Lecture 2: Logical Problems with Choices

Problem Solving

Before writing a programHave a thorough understanding of the problemCarefully plan an approach for solving it

While writing a programKnow what “building blocks” are availableUsing good programming principles

Algorithms

Computing problemsAll can be solved by executing a series of actions in a specific order

Algorithms: An algorithm is a clearly specified set of simple instructions to be followed to solve a problem

Actions to be executedThe order in which these actions are to be executed

Program controlSpecify order in which statements are to be executed

Control Structures

Sequential executionNormally, statements are executed one after the other in the order written

Transfer of controlWhen the next statement executed is not the next one in sequenceOveruse of goto statements led to many problems

All C programs written in term of 3 control structuresSequence structures

Programs executed sequentially by defaultSelection structures

Three types: if, if … else, and switchRepetition structures

Three types: while, do … while and for

Pseudocode

Artificial, informal language that helps develop algorithmsSimilar to everyday EnglishNot actually executed on computersHelp “think out” a program before writing it

Easy to convert into a corresponding C programConsists only executable statement

Definitions are not executable statements

Flowchart

Graphical representation of an algorithmDrawn using certain special-purpose symbols connected by arrows called flowlinesSpecial-purpose symbols

Rectangle (action) symbol:any type of action.Oval symbol:the beginning or end of a programSmall circle (connector) symbol:the entry or exit of a portion of an algorithm.Diamond symbol:indicate that a decision is to be made.

Single-entry/single-exit

Flowcharting C’s sequence stucture

if Selection Statement

Selection structureUsed to choose among alternative courses of actionExample - Pseudocode:if student’s grade is no less than 60

print “Passed”If condition true

Print statement executed and program goes on to next statement.

If condition falsePrint statement is ignored and the program goes onto the next statement

condition

if Selection Statement

Pseudocode statement in Cif ( grade >= 60) printf( “Passed\n” );

C code corresponds closely to the pseudocodeFlow chart for the if selection statement

Diamond symbol (decision symbol)Indicates decision is to be madeContains an expression that can be true or false

Test the condition, follow appropriate path

condition

if … else Selection Statement

if … elseSpecifies an action to be performed both when the condition is true and when it is falseif: Only performs an action if the condition is true.

Problem:

Pseudocode:

Read in 2 numbers and print in non-decreasing order.

Read in two numbers, num1 and num2.If num1 is no larger than num2 Print “num1 num2”else Print “num2 num1”

if … else Selection Statement

Flowchart

begin

Read in two numbers: num1 and num2

num1 <= num2 Print “num1 num2”truefalse

end

Print “num2 num1”

Flowcharting the double-selection if...else statement

if … else Selection Statement

C code

Nested if … else Statements

Test for multiple cases by placing if…else selection statements inside if…else selection statementOnce condition is met, rest of statements skippedExample - Pseudocode

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 Print “F”

Nested if … else Statements

Flowchart

Print “A” truescore>=90

false

score>=80Print “B” true false

score>=70Print “C” true false

score>=60Print “D” truePrint “F”

false

Nested if … else StatementsC code

/* Convert a student's score to grade */

#include <stdio.h>

/* function main begins program execution */int main( void ){ int score; / * score of the student */

printf( "Enter the score\n" ); /* prompt */ scanf( "%d", &score ); /* read an integer */

if ( score >= 90 ) printf( "The grade is 'A'.\n" ); else if ( score >= 80 )

printf( "The grade is 'B'.\n" ); else

if ( score >= 70 ) printf( "The grade is 'C'.\n" ); else if ( score >= 60 ) printf( "The grade is 'D'.\n" ); else printf( "The grade is 'F'.\n" );

return 0; /* indicate that program ended successfully */

} /* end function main */

Compound Statement

Set of statements within a pair of bracesExample:

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.

A compound statement can be placed anywhere in a program that a single statement can be placed.

Write a program that completes the following:Read in three integers and determine the largest.

You should only use if/else statements for the logic in your code.

Submit your maxnum.c file to the dropbox called Maximum Number and complete Program Quiz 2.

In-Class Programming Exercise