Lecture 2: Logical Problems with Choices

Lecture 2: Logical Problems with Choices

Problem Solving • Before writing a program • Have a thorough understanding of the problem • Carefully plan an approach for solving it • While writing a program • Know what “building blocks” are available • Using good programming principles

Algorithms • Computing problems • All 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 executed • The order in which these actions are to be executed • Program control • Specify order in which statements are to be executed

Control Structures • Sequential execution • Normally, statements are 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 C programs written in term of 3 control structures • Sequence structures • Programs executed sequentially by default • Selection structures • Three types: if, if … else, and switch • Repetition structures • Three types: while, do … while and for

Pseudocode • Artificial, informal language that helps develop algorithms • Similar to everyday English • Not actually executed on computers • Help “think out” a program before writing it • Easy to convert into a corresponding C program • Consists only executable statement • Definitions are not executable statements

Flowchart • Graphical representation of an algorithm • Drawn using certain special-purpose symbols connected by arrows called flowlines • Special-purpose symbols • Rectangle (action) symbol: any type of action. • Oval symbol: the beginning or end of a program • Small 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

condition if Selection Statement • Selection structure • Used to choose among alternative courses of action • Example - 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 false • Print statement is ignored and the program goes onto the next statement

condition if Selection Statement • Pseudocode statement in C if ( grade >= 60) printf( “Passed\n” ); • C code corresponds closely to the pseudocode • Flow chart for the if selection statement • 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

if … else Selection Statement • if … else • Specifies an action to be performed both when the condition is true and when it is false • if: 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”

Flowcharting the double-selection if...else statement num1 <= num2 if … else Selection Statement Flowchart begin Read in two numbers: num1 and num2 true false Print “num1 num2” Print “num2 num1” end

if … else Selection Statement C code

Nested if … else Statements • Test for multiple cases by placing if…else selection statements inside if…else selection statement • Once condition is met, rest of statements skipped • Example - 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”

true true true true false false false score>=70 score>=80 score>=60 score>=90 Nested if … else Statements Flowchart Print “A” Print “B” Print “C” false Print “D” Print “F”

Nested if … else Statements /* 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 */ C code

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

In-Class Programming Exercise 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 andcomplete Program Quiz 2.

Lecture 2: Logical Problems with Choices