Giving names to things

In the previous blog post, we have found a somewhat tedious way to compute the Fibonacci sequence. We started by typing 1+1 at the console, we looked at the result, which was 2, we typed 2+1, we got 3, we typed 3+2 and so on and so forth. In this and the following post, we'll discover a less excruciatingly boring way of doing the same computation. In the process, we'll learn two key building blocks of programming: assigning values to variables and looping.

Variable assignment

In Julia (and in any reasonable programming language), you can assign a value to a variable. It works as follows:

• First you decide a name for your variable, for example fib (almost any sequence of characters will do, it's good to choose something that's easy to remember and that clarifies the purpose of that variable)
• Choose a value to assign (for example 1)
• Execute the following code:

julia> fib = 1

and you're all set! Now we can inquire Julia about the value of fib:

julia> fib
1

Once you've assigned a variable, you can reassing it to something else, for example:

julia> fib = 2

And now:

julia> fib
2

Also, you can assign several variables simultaneously, with the following syntax:

julia> fib, next_fib = 1, 2

And now fib is 1 and next_fib is 2.

Back to Fibonacci

fib and next_fib will be (as the names suggest) two consecutive Fibonacci numbers. How can we go from one pair of consecutive Fibonacci numbers to the next? For example, starting with (1, 2), we want to move to (2, 3), then (3, 5), (5, 8) and so on. It's actually quite simple, fib has to be transformed into next_fib and next_fib has to be transformed into fib+next_fib or, in Julia code:

julia> fib, next_fib = next_fib, fib+next_fib

We have a new way of computing the Fibonacci sequence:

julia> fib, next_fib = 1, 2
(1, 2)

julia> fib, next_fib = next_fib, fib+next_fib
(2, 3)

julia> fib, next_fib = next_fib, fib+next_fib
(3, 5)

julia> fib, next_fib = next_fib, fib+next_fib
(5, 8)

julia> fib, next_fib = next_fib, fib+next_fib
(8, 13)

julia> fib, next_fib = next_fib, fib+next_fib
(13, 21)

julia> fib, next_fib = next_fib, fib+next_fib
(21, 34)

julia> fib, next_fib = next_fib, fib+next_fib
(34, 55)

julia> fib, next_fib = next_fib, fib+next_fib
(55, 89)

julia> fib, next_fib = next_fib, fib+next_fib
(89, 144)

Exercises

1. What is the largest Fibonacci smaller than 10000?
2. (1346269, 2178309) is a Fibonacci pair. What is the previous Fibonacci pair? Can you find a smart way to go backward in the Fibonacci sequence?