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
Programm a drawing program. 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.