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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:



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# 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))

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Output:

Example 2:

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# 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))

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Output:




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