SelectFirst

SelectFirst[data,crit]

gives the first ei of data for which crit[ei] is True, or Missing["NotFound"] if none is found.

SelectFirst[data,critprop]

returns the property prop of the selected elements.

SelectFirst[data,crit,default]

gives default if there is no ei of data such that crit[ei] is True.

SelectFirst[crit]

represents an operator form of SelectFirst that can be applied to an expression.

更多信息

  • 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 »
    Associationassociation 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 »
    Allan 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].

范例

打开所有单元关闭所有单元

基本范例  (6)

选择是偶数的第一个元素：

Return the index of the selected element:

给出默认的 x，当找不到偶数时：

Use the operator form of SelectFirst:

SelectFirst 作用于 Association 中的数值上：

Use SelectFirst with a Tabular object:

范围  (15)

Basic Uses  (5)

找到包含 x 的第一对:

Use a pure function to test elements:

Use an operator form as selection criterion:

Use SelectFirst in operator form:

使用自定义的默认值：

Input Data  (5)

SelectFirst 作用于任意头部，不只是 List:

SelectFirst works on values in Association:

SelectFirst 作用于 SparseArray 对象：

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:

Property Forms (5)

Return the selected element:

Return the index of the selected element:

Return the Boolean mask of the data:

Return two forms:

Return the association with all the properties:

应用  (7)

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:

属性和关系  (4)

如果需要，只计算默认值：

SelectFirst 只返回第一个匹配：

Select 返回所有匹配：

SelectFirst 可以使用 Select 和 Replace 的组合实现：

SelectFirst 与 FirstCase 相似，除非它使用函数而不是模式：

选择元素和少于10的第一个列表：

使用 FirstCase 获取相同结果：

可能存在的问题  (1)

The head of the input expression is not preserved:

Wolfram Research (2014)，SelectFirst，Wolfram 语言函数，https://reference.wolfram.com/language/ref/SelectFirst.html （更新于 2025 年）.

文本

Wolfram Research (2014)，SelectFirst，Wolfram 语言函数，https://reference.wolfram.com/language/ref/SelectFirst.html （更新于 2025 年）.

CMS

Wolfram 语言. 2014. "SelectFirst." Wolfram 语言与系统参考资料中心. Wolfram Research. 最新版本 2025. https://reference.wolfram.com/language/ref/SelectFirst.html.

APA

Wolfram 语言. (2014). SelectFirst. Wolfram 语言与系统参考资料中心. 追溯自 https://reference.wolfram.com/language/ref/SelectFirst.html 年

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: 30-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: 30-May-2025 ]}