Control Structures
The if, for, and while Statements
§5.1, §5.4, & §5.5
(Several examples here;
also Lab #4)
1
Basic Kinds of Control
The behavior of main()(and other functions) is
determined by the statements within it.
Statements fall into one of three categories called
control structures:
Statements that simply execute in ___________.
Statements that _________ one of several
alternatives.
See
flowcharts in
§5.4 & 5.5
Statements that _________ another statement.
EVERY PROGRAM CAN BE
WRITTEN USING THESE 3
CONTROL STRUCTURES.
2
Sequential execution
The design of computers in which program instructions
are stored in memory along with the data processed by
the program is known as the ____________________ Named after
mathematician/
architecture. A special register, called the
physicist John
von Neumann
_______________________ (IC), stores the address (see Ch.1, p. 8)
of the memory location where the next instruction to
be executed is found. It is initialized with the address of
the first instruction and execution proceeds as follows:
Fetch the instruction whose address is in the IC
Decode it to find the opcode and the operands
Execute the operation
______ (unless the IC is changed in some other way)
3
The last step (IC++) indicates that the default manner
of executing statements is ______________________.
In C++, a list of statements enclosed in curly braces
{
}
Statement1
Statement2
...
StatementN
STYLE TIP
Indent& align the
statements they enclose.
is known as a (________________________________ or
_________). It is a statement that produces sequential
execution of the statements enclosed by the { and }.
 Note: A compound statement is a single statement
and can thus be used any place a statement is needed .
4
Scope
A variable declared in a block is called a ________
___________. It exists (has memory allocated to it)
only from its declaration to the end of the block.
We say that its ____________ extends from its
declaration to the end of the block.
For example, in the code
...
cin >> item;
while (item != -999)
{
int i = 0;
...
cin >> item;
i++;
}
cout << "Value of i = " << i;
This is one of
several scope
rules. Others will
be described later.
the last line won't compile because local variable i is out of
scope. The "lifetime" of i ends at the }.
5
Selective Execution
(§ 5.4)
In contrast to sequential execution, there are situations
in which a problem’s solution requires that a statement
be executed selectively, based on a ___________ (a
__________ expression):
The C++ ________________ is a statement that causes
selective execution, allowing a program to choose exactly
one of two statements and execute it.
Note again:
An if statement
is a single statement, even if it
extends over
many lines.
if (Condition)
Statement1
else
Statement2
optional
6
The Simple if
The C++ if statement has several different forms.
The first form has no else or Statement2, and is
called the simple if:
if (Condition)
Statement
If Condition is true,
Statement
Statement is ____________ ;
otherwise Statement is _________ .
T
Condition
F
7
Examples:
// if value is negative, make it positive
if (value < 0)
value = -value;
// Display date in mm/dd/yyyy format
if (month < 10)
cout << 0;
cout << month << '/';
if (day < 10)
cout << 0;
cout << day << '/' << year << endl;
8
Examples showing Statement can be a
compound statement:
// if a < b, interchange a and b (integers)
if (a < b)
Common Alternative Style:
{
if (a < b){
int temp = a;
int temp = a;
a = b;
a = b;
b = temp;
b = temp;
}
}
// Stop program if a is 0
// (in quadratic equation solver)
cout is buffered;
cerr is not.
if (a == 0)
{
cerr << "*** Coefficient of x^2 cannot be 0 ***\n";
____________; // Stop execution & return 1 to OS
}
Lab 4:
Exer. 4.3
9
The Two-Branch if
In the second form of if, the
else and Statement2 are present:
if (Condition)
Statement1
else
Statement2
STYLE TIP
Align if and else and
indent & align the
statements in each part..
If Condition is true,
T
Statement1 is __________
and Statement2 is _________;
Statement1
otherwise Statement1 is
___________ and Statement2
is ________________.
The ternary operator ? : is
essentially an inline if of this form.
Condition
F
Statement2
10
Example:
// Continuation of quadratic equation solver
// Legal quadratic equation, so find roots (if any)
double discrim = b*b - 4*a*c;
if (discrim >= 0)
{
double root1 = (-b + sqrt(discrim)) / (2*a),
root2 = (-b - sqrt(discrim)) / (2*a);
cout << "Solutions: " << root1 << ", "
<< root2 << endl;
}
else
cout << "No real roots\n";
11
The Multi-branch if
The final form of the if statement is:
if (Cond1)
Stmt1
else if (Cond2)
Stmt2
...
else if (CondN)
StmtN
else
StmtN+1
STYLE TIP
Align if, else ifs,
and else and indent
& align the statements
in each part.
