Immutable collections

As we have discussed, immutable data structures cannot be changed. So when you run a function over a collection a copy of that collection is returned. Lets see this by running some code in the REPL.

Note Define a data structure called numbers using a vector. Then write a function that uses the map and inc function to increment all the numbers in a vector.

Then check the current value of the numbers data structure by evaluating its name. 

  1. ;; define the data structure 
  2. (defn numbers [1 2 3 4 5])
  4. ;; increment the numbers
  5. (map inc numbers)
  7. ;; see the current value of numbers
  8. numbers

Note Use the conj function to first add the number 5 to the numbers vector from the previous exercise and check the value of numbers. Then add the number 6 to the numbers vector and check the value of numbers.

Finally, use the conj function to add both 5 and 6 to the numbers vector and check the value of numbers

  1. (def numbers [1 2 3 4])
  3. ;; add 5 to the numbers vector
  4. (conj numbers 5)
  6. ;; check the value of numbers
  7. numbers
  8. ;; => [1 2 3 4]
  10. ;; add 6 to the numbers vector
  11. (conj numbers 6)
  13. ;; check the value of numbers
  14. numbers
  15. ;; => [1 2 3 4]
  17. ;; add 5 and 6 to the numbers vector
  18. (conj numbers 5 6)
  20. ;; Alternatively, you can use the threading macro to chain two conj function calls
  21. (-> numbers
  22.     (conj 5)
  23.     (conj 6))
  25. ;; check the value of numbers
  26. numbers
  27. ;; => [1 2 3 4]

So even though we have applied several functions on the numbers data structure it still has the same value.