The Three Basic Selection Structures in C++ Programming Concepts

Lecture 3: Control
Structures - Selection
Author : Aamir Saleem Ansari
Web : www.techora.net

The Plan for Today
 Review from last two weeks
 Flowcharts

Pseudocode
 Data types
 Variables and Constants
 Structure of a C program
 Formatted output: printf( )
 Operators and their precedence
 Review control structures
 Sequence
 Selection
 Repetition
 Selection structures
 If
 If/else
 Switch
 Relational operators
 Selection structure example

Learning Objectives
 Apply concepts for developing algorithms, using
variables, and structuring a C program
 Explain what is meant by a control structure
 Explain the three basic types of control
structures
 Determine the result of relational comparisons
 Apply the if and if/else control structures

Control Structures - Review
 All programs can be written in terms of three
control structures (like building blocks)
 Sequence
 ‘Built-in’ to C
 Unless otherwise directed, one statement after the next is
executed
 Selection (three types)
 Depending on a condition, select between one statement or
another
 If var1 is greater than 10, do this…, else do that…
 Repetition (three types)
 Depending on a condition, execute one or more statements
repeatedly

Selection Structure Overview
 Three kinds of selections structures
 if (also called, ‘single-selection’)
 if condition is true
Perform action
 if condition is false, action is skipped, program continues
 if/else (also called, ‘double-selection’)
 if condition is true
Perform action
 else (if condition is false)
Perform a different action (this will be skipped if condition is true)
 switch (also called ‘multiple-selection’)
 Allows selection among many actions depending on the
integral value of a variable or expression

Single Selection IF - Flowchart
TRUE
FALSE
Speed > 65
connector
flow line
decision symbol
action symbol
Print “You’re
speeding”

Operations Associativity
::
() [] left to right
Function_name() right to left
. -> left to right
‘ ! ` ++ -- + - *
&(type) sizeof
right to left
* / % .* ./ left to right
+ - left to right
<< >> left to right
< <= > >= left to right
== != left to right
& left to right
^ left to right
| left to right
&& left to right
^^ left to right
|| left to right
?: right to left
= += -= *= /= %= |=
<<= >>=
right to left
, left to right
Relational Operators
Important for constructing
the decision expression
5 < 7 result is ____
5 > 7 result is _____
7 <= 7 result is ____
8 >= 7 result is ____
5 == 5 result is ____
5 == 7 result is ____
var1 = 7 result is____
5.0 == 5 result is ___
6 != 5 result is ____
Adapted from H. Cheng chap04.ppt, slide 5
Practice

Double-Selection IF - Flowchart
TRUE
Speed > 65
FALSE
Print “Over
speed limit”
Print “Within
speed limit”

Adapted from Deitel & Deitel, C How to Program, 6th
ed., p. 111
SWITCH - Flowchart

IF statement (single-selection)
 Syntax
if(expression) /* if expression is TRUE (i.e., NOT EQUAL to zero) */
statement1; /* then execute this statement */
statement2; /* execute this statement next*/
 Notes
 Indent the statements for clarity
 Can have multiple statements
 Enclose a ‘block’ of statements using { } (curly braces)
if( x <= 2 )
{
statement1;
statement2;
}
statement3;

IF statement example
 Pseudocode (notice indentation!)
If speed is greater than 65 mph
print “You’re speeding!”
 C code
if(speed > 65)
printf(“You’re speeding!n”);
 C code with multiple statement block
if(speed > 65)
/* statements below executed only if speed > 65 is true */
{
printf(“You’re speeding!n”);
printf(“Slow down!n”);
printf(“Keep speed below 65 MPHn”);
}

IF-ELSE statement - Double Selection
 Syntax
if(expression) /*if expression is TRUE (i.e., NOT EQUAL to zero) */
statement1; /* execute this statement */
else /* else execute the following statement */
statement2;
Notes:
 If expression is non-zero, statement1 is executed, then the
program continues with the statement after statement2,
i.e., statement2 is skipped
 If expression is equal to zero, statement1 is skipped and
statement2 is executed, then the program continues with
the statement after statement2

