# Get array value index by searching value

In a project i'm making, i need to use arrays. Now i have a question:
Is there a way to find a value's index just by searching for the value?

For example:

int arrayVals = {2, 4, -8, 3, 5, 1};

Now for example when i input "-8" into a function, i need it to return the index of -8, which is 2. How would i go about doing that?

Oh and how can i get the length of an array?

To do inverse lookup you need to traverse the array

``````const int arysz = 6;
int ary[arysz] = {4, 32, 6, 2, -1, 5}
int wantedval = -1;
int wantedpos;
for (int i=0; i<arysz; i++) {
if (wantedval = ary[i]) {
wantedpos = i;
break;
}
}
``````

If they’re in order, you can also do a binary search (google it)

For finding array length, the best method is to use the above method and store a separate variable for the length. However, this works, too

``````int ary;
int len = sizeof(ary)/sizeof(ary)
``````

WizenedEE:
To do inverse lookup you need to traverse the array
-code-

Thank you. Now what happens when i search for a value that isn't in the array?

Then wantedval = -1 and you know you didn't find it.
Note make the initial value of wantedval outside the range of numbers you are expecting in the array.

Will this code work if i expand the array during execution of the code?

If your array is ordered, you can shorten search times by doing a binary search.

AWOL:
If your array is ordered, you can shorten search times by doing a binary search.

It's not ordered.

Will this code work if i expand the array during execution of the code?

You can't do that, the array size is fixed by:-

``````int ary[arysz] = {4, 32, 6, 2, -1, 5}
``````

You can't expand this once it has been set.

Useful bit code from Wizend, just what I was looking for.
Works better with

``````if (wantedval == ary[i]) {
``````

, not

``````if (wantedval = ary[i]) {
``````

Had me going for a bit.

You must also have a flag that indicates the value is actually found in the array.

A common way is to initialize the position with -1, as this is (In C++) not a valid array index.

int wantedpos = -1;
... search...

if (wantedPos != -1) // it really is found in the array.

Another problem can be that value occur more than once in an array.
In that case you need to define if you want to find the 1st 2nd .... last instance.

Thankyou WizenedEE , I have used your script above and run very well…I just modified symbol of “=” be “==”
I’ve spent a lot of time to find this script that I use to get index numbering…

const int arysz = 6;
int ary[arysz] = {4, 32, 6, 2, -1, 5}
int wantedval = -1;
int wantedpos;
for (int i=0; i<arysz; i++) {
if (wantedval == ary*) { //modified here*

• wantedpos = i;*
• break;*
• }*
}
Nice job sir…

if (wantedval == ary)

missing the index with ary

Not exactly. The post was made without code tags and turned into italics at that point. The array index was interpreted as a command to use italics, instead of being displayed. This is one of the reasons why code tags MUST be used!

Any way you can get them all to read that?