Programming selection and iteration

Question 1

Selection - if statement

Answer 1

Selection or conditional statements execute a block of code based on the result of some test or condition we have set in the code

Testing to see whether the result is TRUE or FALSE and we can set different actions depending on the result of our test.
Format for an IF statement is:

IF Condition = True
THEN execute instructions 1
END IF

A condition is an expression that has a value of TRUE or FALSE. If the condition is true then the instructions are executed

Question 2

Indentation

Answer 2

Indentation signifies the start and end of a block of code
Correct indentation is crucial, or the program may either not run, or give an incorrect result

Question 3

Selection - if else statement

Answer 3

Sometimes, the IF statement also contains instructions to be carried out if the condition is False

IF Condition = True
THEN execute instructions 1
ELSE
THEN execute instructions 2
END IF

Question 4

Selection: nesting if statements

Answer 4

We can also use IF/ELIF statements to check sub-conditions in a program:
x=5
y=8
if x == y:
print(“x and y are equal.”)
else:
if x < y:
print(“x is less than y.”)
else:
print(“x is greater than y.”)

Question 5

Selection - comparison and logical operators: ==

Answer 5

Equality operator
checks whether both values are the same

Question 6

Selection - comparison and logical operators: !=

Answer 6

Not equal to

Question 7

Selection - comparison and logical operators: >

Answer 7

Greater than

Question 8

Selection - comparison and logical operators: <

Answer 8

Less than

Question 9

Selection - comparison and logical operators: >=

Answer 9

Greater than or equal to

Question 10

Selection - comparison and logical operators: <=

Answer 10

Less than or equal to

Question 11

Selection - comparison and logical operators: AND

Answer 11

Checks whether both conditions are true or false
Returns true only when both conditions are true

Question 12

Selection - comparison and logical operators: OR

Answer 12

Checks whether either of the conditions are true
Returns true when either input is true

Question 13

Selection - comparison and logical operators: NOT

Answer 13

Reverses a Boolean value (true or false)

If true then it becomes false
If false then it becomes true

Question 14

3 basic programming constructs

Answer 14

Sequence
Selection
Iteration

Question 15

Iteration

Answer 15

Allows a group of statements in the code to be executed several times
Useful because we would otherwise have to write the instructions out the correct number of times
There are two different types of iteration – indefinite and definite
Both can be used but in some cases one is more appropriate than the other

Question 16

Iteration - FOR loop - definite or counter controlled

Answer 16

FOR Counter = 1 TO 10
OUTPUT print Counter
ENDFOR

Question 17

Iteration - WHILE loop - Indefinite or conditional

Answer 17

Counter = 1
WHILE Counter < 10
OUTPUT print Counter
Counter = Counter + 1
ENDWHILE

Question 18

Iteration - REPEAT UNTIL loop - Indefinite or conditional
(not covered in Python)

Answer 18

Counter = 1
REPEAT
OUTPUT print Counter
Counter = Counter + 1
UNTIL Counter <10
ENDREPEAT

Question 19

Iteration - while loops

Answer 19

Counter = 1
REPEAT
OUTPUT print Counter
Counter = Counter + 1
UNTIL Counter <10
ENDREPEAT

WHILE loops are also known as conditional loops as they will continue to iterate or loop until a condition, which you have set in your code, is met
Important to make sure that you have written code that will allow your loop to finish and avoid an infinite loop
The condition might be a condition that requires a certain variable to be in a certain range, or a certain statement to be true or false
This type of iteration is characterised by WHILE loops