# Get index of first true value of array in Julia | Array findfirst() Method

Last Updated : 23 Mar, 2020

The `findfirst()` is an inbuilt function in julia which is used to return the index or key of the first true value in the specified array. Here values of index or key start from 1 i.e, for index of 1st element is 1, index of 2nd element is 2 and so on.

Syntax:
findfirst(A)
or
findfirst(predicate::Function, A)

Parameters:

• A: Specified array
• Predicate Function: Determines whether something is true or false based on the specified arguments

Returns: It returns the index or key of the first true value in the specified array.

Example 1:

 `# Julia program to illustrate  ` `# the use of Array findfirst() method ` `  `  `# Finding index of first true value from  ` `# the 1D array A ` `A ``=` `[false, true, true, false] ` `println(findfirst(A)) ` `  `  `# Finding index of first true value from  ` `# the 2D array B of size 2 * 2 ` `B ``=` `[false false; true false] ` `println(findfirst(B)) ` `  `  `# Finding index of first true value from  ` `# the 3D array C of size 2 * 2*2 ` `C ``=` `cat([false false; true false],  ` `        ``[false true; true false], ` `        ``[true false; true true], dims ``=` `3``) ` `println(findfirst(C)) `

Output:

Example 2:

 `# Julia program to illustrate  ` `# the use of Array findfirst() method ` `  `  `# Finding index of first even value from  ` `# the 1D array A ` `A ``=` `[``1``, ``2``, ``5``, ``6``] ` `println(findfirst(iseven, A)) ` `  `  `# Finding index of first even value from  ` `# the 2D array B of size 2 * 2 ` `B ``=` `[``3` `5``; ``6` `7``] ` `println(findfirst(iseven, B)) ` `  `  `# Finding index of first even value from  ` `# the 3D array C of size 2 * 2*2 ` `C ``=` `cat([``1` `2``; ``3` `4``], [``5` `6``; ``7` `8``],  ` `        ``[``9` `10``; ``11` `12``], dims ``=` `3``) ` `println(findfirst(iseven, C)) `

Output:

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