How to pick a random string from an array on mouse press in Processing

I want to do the following in Processing:

Display text on the screen
When mouse is not pressed, always display the first string in the array
When mouse is pressed, choose a random string from the array and have this string persist until the mouse is pressed again

I tried moving "int index = int(random(0, 3));" two draw() but this picks a random string many times a second, and I want it to pick it once until I press the mouse again.

In draw(), the value of index is always zero. It only changes in mousePressed().

Here's my code:

String[] sentences = { // defines an array of three strings
  "Sentence one",
  "Sentence two",
  "Sentence three",
};

int index = 0; // initializes variable "index" to zero

void setup() {
  size(600, 600);
}

void draw() {
  background(255);
  fill(0);
  text(sentences[index], 10, 100); // picks a random sentence from the array
}

void mousePressed() {
  if (mousePressed == true) { // when mouse pressed 
    int index = int(random(0, 3)); // picks a random value for index: either 0,1,2
  }
}

Solution

You're experiencing variable shadowing. This line:

int index = int(random(0, 3)); // picks a random value for index: either 0,1,2

declares index as a local variable, so within that scope only it shadows the global variable.

you probably meant

index = int(random(0, 3)); // picks a random value for index: either 0,1,2

(you're defining and assigning index at the top of your code, but in mousePressed() you just want to assign a different variable to the previously defined variable)

Off topic:

if (mousePressed == true) is the same as if (mousePressed), but in this context (within the mousePressed() function is redundant since it will always be true there
you're not handling the "when the mouse is not pressed" case: mouseReleased() can help with that.

Here's a modified version of your code reflecting the above:

String[] sentences = { // defines an array of three strings
  "Sentence one",
  "Sentence two",
  "Sentence three",
};

int index = 0; // initializes variable "index" to zero

void setup() {
  size(600, 600);
}

void draw() {
  background(255);
  fill(0);
  text(sentences[index], 10, 100); // displays sentence at index from the array
}

// when mouse pressed 
void mousePressed() {
  index = int(random(3)); // picks a random value for index: either 0,1,2
}
// when mouse released
void mouseReleased() {
  index = 0; // always displays the 1st sentence when mouse is not pressed
}