==   equal to
!=   not equal to
>    greater than
<    less than
<=   less than or equal to
>=   greater than or equal to

We can also combine several conditions using logical operators:

	&&    	and
	||    	or
	!     	not

Let's look at some conditions in isolation, prior to using them in 'if' statements.

int n=1, m = 2; // declare a few vars for examples
double x=3.67,y=0.0;

n < 3                    is true
m == 1                   is true
x >= 4.0                 is false

(n < 3) && (m == 1)      is true
(n < 3) || (m == 1)      is true

(n < 3) && (x >= 4.0)    is false
(n < 3) || (x >= 4.0)    is true

!(m == 1)                is false

These examples illustrate that    ||    &&     !     work like boolean operators in maths i.e.

- when we 'or' a series of expressions together, we only need one to be true to make the whole expression true.

- when we 'and' a series of expressions together, each one must be true to make the whole expression true.

Note that the 'equal to ' condition is written

==

 =

The operators cannot be used on strings - only numbers.

The if statement

function void main() { int age = 0; age = getInt("Type your age"); if(age<20) { print ("you are young");} else { print("sorry - old!"); } alert("done"); }

Note:

Following the word 'if' we must put a condition, which is true or false.

If it is true, the statements within the first set of { } will be done, as far as 'else'. If the condition was false, the second group of statements within {...} will be done.

- the condition MUST be enclosed in round brackets.

- The two options can contain any number of instructions.

- If 'else' sounds awkward, interpret it in your head as 'otherwise'.

The second option might not be needed in certain problems, so we may put e.g:

if(age < 12)
{
alert("Too young");
}

Code formatting - pretty

Click the Pretty button. You will see that the code is indented, giving:

function void main()
{
    int age = 0;
    age = getInt("Type your age");
    if(age<20)
    {
        print ("you are young");
    }
    else
    {
        print("sorry - old!");
    }
    alert("done");
}

Choosing and, or

if((age>=16) || (age<=64))  // wrong
{
  alert("age ok");
}

We should use && (and). The overall result is true only if the first and the second condtions are true.

Nested Ifs

{
    {

    }
}

function void main() { int mark = 0; mark = getInt("Type mark"); if(mark>90) { alert("distinction"); } else { if(mark>=40) { alert("pass"); } else { alert("fail"); } } }

In the above, we are expressing the situation where there is a list of conditions, one of which will be true, and a list of actions for each case. The final 'else' is a 'catch-all' if none of the other conditions were met. This pattern is very common. Note the use of {...}. It is essential to pretty-print such code, as we have done.

Using Else

n = getInt("Type a positive number");
if (n<0)
{
    print("Error!");
}
// now process it
if (n>=0)
{
    ... process it
}

n = getInt("Type a positive number");
if (n < 0)
{
    print("Error!");
}
else
{
    ... process it
	
}

Key Points

Consider using extra brackets (...) in conditions to clarify the meaning of the program.

Problems

2. As Q1, but a warning message should be printed. In any case, square the number and print the answer.

3. Write a program to read in the dimensions of a rectangle and print its area. An error message should be printed if any dimension is negative.

4. Write a program to read in two integers, and to print them in ascending order.

5. Input 3 integers, and display an alert stating whether they are all equal or not.

6. Input an age, and display a message stating "Available for work" if in range 16 to 64 inclusive. If not, display "out of range".

7. Input 3 numbers, and print the biggest.