User interactions are one of the reasons p5 is such a great tool for building simple physics sims. Sliders can be used to adjust the value of somethings smoothly, while text boxes can be used to let users enter exact values. Sometimes we just want one or the other, other time we want both.

## 1. Making a slider

First, we’ll make a slider. This is done by creating the slider, we will call it aSlider.

aSlider = createSlider(0,100,50,1)

The arguments in the parenthesis mean that the slider will range from 0 to 100, have an initial value of 50, and use increments of 1. We will position the slider at x = 50 and y=20 by adding the following line:

aSlider.position(50,20)

Run the sketch below to see the slider:

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function setup() {
canvas = createCanvas(300, 300);
aSlider = createSlider(0,100,50,1);
aSlider.position(50,20)
}

function draw() {
background(200);

}



Next we want the slider to do something. Let’s make it change the radius of a circle.

The line

ellipse(width/2,height/2,slider.value())

in the draw function creates and ellipse that located in the center with the third argument, its radius, being defined by the value of the slider: aSlider.value().

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function setup() {
canvas = createCanvas(300, 300);
aSlider = createSlider(0,100,50,1);
aSlider.position(50,20)
}

function draw() {
background(200);
ellipse(width/2,height/2,aSlider.value())
}



## 2. Adding a textbox

We can create a textbox, called aTextbox by adding these two lines to the setup:

aTextBox = createInput('');

aTextBox.position(50,40);

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function setup() {
canvas = createCanvas(300, 300);
aSlider = createSlider(0,100,50,1);
aSlider.position(50,20)

aTextBox = createInput('');
aTextBox.position(50,40);

}

function draw() {
background(200);
ellipse(width/2,height/2,aSlider.value())
}



## 3. Submit button

We also need a button to let the user tell the sketch to use the value from the textbox:

button = createButton('submit');

button.position(textBox.x + textBox.width+10, textBox.y);

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function setup() {
canvas = createCanvas(300, 300);
aSlider = createSlider(0,100,50,1);
aSlider.position(50,20)

aTextBox = createInput('');
aTextBox.position(50,40);
aTextBox.size(50)

button = createButton('submit');
button.position(aTextBox.x + aTextBox.width+10, aTextBox.y);

}

function draw() {
background(200);
ellipse(width/2,height/2,aSlider.value())
}



One more thing for the button: it needs to do something. Let’s make it trigger the function called updateValue(). Adding the line:

button.mousePressed(updateValue);

will take care of that. This is called a method in Javascript. The .mousePressed(aFunction) tells the button to execute the function contained in the mousePressed() argument when clicked.

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function setup() {
canvas = createCanvas(300, 300);
aSlider = createSlider(0,100,50,1);
aSlider.position(50,20)

aTextBox = createInput('');
aTextBox.position(50,40);
aTextBox.size(50)

button = createButton('submit');
button.position(aTextBox.x + aTextBox.width+10, aTextBox.y);
button.mousePressed(updateValue);

background(200);

}

function draw() {
ellipse(width/2,height/2,aSlider.value())
}

function updateValue(){
background('red');
}



## 4. Linking the slider and the textbox

Now we need the textbox value to set the slider, and visa-versa. We can use the following line to tell the slider to call a function (called sliderChange) every time it is changed. We do this by

aSlider.input(sliderChange);,

which is another use of the method construction. Now the method is the input() and it says to execute the sliderChange() function every time the slider value is changed.

And likewise, if the textbox is changed, the slider will adjust because we put

aSlider.value(aTextBox.value())

in the updateValue() function.

Run the sketch to see the linkage between all three things: the slider, the textbox, and the circle.

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function setup() {
canvas = createCanvas(300, 300);
aSlider = createSlider(0,100,50,1);
aSlider.position(50,20)
aSlider.input(sliderChange);

aTextBox = createInput('');
aTextBox.position(50,40);
aTextBox.size(50)
aTextBox.value(aSlider.value());

button = createButton('submit');
button.position(aTextBox.x + aTextBox.width+10, aTextBox.y);
button.mousePressed(updateValue);

}

function draw() {
background(200);
ellipse(width/2,height/2,aSlider.value())
}

function updateValue(){
aSlider.value(aTextBox.value())
}

function sliderChange() {
aTextBox.value(aSlider.value());
}



Ta-Da! Now both the slider and the textbox are linked and both can be used to control the circle’s radius, or whatever parameter you want them too.