freeCodeCamp/guide/english/certifications/javascript-algorithms-and-data-structures/basic-javascript/iterate-through-an-array-with-a-for-loop/index.md

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Iterate Through an Array with a For Loop

Iterate Through an Array with a For Loop

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Hints

Hint 1

Remember the structure of a for loop: for ([initialization]; [condition]; [final-expression]) statement

• The [initialization] part is executed only once (the first time).
• The [condition] is checked on every iteration.
• The [final-expression] is executed along the statement if [condition] resolves to true.

Hint 2

Remember how accumulators work: var x += i

• The variable x is going to act as the accumulator.
• The variable i is the one which value will be stored (and accumulated) inside x
• The expression += is an just abreviation of x = x + i

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Solutions

Solution 1 (Click to Show/Hide)

var total = 0;
for (var i = 0; i < myArr.length; i++) {
  total += myArr[i];
}

Code Explanation

• Inititialization: i gets a value of 0 and its used as a counter.
• Condition: the subsequent code is executed as long as i is lower than the length of myArr (which is 5; five numbers but arrays are zero based).
• Final-expression: i is incremented by 1.
• Statement: The function adds myArr[i]'s value to total until the condition isn't met like so:

total + myArr[0] -> 0 + 2 = 2 
total + myArr[1] -> 2 + 3 = 5
total + myArr[2] -> 5 + 4 = 9
total + myArr[3] -> 9 + 5 = 14 
total + myArr[4] -> 14 + 6 = 20

Solution 2 (Click to Show/Hide)

for (var y = myArr.length - 1; y >= 0; y--) {
  total += myArr[y];
}

Code Explanation

This works similarly to the last solution but it's faster¹ although it might not meet your requirements if order is important.

• Initialization: y gets the myArr.length's value once so the function doesn't need to check it at condition every time the loop is executed.
• Condition: the loop is executed as long as y is greater than 0.
• Final-expression: y is decremented by 1.
• Statement: The function adds myArr[y]'s value to total until the condition isn't met like so:

total + myArr[4] -> 0 + 6 = 6
total + myArr[3] -> 6 + 5 = 11
total + myArr[2] -> 11 + 4 = 15
total + myArr[1] -> 15 + 3 = 18
total + myArr[0] -> 18 + 2 = 20

Relevant Links

1. "Are loops really faster in reverse?", stackoverflow.com

Relevant Links

• "for" - MDN JavaScript reference