# Reversed loop

Hello. Im doing Credit Card Checker https://www.codecademy.com/practice/projects/credit-card-checker it should return valid but it returns invalid

const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8]

``````function validateCred(array) {

emptyArray = [];

everyOther = [];

for (let i=array.length-1; i > 0; i--) {

emptyArray.push(array[i]);

}

for (let j=1; j < emptyArray.length; j += 2) {

everyOther.push(emptyArray[j]);

}

modified = everyOther.map(function multiply(s){

let multiplied = s*2;

if (multiplied > 9){

return multiplied - 9;

}else{

return multiplied;

}

})

let sum = modified.reduce((a, b) => a+b, 0);

if(sum%10 == 0) {

console.log("credit card number is valid")

}else {

console.log("credit card number is invalid")}

return [emptyArray, everyOther, modified, sum];

}

console.log(validateCred(valid1))
``````
1 Like

I believe the issue is is in your very first loop. According to the exercise the first digit on the right is a check digit, then you iterate backwards. However, in your first for loop you are starting with the check digit (`index length-1`), and leaving out the first digit (`index 0`). You want to write the loop in such a way that youâ€™re excluding the check digit, and including the first digit. Then it should give you the correct result.

1 Like

replaced i > 0 with i >=0, but still says â€śinvalid numberâ€ť even though computation is correct

``````function validateCred(array) {

emptyArray = [];

everyOther = [];

array2 = [];

for (let x=0; array.length >=)

for (let i=array2.length-1; i >= 0; i--) {

emptyArray.push(array2[i]);

}

for (let j=1; j < emptyArray.length; j += 2) {

everyOther.push(emptyArray[j]);

}

modified = everyOther.map(function multiply(s){

let multiplied = s*2;

if (multiplied > 9){

return multiplied - 9;

}else{

return multiplied;

}

})

let sum = modified.reduce((a, b) => a+b, 0);

if(sum%10 == 0) {

console.log("credit card number is valid")

}else {

console.log("credit card number is invalid")}

return [emptyArray, everyOther, modified, sum];

}

console.log(validateCred(valid1))
``````

Ah I think I see the issue. `modified` is the array of every other element, however you still need to add the rest of the elements that didnâ€™t need to be doubled. So what you should be getting as your final sum for this number is `80`, however itâ€™s `25` as youâ€™re only adding the ones you doubled.
So youâ€™ll need to find a way of getting the sum of the remaining elements and then adding that onto your `sum`, which should then make it valid.

1 Like

Itâ€™s always good to get visuals on whatâ€™s going on in loops. Not only does it solve problems, but it strengthens recognition of patters

1 Like

Canâ€™t figure out how to add â€śmodifiedâ€ť to â€śsumArrayâ€ť

``````const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8];
const valid2 = [5, 5, 3, 5, 7, 6, 6, 7, 6, 8, 7, 5, 1, 4, 3, 9];
const valid3 = [3, 7, 1, 6, 1, 2, 0, 1, 9, 9, 8, 5, 2, 3, 6];
const valid4 = [6, 0, 1, 1, 1, 4, 4, 3, 4, 0, 6, 8, 2, 9, 0, 5];
const valid5 = [4, 5, 3, 9, 4, 0, 4, 9, 6, 7, 8, 6, 9, 6, 6, 6];

// All invalid credit card numbers
const invalid1 = [4, 5, 3, 2, 7, 7, 8, 7, 7, 1, 0, 9, 1, 7, 9, 5];
const invalid2 = [5, 7, 9, 5, 5, 9, 3, 3, 9, 2, 1, 3, 4, 6, 4, 3];
const invalid3 = [3, 7, 5, 7, 9, 6, 0, 8, 4, 4, 5, 9, 9, 1, 4];
const invalid4 = [6, 0, 1, 1, 1, 2, 7, 9, 6, 1, 7, 7, 7, 9, 3, 5];
const invalid5 = [5, 3, 8, 2, 0, 1, 9, 7, 7, 2, 8, 8, 3, 8, 5, 4];

// Can be either valid or invalid
const mystery1 = [3, 4, 4, 8, 0, 1, 9, 6, 8, 3, 0, 5, 4, 1, 4];
const mystery2 = [5, 4, 6, 6, 1, 0, 0, 8, 6, 1, 6, 2, 0, 2, 3, 9];
const mystery3 = [6, 0, 1, 1, 3, 7, 7, 0, 2, 0, 9, 6, 2, 6, 5, 6, 2, 0, 3];
const mystery4 = [4, 9, 2, 9, 8, 7, 7, 1, 6, 9, 2, 1, 7, 0, 9, 3];
const mystery5 = [4, 9, 1, 3, 5, 4, 0, 4, 6, 3, 0, 7, 2, 5, 2, 3];

// An array of all the arrays above
const batch = [valid1, valid2, valid3, valid4, valid5, invalid1, invalid2, invalid3, invalid4, invalid5, mystery1, mystery2, mystery3, mystery4, mystery5];

function validateCred(array) {
emptyArray = [];
everyOther = [];
everyAnother = [];
sumArray = [];

for (let i=array.length-1; i >= 0; i--) {
emptyArray.push(array[i]);
}

for (let j=1; j < emptyArray.length; j += 2) {
everyOther.push(emptyArray[j]);

}
for (let x=0; x < emptyArray.length; x +=2){
everyAnother.push(emptyArray[x]);
sumArray.push(emptyArray[x]);
}
modified = everyOther.map(function multiply(s){
let multiplied = s*2;
if (multiplied > 9){
return multiplied-9
}else{
return multiplied;
}
})
console.log(typeof modified)
for (let p=0; p < modified; p++) {
sumArray.push(modified[p]);
}
let sum = sumArray.reduce((a, b) => a+b, 0);
if(sum%10 == 0) {
console.log("credit card number is valid")
}else {
console.log("credit card number is invalid")}
return [emptyArray, everyOther, everyAnother, modified, sumArray, sum];
}

console.log(validateCred(valid1))
``````