31 | Parts of Lists |

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[[...]] is an alternative notation.

Use [[2]] to pick out element 2 from a list:

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You can ask for a list of parts by giving a list of part numbers.

Pick out parts 2, 4 and 5:

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Pick out parts 2 through 5:

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Take the last 2 elements from a list:

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Drop the last 2 elements from a list:

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Pick out the second sublist, corresponding to the second row, in the array:

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This picks out element 1 on row 2:

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Pick out the first column, by getting element 1 on all rows:

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This gives a list of all positions at which x appears:

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The positions at which “a” occurs in a list of characters:

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Replace part 3 with x:

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Replace two parts:

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Sometimes one wants particular parts of a list to just disappear. One can do this by replacing them with Nothing.

Nothing just disappears:

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Replace parts 1 and 3 with Nothing:

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Take 50 random words, dropping ones longer than 5 characters, and reversing others:

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Take takes a specified number of elements in a list based on their position. TakeLargest and TakeSmallest take elements based on their size.

Take the 5 largest elements from a list:

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TakeLargestBy and TakeSmallestBy take elements based on applying a function.

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Part[list,n] | part n of a list | |

list[[n]] | short notation for part n of a list | |

list[[{n_{1},n_{2},...}]] | list of parts n_{1}, n_{2}, ... | |

list[[n_{1};;n_{2}]] | span (sequence) of parts n_{1} through n_{2} | |

list[[m,n]] | element from row m, column n of an array | |

list[[All,n]] | all elements in column n | |

Take[list,n] | take the first n elements of a list | |

TakeLargest[list,n] | take the largest n elements of a list | |

TakeSmallest[list,n] | take the smallest n elements of a list | |

TakeLargestBy[list,f,n] | take elements largest by applying f | |

TakeSmallestBy[list,f,n] | take elements smallest by applying f | |

Position[list,x] | all positions of x in list | |

ReplacePart[list,nx] | replace part n of list with x | |

Nothing | a list element that is automatically removed |

31.1Find the last 5 digits in 2^1000. »

31.2Pick out letters 10 through 20 in the alphabet. »

31.3Make a list of the letters at even-numbered positions in the alphabet. »

31.4Make a line plot of the second to last digit in the first 100 powers of 12. »

31.5Join lists of the first 20 squares and cubes, and get the 10 smallest elements of the combined list. »

31.8Make a list of the first 100 cubes, with every one whose position is a square replaced by Red. »

31.9Make a list of the first 100 primes, dropping ones whose first digit is less than 5. »

31.10Make a grid starting with Range[10], then at each of 9 steps randomly removing another element. »

31.12Find the 5 longest integer names for integers up to 100. »

31.13Find the 5 English names for integers up to 100 that have the largest number of “e”s in them. »

Usually “

*list*part*n*” or “*list*sub*n*”. The second form (with “sub” short for “subscript”) comes from thinking about math and extracting components of vectors.What happens if one asks for a part of a list that doesn’t exist?

One gets a message, and the original computation is returned undone.

Can I just get the first position at which something appears in a list?

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