An Elementary Introduction to the Wolfram Language
31Parts of Lists
Part lets you pick out an element of a list.
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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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;; lets you ask for a span or sequence of parts.
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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The function Position finds the list of positions at which something appears.
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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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The function ReplacePart lets you replace parts of a list:
Replace part 3 with x:
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Replace two parts:
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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[[{n1,n2,...}]] list of parts n1, n2, ...
list[[n1;;n2]] span (sequence) of parts n1 through n2
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. »
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31.2Pick out letters 10 through 20 in the alphabet. »
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31.3Make a list of the letters at even-numbered positions in the alphabet. »
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31.4Make a line plot of the second to last digit in the first 100 powers of 12. »
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31.5Join lists of the first 20 squares and cubes, and get the 10 smallest elements of the combined list. »
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31.6Find the positions of the word software in the Wikipedia entry for computers»
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31.7Make a histogram of where the letter e occurs in the words in WordList[ ]»
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31.8Make a list of the first 100 cubes, with every one whose position is a square replaced by Red»
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31.9Make a list of the first 100 primes, dropping ones whose first digit is less than 5. »
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31.10Make a grid starting with Range[10], then at each of 9 steps randomly removing another element. »
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31.11Find the longest 10 words in WordList[ ]»
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31.12Find the 5 longest integer names for integers up to 100. »
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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 doesnt 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?
Tech Notes
  • First and Last are equivalent to [[1]] and [[-1]].
  • In specifying parts, 1;;-1 is equivalent to All.
 
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