pxt-calliope/docs/projects/stop-watch.md

Stop watch

This project turns the @boardname@ into a simple stop-watch. You can use it the [Watch project] if you've build it already. Pressing A will start the timer and pressing A again will display the resulting time. Let's get started.

Program state

The stop watch has two states: it is either stopped or measuring time. Let's create a new variable start_time to track the state:

• if start_time is equal to 0, the watch is stopped.
• if start_time is not equal to 0, the watch is running and the value of start_time is the time it started.

Pseudo code

The rough outline of the code is as follows: when a user presses A, we start by determining in which state we are. If the watch is stopped (start time is 0), we start the counter and store the current time. If the watch is running (start time is not 0), we compute the duration and reset start time to 0.

The ||input:runningTime|| block returns the number of milli-seconds elapsed since the @boardname@ was turned on. To compute the duration in seconds, we use the following formula:

duration in seconds = (current time - start time) / 1000.0

In pseudo code, this could look like this:

on button pressed
    if start time is 0
        store current time into start time      
    else
        show duration
        reset start time

If you translate the pseudo-code line by line into blocks, it might end up like this.

let start_time = 0
input.onButtonPressed(Button.A, function () {
    // is the watch running?
    if (start_time == 0) {
        // store current time
        start_time = input.runningTime()
        basic.showIcon(IconNames.Butterfly)
    } else {
        // compute duration and display it
        basic.showNumber(Math.idiv(input.runningTime() - start_time, 1000))
        // reset watch state
        start_time = 0
    }
})
start_time = 0