Lesson #3 and #4. Conditions

Theory

Lection # 3 in pdf format

IF Statements

• If statements are concerned with Boolean logic. If the statement is true, the block of code associated with the if statement is executed. If the statement is false, control either jumps to the line after the if statement, or after the end keyword of an if statement block.

var numb:= 1; if numb = 1 then begin // statements that will execute if the value of the numb variable is 1, // will be placed here. end;

var numb:= 1; if numb = 1 then begin // statements that will execute if the value of the numb variable is 1, // will be placed here. end;

• You can remove the begin and end keywords if your statement to execute is a single line statement. PascalAbc understands that if no begin and end are used, the line immediately after the if (condition) will be executed if the condition is true.
Else clauses
• IF statements can also have associated else clauses. The else clause executes when the if statement is false:

var numb:= 1; if numb = 1 then begin // Block of code executes if the value of the numb variable is 1. end else begin // Block of code executes if the value of the numb variable is not 1. end

var numb:= 1; if numb = 1 then begin // Block of code executes if the value of the numb variable is 1. end else begin // Block of code executes if the value of the numb variable is not 1. end

Else if clauses (or Chained If statements)
• If statements can also have associated else if clauses. The clauses are tested in the order that they appear in the code after the if statement. If any of the clauses returns true, the block of code associated with that statement is executed and control leaves the block of code associated with the entire if construct.

var numb:= 1; if numb = 1 then begin // Block of code executes if the value of the numb variable is 1. end else if numb = 0 then begin // Block of code executes if the value of the numb variable is 0. end else begin // Block of code executes if the value of the numb variable is neither above answers. end

var numb:= 1; if numb = 1 then begin // Block of code executes if the value of the numb variable is 1. end else if numb = 0 then begin // Block of code executes if the value of the numb variable is 0. end else begin // Block of code executes if the value of the numb variable is neither above answers. end

• The semicolon before else is not needed!
• You can create as many else if blocks as necessary.
Sample:
To do: A number of season (Winter is the first) is given. Output a name of the entered season number. It is better to use the chained If statements.

✍ Algorithm:

Logical (boolean) operations

We can define variables of boolean type:

var A,B: boolean; A := True; B := False; Print(A and B); Print(A or B); Print(not A);

var A,B: boolean; A := True; B := False; Print(A and B); Print(A or B); Print(not A);

• A and B is True if A is True and B is True at the same time. In other cases A and B is False
• A or B is False if A is False and B is False. In other cases A or B is True
• not A has opposite value: not A is True if A is False
Logical (boolean) operations in if statements:
• Conditions may consist of logic operations: not, or, and. If there are more than one condition, so each condition must be surrounded by round brackets.

For example if we must check two conditions:

if (year < 20) or (year > 18) then begin // if body end

if (year < 20) or (year > 18) then begin // if body end

If one of the conditions or both conditions are True then if body executes.
• Or another example:

var a: = 5; if (not (a<4)) and (7>5) then // ← True begin // if body end

var a: = 5; if (not (a<4)) and (7>5) then // ← True begin // if body end

• The statements write and print can also return True or False:

//... a = 5; write (a >= 5); // returns True // ...

//... a = 5; write (a >= 5); // returns True // ...

• You don’t need to write semicolon before else clause.
• Don’t forget about begin..end in the cases when there is more than one lines of code after then or else.

Case statement

If there are too many else if statements, code can become difficult to understand. In this case, a better solution is to use a switch statement:

var numb:= 1;
case numb of  
1,2 : writeln ('1 or 2'); //  Block of code executes if the value of numb is 1 or 2.
 3: writeln('3'); //  Block of code executes if the value of numb is 2.
 4: writeln('4'); //  Block of code executes if the value of numb is 3.
 5: writeln('5'); //  Block of code executes if the value of numb is 5.
 else writeln('nothing matches'); //  Block of code executes if nothing matches.
end

• A block labeled else will execute when none of the other blocks match.

Using String type

Case statement can check the variable of String type.

