Question

I’ve never had much need for writing large quantities of formal pseudo-code but the need has arisen, so I thought I’d pick some standards in order to stay consistent across code.

To that effect I picked up some “iTunes U” courseware videos, amongst other things the 6.046J / 18.410J Introduction to Algorithms (SMA 5503).

In the very first lecture video, the lecturer writes Insertion Sort on the blackboard, and he writes this:

Insertion-Sort(A, N) // Sorts A[1..n]
  for j ← 2 to n
    do key ← A[j]
      i ← j-1
      while i > 0 and A[i] > key
        do A[i+1] ← A[i]
          i ← i-1
      A[i+1] ← key

So, my questions:

• Why i ← j-1 when A[i+1] = key? That is, why ← in some cases, and = in another? Note that in the above code, the ← is used for the latter too, but in the handouts, available on the web, = is used, is this simply a typo? (I assume so)
• More important, why do key ← A[j] when i ← j-1? What is so special that it requires a do command like that, and an indentation?

In other words, why isn’t the above pseudo-code written like this (with my highlights):

Insertion-Sort(A, N) // Sorts A[1..n]
  for j ← 2 to n
    key ← A[j]                  <-- lost the do here
    i ← j-1                     <-- no indentation
    while i > 0 and A[i] > key
      A[i+1] ← A[i]             <-- lost the do here
      i ← i-1                   <-- no indentation
    A[i+1] ← key

Final question: Does anyone have a code standard for pseudo-code handy somewhere? My main goal is consistency, so that I only have to “teach” the recipients once.

Answer 1

Structured English is a ‘standardised’ pseudo-code language.