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FirstPosition

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FirstPosition[expr, pattern]

gives the position of the first element in expr that matches pattern, or Missing[“NotFound”] if no such element is found.

FirstPosition[expr, pattern, default]

gives default if no element matching pattern is found.

FirstPosition[expr, pattern, default, levelspec]

finds only objects that appear on levels specified by levelspec.

• • FirstPosition[{a, b, a, a, b, c, b}, b]

• • FirstPosition[{{a, a, b}, {b, a, a}, {a, b, a}}, b]

• • FirstPosition[{x, y, z}, b]

Find the first position at which x^2 to appears:

• • FirstPosition[{1 + x^2, 5, x^4, a + (1 + x^2)^2}, x^2]