Control Structures (A)
Topics to cover here:
• Introduction to Control Structures in the
algorithmic language
• Sequencing
Control Structures
• In your algorithm (or program), you can use
different types of statements.
• Each statement starts with a specific keyword
(except for the assignment statement (see
later)).
• There are 3 categories of control structures:
1- Sequencing
2- Selection
3- Repetition
1- Sequencing
• It is a sequential control given in a compound
statement or a block.
• A compound statement (or a block) is a
sequence of statements ordered in a way to give
a solution: e.g.
S1
S2
S3
is a sequence of 3 statements
In C++, the ; symbol is used to separate
statements.
1- Sequencing (Cont.)
• The statements that have a sequential control:
1- INPUT/OUPUT statements
2- Assignment statement
INPUT/OUPUT statements
INPUT statement (in pseudo code):
Use Input statement to input data into variables
from the standard input device (e.g. a keyboard).
INPUT Statement
Syntax: INPUT List of variables
where, List of variables contains one or more
variables
e.g.
INPUT x
INPUT a, b
The semantics (execution) of this statement:
You can enter the values you want for the
variables in the statement from the keyboard and
the computer will assign these values into the
variables (stores them in memory).
OUPUT Statement (in pseudo code)
• The OUTPUT statement has many uses.
• Use OUTPUT statement to output
- The values of variables stored in memory.
- A message (i.e. a string of characters).
- The value of an expression.
OUPUT Statement (in pseudo code).. cont.
Syntax:
1- OUTPUT List of variables
where, List of variables contains one or more variables
e.g. OUTPUT x
OUTPUT a, b
The semantics (execution) of this statement:
This statement allows the computer to access the
locations of the variables mentioned in the
statement and displays their contents on an output
device (e.g. a screen).
OUPUT Statement (in pseudo code).. cont.
2- OUTPUT message
where message may by any string of
characters enclosed with double quotas.
Use such statement
- for making your output clearer
- for helping the user of your program to interact
easily with it.
e.g. OUTPUT “Enter 3 values”
OUPUT Statement (in pseudo code) (Cont.)
The semantics (execution) of this statement:
This statement will display the message on the screen.
3- OUTPUT expression
where expression is any arithmetic expression
e.g. OUTPUT
OUTPUT
3+6
x–y
The semantics (execution) of this statement:
First, the expression is evaluated, then the result will
be displayed on the screen. For the first example, it
will display 9.
OUPUT Statement (in pseudo code).. cont.
NOTE
You can mix between the different types of the
OUTPUT statements.
e.g.
OUTPUT “Length = “ , length
The semantics (execution) of this statement:
This statement will display the message
Length = on the screen and on the same line it
will display the value of the variable length.
Assignment Statement (in pseudo code)
• Storing a new value in a memory location is called
assignment.
• We use the operator  as assignment operator.
Syntax:
Variable  Expression
The semantics (execution) of this statement:
1- The Expression on the right of  is evaluated
2- The result of the expression is assigned to the variable
on the left of  .
e.g.
X  10
(This means that X has 10 now )
Y  X + 5 (This means that Y has 11 now, if X is 6)
Assignment Statement (Cont.)
NOTE:
The right hand side (RHS) of the assignment
statement should be of the same data type of the left
hand side (LHS).
e.g.
1- T  true
This will be correct if T is of Boolean type.
2- A  x + y * 2
This will be correct if A has a numeric data type (e.g.
integer, or real) and the value of the expression on
(RHS) has the same numeric data type.
Assignment Statement (Cont.)
• How to execute a statement like X  X + 1 ?
Suppose we have:
X5
Then to execute X  X + 1, we proceed as follows:
X
5
6
X5
XX+1
Assignment Statement .. cont.
• Dereferencing:
If we want to copy a value from one memory location (say,
X) into another location (say, Y), we say that we
dereference a variable.
e.g.