IF-ELSE statement example
 Pseudocode (notice indentation!)
If speed is greater than 65 mph
print “Over speed limit!”
else
print “Within speed limit”
 C code
if(speed > 65)
printf(“Over speed limit!n”);
else
printf(“Within limitn”);

Compound Condition - &&
 Logical operators for more complex
decisions
 Logical AND operator && (double ampersand)
 if(switch1 = = 0 && switch2 = = 1)
turn on the motor
 The condition evaluates to TRUE if and only if BOTH
expressions on either side of && evaluate to TRUE
 Note operator precedence
 Otherwise condition evaluates to FALSE
 Beware of ‘short circuit evaluation’
 Make the condition most likely to be FALSE the left-
most condition

Compound Condition - | |
 Logical operators for more complex
decisions, cont.
 Logical OR operator | | (double vertical bar)
 if(switch1 = = 0 || switch2 = = 1)
turn on the motor
 The condition evaluates to TRUE if one or the other
or both expressions on either side of && evaluate to
TRUE
 Note operator precedence
 Otherwise condition evaluates to FALSE
 Beware of ‘short circuit evaluation’
 Make the condition most likely to be TRUE the left-most
condition

Grade Determination for Overall Percentage (OP)
OP >= 90 ‘A’
80 <= OP < 90 ‘B’
70 <= OP < 80 ‘C’
60 <= OP < 70 ‘D’
OP < 60 ‘F’
Nesting selection structures
 Selection structures can
be stacked and nested
to handle more
sophisticated
decision/action
functionality
 Ex. Figuring grades
 Pseudocode 
Notes:
 “an else is always
associated with the
nearest previous if”
(Darnell & Margolis, 1996)
 Use braces ({ })to clarify
the association of the
else for other situations
where the decision
structure is more
complicated
Adapted from Deitel & Deitel, C How to Program, 3rd
ed., p.
64

Nesting If/else – C Code – Two Ways
Or
Adapted from Deitel & Deitel, C How to Program, 3rd
ed., p.
64

SWITCH
 Good when faced with
testing multiple
alternatives that depend
on a single variable
 The test is done once
 Must be an integral
expression
 int or char
 NOT float, double
 case items must be
constant integral
expressions
 No variables
 The structure is very
organized and readable

Practice - 1
 Pair up with someone next to you that you do
not know
 Develop an algorithm for:
 Finding and printing out the largest of two numbers
 (3 min) One person work on the pseudocode,
the other on a flowchart
 (1 min) Compare pseudocode and flowchart
 (3 min) Write out the algorithm in C

Practice - 2
 Develop an algorithm for the ignition
control in a car:
Requirements:
 The starter will only start when:
 Key must be in the ignition slot
 Transmission selector must be in ‘Park’
 Key must be turned to ‘Start’ position
 The starter is energized with the statement
starter_on();

References
 Darnell, P. A. & Margolis, P. E. (1996) C, a
software engineering approach, 3rd ed.,
Springer, New York.
 Cheng, H. H. (2010). C for Engineers and
Scientists: An Interpretive Approach,
McGraw-Hill, New York.
 Deitel, H. M. & Deitel, P. J. (2001). C How
to Program, 3rd ed., Prentice-Hall, New
Jersey.

Nesting selection structures
 Selection
structures can be
stacked and nested
to handle more
sophisticated
decision/action
functionality
/* File: ifc.c */
#include <stdio.h>
int main ()
{
int i;
i = 10;
if(i==2 || i == 4)
{
printf("i = 2 or 4n");
}
else if(i == 10)
{
printf("i = 10n");
}
else
{
printf("i = %dn", i);
}
return 0;
}

Operators Operations Associativity
::
() [] left to right
Function_name() right to left
. -> left to right
‘ ! ` ++ -- + - *
&(type) sizeof
right to left
* / % .* ./ left to right
+ - left to right
<< >> left to right
< <= > >= left to right
== != left to right
& left to right
^ left to right
| left to right
&& left to right
^^ left to right
|| left to right
?: right to left
= += -= *= /= %= |=
<<= >>=
right to left
, left to right

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