Conditions in a programming language are instructions that direct the flow of a program. This works a bit like rail traffic: a train travels on a rail until it hits a switch, then the train changes its direction of travel. Conditions therefore change the course of a program. But how is the direction actually decided? The answer to this is logical operations based on the Boolean logic (remember the logic gates exercise?) . We use logical operations in everyday life, here are some examples:

When I'm hungry or thirsty, I go to the kitchen and get something
If I have enough money and I have time, then I'll make a trip to India

In a programming language the spelling is different but the logic behind it remains the same.

equal to ==
not equal !=
greater than >
less than <
greater than or equal to >=
less than or equal to <=
logical And &&
logical Or ||

In code it looks like this:

int x = 10;
if(x == 10)
{
   println("x is the same as 10");
}
else
{
   println("x is not the same as 10");
}

The code can almost be read out as intelligible language: If x is 10, write "x is equal to 10", if not then write "x is not equal to 10".

The syntax of a condition is always:

if([boolean expression])
 {
 // code that will be executed if the answer is true
 }
 else
 {
 // code that will be executed if the answer is false
 }

Of course there are also some variants of writing style:

int x=10;
  
// Conditional without brackets 
// the conditional will run until the next semicolon
// The "else" is optional 
if(x == 10)
println("x ist gleich 10");
  
// Here is a conditional with multiple cases
// pay attention to the use of 'else if'
if(x == 10)
println("x is the same as 10");
else if(x == 9)
println("x is the same as 9");
else if(x == 8)
println("x is the same as 8");
else
println("x is not 10,9 or 8");

For the multiple cases, there is a simpler version:

int x=10;
switch(x)
{
case 10:
println("x is the same as 10");
break;
case 9:
println("x is the same as 9");
break;
case 8:
println("x is the same as 8");
break;
default:
println("x is the same as 10,9 or 8");
break;

Exercise 5

Programm a drawing app. The color can be changed with the buttons '1' - '5'. The left mouse button is draws and the right mouse button is used to erase it.

Conditionals (en)

Exercise 5