SelectFirst
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SelectFirst
gives the first ei of data for which crit[ei] is True, or Missing["NotFound"] if none is found.
gives default if there is no ei of data such that crit[ei] is True.
represents an operator form of SelectFirst that can be applied to an expression.
Details

- SelectFirst keeps the first element for which the given criterion crit is True:
- SelectFirst[data,crit] is equivalent to First[Select[data,crit]].
- The data can have the following additional forms and interpretations:
-
{e1,e2,…} list of values » f[e1,e2,…] any head f » Association association of values » QuantityArray[…] quantity array or other structured array Tabular[…] type-consistent tabular data » TabularColumn[…] type-consistent column data » Dataset[…] general hierarchical data - The property prop can have the following forms and interpretations:
-
"Element" the selected values » "Index" indices of the selected values » "BitVectorMask" Boolean mask returning True for slected values and False otherwise » {prop1,prop2,…} a list of multiple forms » All an association giving element, index and bit vector mask » - In SelectFirst[data,crit,default], default is only evaluated if there is no ei in data such that crit[ei] is True.
- SelectFirst[crit][data] is equivalent to SelectFirst[data,crit].
Examples
open allclose allBasic Examples (6)Summary of the most common use cases
Select the first element that is even:

Return the index of the selected element:

Give the default x when no even numbers are found:

Use the operator form of SelectFirst:

SelectFirst operates on values in an Association:

Use SelectFirst with a Tabular object:



Scope (15)Survey of the scope of standard use cases
Basic Uses (5)
Find the first pair containing x:

Use a pure function to test elements:

Use an operator form as selection criterion:

Use SelectFirst in operator form:


Input Data (5)
SelectFirst works with any head, not just List:

SelectFirst works on values in Association:

SelectFirst works with vector SparseArray objects:


SelectFirst works with TabularColumn objects:

Select the first date in November:

Use SelectFirst on a Tabular object with named columns:

Select the first row with last column date being a business day:


Applications (7)Sample problems that can be solved with this function
Select the first number between 9 and 99 that equals 1 modulo both 3 and 5:

Select the first 4-tuple that reads the same in reverse:

Find the first 3×3 matrix of 0s and 1s that has determinant 1:

Select an eigenvalue that lies within the unit circle:

Find the first built-in Wolfram Language object whose name is at least 36 characters long:

Select the first numeric quantity from a product:

Find the position of the first prime in a random sample from a PoissonDistribution:

Find the distribution of positions of first primes:


Properties & Relations (4)Properties of the function, and connections to other functions
The default value is only evaluated if needed:


SelectFirst returns only the first match:

Select returns all matches:

SelectFirst can be implemented with a combination of Select and Replace:




SelectFirst is similar to FirstCase except that it uses a function instead of a pattern:
Select the first list that has a sum of elements less than 10:

Use FirstCase to get the same result:

Wolfram Research (2014), SelectFirst, Wolfram Language function, https://reference.wolfram.com/language/ref/SelectFirst.html (updated 2025).
Text
Wolfram Research (2014), SelectFirst, Wolfram Language function, https://reference.wolfram.com/language/ref/SelectFirst.html (updated 2025).
Wolfram Research (2014), SelectFirst, Wolfram Language function, https://reference.wolfram.com/language/ref/SelectFirst.html (updated 2025).
CMS
Wolfram Language. 2014. "SelectFirst." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2025. https://reference.wolfram.com/language/ref/SelectFirst.html.
Wolfram Language. 2014. "SelectFirst." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2025. https://reference.wolfram.com/language/ref/SelectFirst.html.
APA
Wolfram Language. (2014). SelectFirst. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/SelectFirst.html
Wolfram Language. (2014). SelectFirst. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/SelectFirst.html
BibTeX
@misc{reference.wolfram_2025_selectfirst, author="Wolfram Research", title="{SelectFirst}", year="2025", howpublished="\url{https://reference.wolfram.com/language/ref/SelectFirst.html}", note=[Accessed: 29-May-2025
]}
BibLaTeX
@online{reference.wolfram_2025_selectfirst, organization={Wolfram Research}, title={SelectFirst}, year={2025}, url={https://reference.wolfram.com/language/ref/SelectFirst.html}, note=[Accessed: 29-May-2025
]}