________________ of the statements Stmti will
be selected and executed, namely, the one
corresponding to the first Condi that is true.
12
The intent is to implement a multi-alternative selection
structure of the following form, where exactly one of the
alternatives is selected and executed:
Stmt1
Stmt2
Stmt3
StmtN
StmtN+1
13
Actually, however, it implements a "waterfall"
selection structure of the following form:
T
Stmt1
Cond1
T
F
Cond2
F
...
Stmt2
T
StmtN
CondN
F
StmtN+1
14
And it is treated by the compiler as a sequence of
______________ in which each else clause (except
the last) is another if-else statement:
if (Cond1)
Stmt1
else
if (Cond2)
Stmt2
else
if (Cond3)
Stmt3
...
else
if (CondN)
StmtN
else
StmtN+1
This form is surely more difficult to type with all its staggered
indents. It also does not display as clearly the different
alternatives and that exactly one of them will be selected.
15
If Cond1 is true, Stmt1 is executed and
the remaining statements are skipped;
otherwise, control moves to Cond2;
if Cond2 is true, Stmt2 is executed and
the remaining statements are skipped;
otherwise, control goes to the next
condition
...
if CondN is true, StmtN is executed and
StmtN+1 is skipped;
otherwise, StmtN+1 is executed.
if (Cond1)
Stmt1
else if (Cond2)
Stmt2
...
else if (CondN)
StmtN
else
StmtN+1
16
Example: Assigning letter grades:
Using the nested-if form:
if (score > 100 || score < 0)
cerr << "Invalid score!\n";
else
if (score >= 90)
grade = 'A';
else
if (score >= 80)
grade = 'B';
else
if (score >= 70)
grade = 'C';
else
if (score >= 60)
grade = 'D';
else
grade = 'F';
17
... or the preferred if-else-if form:
if (score > 100 || score < 0)
cerr << "Invalid score!\n";
else if (score >= 90)
Note the simple conditions;we don't
grade = 'A';
need
(score <= 100 && score >= 90)
else if (score >= 80)
(score < 90 && score >= 80)
grade = 'B';
(score < 80 && score >= 70)
else if (score >= 70)
(score < 70 && score >= 60)
grade = 'C';
Do you understand why?
else if (score >= 60)
grade = 'D';
else
Here's an example of program
grade = 'F';
efficiency — not doing unnecessary
computations.
18
Repetitive Execution
(§ 5.5)
Finally, there are situations where solving a problem
requires that a statement be repeated, with the
repetition being controlled by a ___________.
•There are three parts to the repetition
mechanism:
– Initialization
– Repeated execution
– Termination
•Now we look at one repetition statement in
C++, the for statement:
19
Does the
initialization
Causes termination —
think "while this is true,
do the following"
Usually modifies
something each time
through the loop
for (InitializerExpr; LoopCondition; ModifierExpr)
Statement
where Statement can be
either a single statement, or
a compound statement.
And again, note:
A for statement is a
single statement, even
if it extends over many
lines.
Need
not all
be on
one line
Compared to for
statements in other
programming
languages, C++'s has
an unusual syntax.
20
The for Loop
for (InitializerExpr; LoopCondition; ModifierExpr)
Statement
Statement will be executed
so long as ____________
____________.
Statement is often called
the _
InitializerExpr
F
LoopCondition
T
Statement
of the loop.
ModifierExpr
STYLE TIP
Indent and align the
statements in the
body of the loop.
21
for (InitializerExpr; LoopCondition; ModifierExpr)
_____ _____
Statement
InitializerExpr
Each execution of
LoopCondition
Statement
ModifierExpr
is called one repetition or
iteration of the loop.
F
LoopCondition
T
Statement
Modifierxpr
22
for (InitializerExpr; LoopCondition; ModifierExpr)
Statement
When LoopCondition becomes
false,control proceeds to the
statement ____________
______________.
Note: if the LoopCondition
is initially false, then the
body of the loop will
_____________executed.
For this reason this is
called a ___________ loop.
InitializerExpr
F
LoopCondition
T
Statement
ModifierExpr
23
Counting
The "normal" use of the for loop is to count:
Declare and
initialize the loopcontrol variable
Check if loop-control
variable has gone
through all its values
Inc-/dec-rement
loop-control
variable
int limit;
cin >> limit;
for (______________________________________)
{
Could omit
cout << count << endl;
{ }; loop
body contains
a single
}
statement
Output (suppose limit = 5):
What if limit is 1?
limit is 0?
