How to calculate grades in an array using while loop in javascript

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I'm trying to calculate the total grade and store it in an array and calculate the average grade. The total grade calculation is working fine but the average shows NaN. I don't understand why. Also The Average calculation ignores failing grades (i.e., grades less than 50).

What I have tried:

JavaScript

courseNum=prompt("Enter the number of courses you have taken: ");
grades=[];
totalGrade=0;
grades=0;

i=1;

while (i<=courseNum){
	courseGrade=parseInt(prompt("Enter the grade for course #" + i));
	if (courseGrade>=50 && courseGrade<=100){
		console.log(totalGrade+=courseGrade);
	}
	i++;
}

console.log(grades[i]=totalGrade);
console.log(totalGrade/grades[i]); //this statement

Posted 17-Sep-22 7:25am

Cloverr

Updated 17-Sep-22 18:31pm

OriginalGriff

v2

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Mike Hankey 17-Sep-22 14:01pm

Shouldn't it be;

console.log(totalGrade/i); //this statement

Cloverr 17-Sep-22 17:31pm

i tried that.. doesn't work ☹️

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OriginalGriff · Answer 1 · 2022-09-17T18:31:00

There are a couple of reasons.
Firstly because arrays in Javascript (like most other languages) are zero based: the valid indexes are 0 to N - 1 where N is the number of elements.
So if you have an array of 3 elements, the valid indexes are 0, 1, and 2 - all other values are invalid.

Your while loop starts with i as 1, and exits when it is equal to coursenum + 1
So if you use i to index into the array it will try to fetch a value that doesn't exist, and you get NAN.

And secondly, because you don't store anything into your array at all anyway! You read a value into courseGrade inside the loop, but at no point do you do anything with it to store it in grades

Either get rid of grades entirely - you don't need it for this anyway unless you want to list the actual values entered later in which case they need to be stored in the array - or store the values in your loop.

Finally, I'm not sure exactly what the intent of the calculation is anyway: are you to report the average passing grade, or the average grade? Because that code does neither, even when working.
The average passing grade is the total of passing grades / number of passing grades, you return the total of passing grades / number of all grades.
so if you have five grades: 25, 25, 25, 25, 100 the average grade is 200 / 5 = 40 but the average passing grade is 100 / 1 = 100 You are returning 100 / 5 which is ... not very sensible!

Have a think about exactly what you are expected to do and decide if you need the array at all and if you perhaps need a second counter?