✍ Algorithm:

Using ranges and enumerations

✍ Algorithm:

Labs and tasks

True or false?

please enter two integers
>>> 23
>>> 1
23 is greater than 1 is True 

please enter two integers
>>>5 
>>>5
5 is not equal to 5 is False

please enter three integers
>>>3 >>>6 >>>2
3 is less than 6 less than 2 is False

please enter three integers
>>>2 >>>5 >>>7
2 is less than 5 less than 7 is True

please enter a three digit number
>>> 854
8 is less than 5 and 4 is : False

// The function odd returns True when its argument is odd integer:
print (odd(5)); // true 
print (odd(6)); // false .

-5,  8 => True
 12, 0 => False
 6, -1 => True
 11, 7 => True

please enter two integer numbers
>>> 9 >>> 2
Either 9 or 2 is an odd number. This is : True

If statement

-3 => -3
 0 =>  0
 1 =>  2
 5 =>  6

please enter an integer number
>>> -77
The result is -77

2   => 20
1   =>  1
-10 => -100

please enter an integer number
>>> 8
The result is 80

4 => 12
9 =>  3
-10 => -30

please enter an integer number
>>> 12
The result is 36

-3 => -5
 0 =>  1
 1 =>  2
 5 =>  6

please enter an integer number
>>> 48
The result is 49

Chained If statements and Logical operations in if statements

how old are you?
>>> 8
you should watch the cartoon

how old are you?
>>> 15
you can take a walk

what's your grade?
>>> 2
"it's very bad"

what's your grade?
>>> 4
"it's good"

what's time of the day?
>>> 2
"night"

what's time of the day?
>>> 24
"midnight"

please enter two integer numbers
 3 5
The result is A = 8, B = 8

please enter two integer numbers
4 4
The result is A = 0, B = 0 

N: 
>>> 16 
result =  416

N: 
>>> 86
result = 286

N:
>>> 31
result = 310 

{0.5 points} Task 15:
To do: Integer number x is entered. Calculate the value of the function f:

Note: The ∪ sign means boolean or.

Expected output:

Enter an integer: 
>>> 4
Result is 8

[Program name: L3task15.pas]

{0.5 points} Task 16:
To do: Real number x is entered. Calculate the value of the function f:

Expected output:

please, enter an x value: -2
result of 6-x =  8 

please, enter an x value: 5
result of 2sin(x) =  -1.91784854932628 

[Program name: L3task16.pas]

Enter three-digit integer:
>>> 602 
True 

Enter three-digit integer:
>>> 311 
False

Enter three-digit integer:
>>> -100
True

Switch statement

what's your grade?
>>> 2
"it's very bad"

what's your grade?
>>> 4
"it's good"

please enter an integer from 1-7, for day of the week
4
Thursday

please enter an integer from 1-7, for day of the week
9
such day does not exist

Enter arithmetic operation, please (from 1 till 4):
>>> 1  
Enter two real numbers:  
>>> 2.2  >>> 5.0 
the result is 2.2 + 5.0 = 7.2

Enter arithmetic operation, please (from 1 till 4):
>>> 2  
Enter two real numbers:
>>> 5.3  >>> 3.2 
the result is 5.3 - 3.2 = 2.5

enter body weight 
>>> 4
enter mass 1..5 
>>> 4
in kilograms = 4000 

N :
>>> -23 
C : 
>>> 1
True

N :
>>> 103
C :
>>> 3
D : 
>>> 7 
False

Using a String type

enter the time of the day
>>> night
result: a range of hours for night is 1..4

Using ranges and enumerations

Enter a time of a day in hours (from 1 to 24), please: 
>>> 14 
result: 14 hours belongs to day

Enter a time of a day in hours (from 1 to 24), please: 
>>> 24
result: 24 hours belongs to midnight

how old are you?
>>> 6 
preschool

how old are you?
>>> 37 
adulthood

Hometask

Complete this week's tasks that you didn't have time to do in class. The file names are specified in the tasks. You should upload your files to Microsoft Teams.