Control Structures
The if, for, and while Statements
§5.1, §5.4, & §5.5
(Several examples here;
also Lab #4)
1
st. line
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 sequence.
Statements that select one of several
alternatives.
Statements that repeat another statement.
EVERY PROGRAM CAN BE
WRITTEN USING THESE 3
CONTROL STRUCTURES.
See
flowcharts in
§5.4 & 5.5
2
List seq, sel, rep
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 von Neumann
Named after
mathematician
architecture. A special register, called the
/physicist John
von Neumann
instruction counter (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
IC++ (unless the IC is changed in some other way)
3
Diagram: IC, memory, CPU
The last step (IC++) indicates that the default manner
of executing statements is sequential.
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 compound statement (or
block). 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
Nested blocks;
Pascal: BEGIN…END
Scope
A variable declared within a block is called a local
variable. It exists (has memory allocated to it)
only from its declaration to the end of the block.
We say that its scope extends from its
declaration to the end of the block.
This is one of
For example, in the code
several scope
...
cin >> item;
while (item != -999)
{
int i = 0;
...
cin >> item;
i++;
}
cout << "Value of i = " << i;
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
Move int I = 0-;
Will have blocks inside blocks
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 condition (a
boolean expression):
The C++ if statement 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
Statements can be blocks
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
T
If Condition is true,
Statement is executed;
otherwise Statement is skipped.
Statement
Condition
F
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Proceed or bypass
Some programmers use else;
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
Efficient alternative to abs(value);
endl
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";
exit(1);
// Stop execution & return 1 to OS
}
Lab 4:
Exer. 4.3
9
Visual aligns either format;
User-friendly assert
The Two-Branch if
In the second form of if, the
else and Statement2 are present:
STYLE TIP
Align if and else and
indent & align the
statements in each part.
if (Condition)
Statement1
else
Statement2
If Condition is true,
Statement1 is executed
and Statement2 is skipped;
otherwise Statement1 is
skipped and Statement2
is executed.
T
Statement1
Condition
F
Statement2
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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";
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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.
Exactly one of the statements Stmti will
be selected and executed, namely, the one
corresponding to the first Condi that is true.
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The intent is to implement a multi-alternative selection
structure of the following form, where exactly one of the
alternatives is selected and executed:
T
Stmt1
T
Stmt2
F
F
F
F
T
Stmt3
T
StmtN
StmtN+1
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Click
switch
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
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And it is treated by the compiler as a sequence of
nested ifs 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
move if up
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
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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';
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... 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
menu processing - Lab 4
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 condition.
There are three parts to the repetition mechanism:
• Initialization
• Repeated execution
• Termination
Now we look at one repetition statement in C++,
the for statement:
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for, while, do-while
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.
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for (int I = 1; I <= 10; i++)
The for Loop
for (InitializerExpr; LoopCondition; ModifierExpr)
Statement
Statement will be executed
so long as LoopCondition
is true.
InitializerExpr
F
LoopCondition
T
Statement is often called
Statement
the body of the loop.
ModifierExpr
STYLE TIP
Indent and align the
statements in the
body of the loop.
21
for (InitializerExpr; LoopCondition; ModifierExpr)
true
false
Statement
InitializerExpr
Each execution of
LoopCondition
Statement
ModifierExpr
is called one repetition or
iteration of the loop.
F
LoopCondition
T
Statement
ModifierExpr
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for (InitializerExpr; LoopCondition; ModifierExpr)
Statement
When LoopCondition becomes
false,control proceeds to the
statement following
the loop.
InitializerExpr
F
LoopCondition
T
Statement
Note: if the LoopCondition
is initially false, then the
body of the loop will
not be executed.
For this reason this is
called a pretest loop.
ModifierExpr
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Counting
InitializerExpr
F
LoopCondition
T
Statement
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 (int count = 1; count <= limit; count++)
{
cout << count << endl;
}
Output (suppose limit = 5):
1
2
3
4
5
What if limit is 1?
limit is 0?
How get 1, 3, 5, . . . ?
1
no output
count += 2
ModifierExpr
Could omit
{ }; loop
body contains
a single
statement
Scope
Rule
24
Scope: Need count after loop?
Move initializer up
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
Table output
Input Loops
There are two different types of input loops:
1. Counting 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 betwe en the mean and 50% is the C- cutoff.
50% is the D- cutoff.
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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;
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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%
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The second kind of input loop:
2. Sentinel 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 while loop is commonly used in many
programming languages.
-This method requires the availability of an
appropriate sentinel value.
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Second part of lab
The while Loop
Pattern:
while (Condition)
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
F Condition
T
Statement
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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;
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cout << "Enter a test score (-1 to quit): ";
cin >> score;
while (score >= 0)
{
count++;
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
Enter
Enter
Enter
Enter
Enter
A-:
B-:
C-:
D-:
a test score (-1 to quit): 77
another score (-1 to quit): 88
another score (-1 to quit): 66
another score (-1 to quit): 77
another score (-1 to quit): 99
another score (-1 to quit): -1
90.7%
81.4%
65.7%
50%
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Textbook: Chap. 1, pages 8-9
In other early computers, the instructions were stored outside the
machine on punched cards or some other medium and were
transferred into the machine one at a time for interpretation and
execution.
In 1945, Princeton mathematician John von Neumann wrote
“First Draft of a Report on the EDVAC (Electronic Discrete Variable
Automatic Computer)” computer in which he described a scheme that
required program instructions to be stored internally before execution.
This led to his being credited as the inventor of the stored-program
concept. The architectural design he described is still known as the von
Neumann architecture. The advantage of executing instructions from
a computer’s memory rather than directly from a mechanical input
device is that it eliminates time that the computer must spend waiting
for instructions. Instructions can be processed more rapidly and more
importantly, they can be modified by the computer itself while
computations are taking place. The introduction of this scheme to
computer architecture was crucial to the development of generalpurpose computers.
Using while for an Input Loop
while (value != sentinel)
{
Prompt for a value
Read value
}
Process the value
F Condition
T
Statement
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