for (key in array)
    string = (string ==  "" ? array[key] : string "," array[key])

print string

string = array[1]
for (i = 2; i <= length(array); ++i)
    string = string "," array[i];

print string

OFS="," # (would probably do this in BEGIN)

n = 0
for (key in array)
    $(++n) = array[key]

print string

OFS = "," # (would probably do this in BEGIN)

n = 0
for (key in array)
    $(++n) = array[key]

NF = n
print string

OFS = "," # (would probably do this in BEGIN)

$0 = ""
n = 0
for (key in array)
    $(++n) = array[key]

print string