input the ten quiz grades and count passes and failures to While student counter is less than or equal to ten Input the next exam result If the student passed Add one to passes else Add one to failures Add one to student counter • Refine Print a summary of the exam results and decide if tuition should be raised to Print the number of passes Print the number of failures If more than eight students passed Print “Raise tuition”

1/* 2 Analysis of examination results */ 3#include <stdio. h> 4 5 int main() 6{ 7 /* initializing variables in declarations */ 8 int passes = 0, failures = 0, student = 1, result; 9 10 /* process 10 students; counter-controlled loop */ 11 while ( student <= 10 ) { 12 printf( "Enter result ( 1=pass, 2=fail ): " ); 13 scanf( "%d", &result ); 14 15 if ( result == 1 ) /* if/else nested in 16 passes = passes + 1; while */ 17 else 18 failures = failures + 1; 19 20 student = student + 1; 21 } 22 23 printf( "Passed %dn", passes ); 24 printf( "Failed %dn", failures ); 25 26 if ( passes > 8 ) 27 printf( "Raise tuitionn" ); 28 29 return 0; /* successful termination */ 30 } Outline 56 1. Initialize variables 2. Input data and count passes/failures 3. Print results

Outline Enter Result Enter Result Enter Result Passed 6 Failed 4 (1=pass, 2=fail): (1=pass, 2=fail): (1=pass, 2=fail): 1 2 2 1 1 1 2 Program Output 57

11 Assignment Operators 58 • Assignment operators abbreviate assignment expressions c = c + 3; can be abbreviated as c += 3; using the addition assignment operator • Statements of the form variable = variable operator expression; can be rewritten as variable operator= expression; • Examples of other assignment operators: d e f g -= *= /= %= 4 5 3 9 (d (e (f (g = = d e f g * / % 4) 5) 3) 9)

59 12 Increment and Decrement Operators • Increment operator (++) – Can be used instead of c+=1 • Decrement operator (--) – Can be used instead of c-=1 • Preincrement – Operator is used before the variable (++c or --c) – Variable is changed before the expression it is in is evaluated • Postincrement – Operator is used after the variable (c++ or c--) – Expression executes before the variable is changed

60 12 Increment and Decrement Operators • If c equals 5, then printf( "%d", ++c ); – Prints 6 printf( "%d", c++ ); – Prints 5 – In either case, c now has the value of 6 • When variable not in an expression – Preincrementing and postincrementing have the same effect ++c; printf( “%d”, c ); – Has the same effect as c++; printf( “%d”, c );

61 Exercise - 1 • Write pseudocode and draw a flowchart that calculate the average of the 10 numbers. • Input = ? • Output = ? Read 10 integer numbers from the screen. • Logic = ? Show the user the avarage of these 10 integer numbers. 1. Find the cumulative value of the numbers and 2. divide this number with the count of numbers. Draw flow chart and write pseudocode.