X5
Y  10
XY
// now X has the value 10
X
5
10
Y
10
Examples on Simple Algorithms
Example 1
Write an algorithm to determine the total cost of
apples given the number of kilos of apples
purchased and the cost per kilo of apples.
First, we have to analyze the problem to
understand what is the input, output of the
problem, and which formula to use to solve the
problem (if any).
Example1 .. cont.
1- Analysis stage:
• Problem Input:
- Quantity of apples purchased (in kilos)
- Cost per kilo of apples (in dinar/fils per kilo)
•
•
Problem Output:
- Total cost of apples (in dinar/fils)
Formula:
Total cost = Number of kilos of apples × Cost per kilo
Example1 .. cont.
2- Algorithm Design
We write the algorithm by using the pseudo code
keywords (written in bold type). For algorithm
writing, please refer to the attachment given with
the syllabus that is handed to you, previously.
ALGORITHM apples
INPUT quantity, cost
total_cost  quantity * cost
OUTPUT “Total cost = “ , total_cost
END apples
Example1 .. cont.
3- Testing the algorithm
We give a sample data to see whether the algorithm solves the
problem correctly or not.
To give a sample data, proceed as follows:
1- Prepare a table to contain all variables of the algorithm.
2- Give any data you like as input.
3- Calculate any formula in the algorithm using these data.
4- Show the output
e.g.
quantity cost
total_cost
1. 3
0.9
2.
2.7
The output:
Total cost = 2.7
Example 2 **
•The problem statement:
Write an algorithm that converts a given length from
feet and inches into centimeters.
1- Analysis stage:
• Problem Input:
- Length (in feet and inches)
• Problem Output:
- Length in centimeters
• Formula:
Length in centimeters = 2.54 × Length in feet & inches
Example 2 .. cont.
2- Algorithm Design
ALGORITHM Convert
INPUT length
centimeters  2.54 * length
OUTPUT “Length in centimeters= “,
centimeters
END Convert
Example 2 .. cont.
3- Testing the algorithm
length
centimeters
2.5
--6.35
The output:
Length in centimeters = 6.35
------------------------------------------------------------------------length
centimeters
0.4
--1.016
The output:
Length in centimeters = 1.016
Example 3
•The problem statement
Write an algorithm that calculates the wage of an
employee, if the number of hours that he worked and the
rate of each hour are given.
1- Analysis stage:
• Problem Input:
- Hours (the employee has worked)
- Rate of money per hour
• Problem Output:
- The wage that the employee will receive.
• Formula:
Wage = Hours × Rate of money per hour
Example 3 .. cont.
2- Algorithm Design
ALGORITHM Pays
INPUT hours, rate
wage  hours * rate
OUTPUT “Employee wage = “,
wage
END Pays
Example 3 .. cont.
3- Testing the algorithm
hours rate wage
30
1.5
--45
The output
Employee wage = 45
--------------------------------------------------------------------------------------------------------------------------------------------------------
hours rate
37
2.3
wage
--85.1
The output
Employee wage = 85.1
Example 4
• The problem statement
Suppose that a store makes discount on its items.
Design an algorithm that calculates the amount of
money that a customer has to pay on purchasing an
item adding to it the required tax, where the following
data are given: the price of the item before discount,
the discount rate, and the tax rate.
1- Analysis stage:
• Problem Input:
- Original Price of the item
- Discount rate for the item
- Tax rate
Example 4 .. cont.
•Problem Output:
- Price of the item after discount
• Formulae:
discount = original price × discount rate
price
= original price – discount
tax
= price × tax rate
price
= price + tax
Example 4 .. cont.
2- Algorithm Design
ALGORITHM Purchases
INPUT price, discount_rate, tax_rate
discount  price * discount_rate
price  price - discount
tax  price * tax_rate
price  price + tax
OUTPUT “Price = “, price
END Purchases
Example 4 .. cont.
3- Testing the algorithm
price discount_rate tax_rate
30
0.20
0.16
discount
--6
tax
---
24
3.84
27.84
The output:
Price = 27.84