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Cycles while and for in JS, help me!

Brenda Limón on August 12, 2018

Hi! I'm trying to learn how to draw in Canvas, as an exercise to learn about cycles in JS, the exercise is that we have to use while to create a cy...

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Chris Saldivar • Aug 13 '18 • Edited

Loops are always a bit tricky so I like to show my students three basic rules for loops.

Initialize a loop control variable (Used to start and stop the loop)
Write a test condition (Otherwise how will the loop know when to exit)
Update the loop control variable (Without this we'll have an infinite loop)

I've copied the relevant section of your code below and added comments:

var l = 0;               // 1. Setup the loop control variable

while (l < lineas) {     // 2. The test condition that will start/stop the loop
    yi = 10 * l;
    xf = 10 * (l + 1);
    dibujarLinea(colorcito, 0, yi, xf, 300);
    console.log("Linea " + l)
    l = l + 1;           // 3. Update the loop control variable
}

A for loop is a control structure that allows us to perform all three steps on a single line. It really doesn't add much more functionality than a while loop does since you can simulate a for loop by writing code that way you did for the first drawing. You can achieve the same results as the code above with the following for loop:

//   1. init      2. test   3. update
for (let l = 0; l < lineas; l++){
    yi = 10 * l;
    xf = 10 * (l + 1);
    dibujarLinea(colorcito, 0, yi, xf, 300);
    console.log("Linea " + l)
}

I hope this clears things up! Also can I ask where that tutorial is from?

Brenda Limón • Aug 14 '18

Thanks Chris! You’re a great teacher! It’s a course of Platzi, a YC startup that teaches programming in Spanish is platzi.com if you wanna check it :)
Thanks a lot lot lot a thousand lot! 🤗🤗

Chris Saldivar • Aug 15 '18

Glad I could be of help! I'm actually not a teacher, I was an undergrad teaching assistant for 2 years and then a grad assistant for the past 3 semesters. I just quit begin a GA for my last year of grad school because I'm making an online course for learning web development called Let's Learn: Fullstack. However, it's aimed at people that have a familiarity with web development and I'd like for it to be accessible to beginners. So, I'm working on a primer course that covers the fundamentals of web development.

Good luck on your web development journey!

Brenda Limón • Aug 15 '18

When your course is ready let me know! I'll like to take it! :D

Chris Saldivar • Aug 16 '18

Awesome I'll be sure to!

John Alcher • Aug 12 '18

var d = document.getElementById("dibujito");
var lienzo = d.getContext("2d");
var lineas = 30;
var l = 0;
var yi, xf;
var colorcito = "pink";

while (l < lineas) {
    yi = 10 * l;
    xf = 10 * (l + 1);
    dibujarLinea(colorcito, 0, yi, xf, 300);
    console.log("Linea " + l)
    l = l + 1;
}

dibujarLinea(colorcito, 299, 1, 299, 299);
dibujarLinea(colorcito, 1, 1, 299, 1);
dibujarLinea(colorcito, 1, 1, 1, 299);
dibujarLinea(colorcito, 1, 299, 299, 299);

function dibujarLinea(color, xinicial, yinicial, xfinal, yfinal) {
    lienzo.beginPath();
    lienzo.strokeStyle = color;
    lienzo.moveTo(xinicial, yinicial);
    lienzo.lineTo(xfinal, yfinal);
    lienzo.stroke();
    lienzo.closePath();
}

Formatted your code. You can enclose your code in triple backticks (`) so it's easier to read

Brenda Limón • Aug 12 '18

Thanks a lot! :)

Roberto Sanchez • Aug 12 '18 • Edited

The for loop works like this: initial value, condition to keep the loop going, increment

for(let i = 0 ; i<30;i++){
    yi = 10 * i;
    xf = 10 * (i + 1);
    dibujarLinea(colorcito, 300, yi, xf, 0);
    console.log("Linea " + i)
}

Brenda Limón • Aug 14 '18

Oooo this looks so easy! Thanks a lot!!! 🐶🤗

John Alcher • Aug 12 '18

You're welcome! Coming from a country with Spanish heritage, I can kinda infer what your code does. But it would be really nice if you name your variables in English to get better feedback.

Brenda Limón • Aug 14 '18

You know, I haven’t thought about it, but your right!

Ben Sinclair • Aug 13 '18

Hi there. I don't have anything to say right now about the point of your post (sorry) but could I suggest/ask that you don't use l as a variable name? On dev.to (and probably in most people's editors) it looks identical to the number 1.