How get 1, 3, 5, . . . ?
_____
__________
Scope
Rule
24
Nested Loops
Loops can also be nested:
Could
omit { }
since
each loop
body
contains a
single
statement
for (int val1 = 1; val1 <= limit1; val1++)
{
for (int val2 = 1; val2 <= limit2; val2++)
{
cout << val1 << '*' << val2 << " = "
<< val1 * val2 << endl;
}
}
Output (suppose limit1 = 2, limit2 = 3):
1*1
1*2
1*3
2*1
2*2
2*3
=
=
=
=
=
=
1
2
3
2
4
6
25
Input Loops
There two different types of input loops:
1. ____________approach: Ask for the number of
inputs to be entered and use a for loop.
– This method requires knowing in advance
how many inputs there are.
Example: The Nyhoffian method of assigning letter grades:
Find the mean of the numeric scores. This is the B- cutoff.
Halfway between the mean and 100% is the A- cutoff.
Halfway between the mean and 50% is the C- cutoff.
50% is the D- cutoff.
26
Example
/* Program to display a grading scale (Nyhoffian) for a test.
Input: A list of test scores
Output: Letter grade cutoffs
-----------------------------------------------------------*/
#include <iostream>
using namespace std;
int main()
{
cout << "Program finds letter grade cutoffs for a test.\n\n";
// First find the average
double score,
sum = 0.0,
average;
int numScores = 0;
27
cout << "Enter number of scores: ";
cin >> numScores;
for (int count = 1; count <= numScores; count++)
{
cout << "Enter a score: ";
cin >> score;
sum += score;
}
if (numScores > 0)
average = sum / numScores;
else
{
cerr << "\n*** No scores! Stopping program ***\n";
exit(1);
}
// Compute and display letter grade cutoffs.
cout <<
<<
<<
<<
}
"\nA-:
"B-: "
"C-: "
"D-: "
" << (100 + average) /2 << "%\n"
<< average << "%\n"
<< (average + 50) / 2 << "%\n"
<< 50 << "%\n";
28
Execution of our example program:
Program finds letter grade cutoffs for a test.
Enter
Enter
Enter
Enter
Enter
Enter
A-:
B-:
C-:
D-:
number of scores: 5
a score: 77
a score: 88
a score: 66
a score: 77
a score: 99
90.7%
81.4%
65.7%
50%
29
The second kind of input loop:
2. ___________ approach: Values are entered
until the end of input is signaled by entering a
sentinel (or flag) ― a value that signals the
end of input.
- A _________ loop is commonly used in many
programming languages.
-This method requires the availability of an
appropriate sentinel value.
The while Loop
Pattern:
while (Expression)
Statement
Statement can be either
a single or a compound
statement, but is almost
always compound.
Repetition continues so long
as Condition is true.
F
Condition
T
Statement
31
Using while for an Input Loop
Pattern:
Prompt for first value
Read value
while (value != sentinel)
{
Process the value
}
Need the first input
value so it can be
compared with
sentinel
Prompt for another value
Read value
Get the
next value
32
Example
/* Program to display a grading scale (Nyhoffian) for a test.
Input: A list of test scores
Output: Letter grade cutoffs
-----------------------------------------------------------*/
#include <iostream>
using namespace std;
int main()
{
cout << "Program finds letter grade cutoffs for a test.\n\n";
// First find the average
double score,
sum = 0.0,
average;
int numScores = 0;
•33
cout << "Enter a test score (-1 to quit): ";
cin >> score;
while (score >= 0)
{
numScores++;
sum += score;
cout << "Enter another score (-1 to quit): ";
cin >> score;
}
if (numScores > 0)
average = sum / numScores;
else
{
cerr << "\n*** No scores! Stopping program ***\n";
exit(1);
}
sentinel
// Compute and display letter grade cutoffs.
cout <<
<<
<<
<<
}
"\nA-:
"B-: "
"C-: "
"D-: "
" << (100 + average) /2 << "%\n"
<< average << "%\n"
<< (average + 50) / 2 << "%\n"
<< 50 << "%\n";
•34
Execution of our example program, but using the while-loop version:
Program finds letter grade cutoffs for a test.
Enter a test score (-1 to quit): 77
Enter another score (-1 to quit): 88
Enter another score (-1 to quit): 66
Enter another score (-1 to quit): 77
Enter another score (-1 to quit): 99
Enter another score (-1 to quit): -1
A-: 90.7%
B-: 81.4%
C-: 65.7%
D-: 50%
35