Pseudocode – Meaning of ‘i, j <-- 1' in Pseudocode

pseudocode

I'm trying to understand what this pseudocode does and I can't seem to understand what the comma is suppose to do here.

I know that j <- 1 is j = 1 but what about the i? And what does j <- j do in this case? 

 fib iter(n) 
    i,j ← 1 
      for k ← 1 to n−1 
      i,j ← j,i + j 
    return j

Best Answer

Assuming this is the algorithm for a Fibonacci number generator, it represents simultaneous/parallel assignment; the comma is not separating two statements, but two operands to the <- operator.

In i, j <- 1, both i and j are set to 1; it's just shorthand for i <- 1 and j <- 1, which could happen in any order.

In i,j ← j,i + j, it means i <- j and simultaneously j <- i+j. It's necessary for both to happen at once, because you need to use the original values of both variables.

In a language without that facility, you'd have to introduce a temporary variable: 

i2 <- i
i <- j
j <- i2 + j

or

j2 <- j
j <- i + j
i <- j2

This may be based on a real language with this syntax, or it may just be a convenience for the algorithms being discussed. For instance, sort algorithms will often be written in a pseudocode with a "swap" operator of some sort, rather than the full set of instructions needed in a particular language, in order to focus on the most relevant details.

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