Branching and Looping Examples, cont’d

Remember the generic triple jump world…

Let’s change this to be interactive.

User input functions in Alice

• ask user for a number• ask user for yes or no• ask user for a string

• Start Alice and open genericTripleJump

• Look at world. my first method

• Note that the statements there form a linear sequence.

• We want to modify this algorithm.

• The first step is to add an If/Else instruction to the method.

• Drag a copy of the If/Else tile from the bottom of the editor area.

• Drop it into the method just below the three jump instructions.

Adding If/Else instructions

• When adding an If/Else, you will be prompted to choose between true “and” “false” as an initial value for the condition.

• This value is merely a placeholder.

• We will always replace this value with a Boolean expression of a condition we want to test.

• Replace the placeholder condition in the If/Else instruction with a function.

• We need the function that will allow us to ask the user a yes/no question.

• We want Alice to jump only if the user answers yes to our query.

• Need to move the instruction that causes Alice to jump to the If clause of our If/Else instruction.

• genericTripleJumpWhile.a2w

• genericTripleJumpLoop.a2w

A Sentinel Sailing Loop

• Sailboat will turn to face the object• Will sail one meter forward• This process continues until the sailboat is within 5

meters of the object

While (NOT (the sailboat is within 5 meters of the [object]))

{turn to face [object]move forward 1 meter

}

A Sentinel Sailing Loop

• Let’s create a sail to method that will work with any object.

• It should accept the target object as an input parameter.

• This will be similar to the way the primitive move method accepts direction and amount

A Sentinel Sailing Loop

• Sailboat will turn to face the object• Will sail one meter forward• This process continues until the sailboat is within 5

meters of the object

While (NOT (the sailboat is within 5 meters of the [object]))

{turn to face [object]move forward 1 meter

}

A Sentinel Sailing Loop

• Sailboat will turn to face the object• Will sail one meter forward• This process continues until the sailboat is within 5

meters of the object

While (NOT (the sailboat is within 5 meters of the [object]))

{turn to face [object]move forward 1 meter

}

A Sentinel Sailing Loop

• Sailboat will turn to face the object• Will sail one meter forward• This process continues until the sailboat is within 5

meters of the object

While (NOT (the sailboat is within 5 meters of the [object]))

{turn to face [object]move forward 1 meter

}

Let’s generalize sailTo so that we can sail to any object.