Manipulating Lists |
There are a number of ways to manipulate lists in Python. The following table describes them:
| Function or Operator | Description |
|---|---|
| + | Join two lists together |
| append(value) | Add value to the end of a list |
| del list[index] | Delete the value at index index from list |
| remove(value) | Remove value from a list |
| reverse() | Reverse the order of a list |
| sort() | Sort a list in numeric or alphabetical order |
| len(list) | Return the number of values in list |
| index(value) | Return the index of value in a list |
| pop(index) | Remove the value at index index from a list and return it |
You can use + to join two lists together:
idlist = [1000, 2000] + [3000, 4000] # idlist now contains [1000, 2000, 3000, 4000] |
You can use append() to add a value to the end of a list:
idlist = [1000, 2000, 3000] idlist.append(4000) # idlist is now [1000, 2000, 3000, 4000] |
To delete a value from a list, use del, which expects the index of the list value it is deleting:
idlist = [1000, 2000, 3000, 4000] del idlist[1] # idlist is now [1000, 3000, 4000] - recall that index 1 refers to the # second element of the list, not the first |
If you know what element you want to delete, but do not know where in the list it is, use remove():
idlist = [1000, 2000, 3000, 4000] idlist.remove(2000); # idlist is now [1000, 3000, 4000] |
To reverse the order of a list, use reverse():
idlist = [1000, 2000, 3000, 4000] idlist.reverse() # idlist is now [4000, 3000, 2000, 1000] |
sort() sorts a list of numbers in numeric order, or a list of strings in alphabetical order:
idlist = [3000, 1000, 4000, 2000] idlist.sort() # idlist is now [1000, 2000, 3000, 4000] namelist = ['Doe,John', 'Smith,Mary', 'Jones,Charlie', 'Public,John'] namelist.sort() # namelist is now ['Doe,John', 'Jones,Charlie', 'Public,John', 'Smith,Mary'] |
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| sort() does not do anything if the list is a mixture of numbers and strings. | ||
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len() returns the number of values in a list:
idlist = [1000, 2000, 3000, 4000] idcount = len(idlist) # idcount is now 4 |
index() locates an item in a list, and returns its index:
patientinfo = ['JohnDoe', 42446, 'Admitted', 1000]
statusindex = patientinfo.index('Admitted')
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In this example, index() finds the first occurrence of the string Admitted in the list. Since this is the third element of the list, statusindex is assigned 2. (Recall that indexing always starts from zero.)
pop() removes an item from a list, specified by index, and allows you to assign it to another variable:
idlist = [1000, 2000, 3000, 4000] patientid = idlist.pop(1) # idlist is now [1000, 3000, 4000], and patientid now contains 2000 |
If you do not specify an index, pop() removes the last element of the list and assigns it:
idlist = [1000, 2000, 3000, 4000] patientid = idlist.pop() # idlist is now [1000, 2000, 3000], and patientid now contains 4000 |