Meeting 11
Building Your Own Objects
11.1 Homework Discussion
Professor: Are we getting a little arithmetic practice today?
Mike: I don’t know. Our generator still nee ds some cosmetic touch-ups but I think it’s
working.
Professor: Don’t worry about that. Please, go on, show me what y ou’ve got.
Mike: This is our function tha t produce s a single equation:
/*
* Returns a randomly generated equation together with the solution.
* Parameters:
* oper: the desired operator in form of a string ("+", "-",
* "*", or "/")
* maxVal: the maximum allowed value for the two operands
* Returns:
* An array with two elements:
* -The generated equation as a string
* -The solution as a number
* Example:
* generateEquation("+", 10, 10) may return the array
* ["8+2=_____", 10]
*/
var generateEquation = function(oper, maxVal) {
var output = [];
var firstOper = Math.round(Math.random() * maxVal);
var secondOper = Math.round(Math.random() * maxVal);
var solution, tmp;
switch (oper) {
case "+":
solution = firstOper + secondOper;
break;
205
case "-":
if (firstOper < secondOper) {
//The difference shouldn’t be negative, so swap the operands.
tmp = firstOper;
firstOper = secondOper;
secondOper = tmp;
}
solution = firstOper - secondOper;
break;
case "*":
solution = firstOper * secondOper;
break;
case "/":
if (secondOper == 0) {
//Shouldn’t divide by zero.
secondOper++;
}
//Uses the product in place of the dividend, so the result is
//always an integer with no remainder.
tmp = firstOper * secondOper;
solution = firstOper;
firstOper = tmp;
}
output[0] = firstOper + oper;
output[0] += secondOper + "=_____";
output[1] = solution;
return output;
};
Professor: Very nice indeed! I like how you commented the function.
Maria: We found some recommendations about how to write comments for functio ns
on the Internet. We discovered that it is extremely useful to have our fun ctions com-
mented in this way, so we don’t have to wonder about how they work every time we
want to use them.
Professor: I don’t think the code needs any further explanation.
Mike: In the end we tested the function in the following man ner:
var i;
for (i = 0; i < 20; i++) {
document.write("<div>" + generateEquation("-", 10)[0] + "</div>");
}
Because the functio n returns an array, we used an array access operator directly on the
function call in orde r to extra ct equations and leave out the solutions. Here is how the
resulting worksheet looks.
206 Meeting 11. Building Your Own Objects
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