Make a[x] array access set the element if not present (per POSIX spec)

This seems very unintuitive to me, but it's very clear in the spec, and
awk/gawk/mawk all do this.

Also fix delarpm2 gawk test as (again per POSIX) "The results of adding
new elements to array within such a for loop are undefined."

Author: benhoyt

TODO.txt

@@ -1,4 +1,2 @@
-* "exit" tests, in BEGIN, end, etc (see gawk exitval tests)
-* fix new TODO tests in interp_test.go
 * should interp have a configurable "safe" mode that doesn't run
   system(), redirection, etc?

goawk.go

@@ -44,7 +44,7 @@ import (
 )
 
 const (
-	version = "v1.2.0"
+	version = "v1.2.1"
 )
 
 func main() {

interp/interp.go

@@ -1037,7 +1037,15 @@ func (p *interp) getArrayIndex(scope VarScope, index int) int {
 // Get a value from given array by key (index)
 func (p *interp) getArrayValue(scope VarScope, arrayIndex int, index string) value {
 	resolved := p.getArrayIndex(scope, arrayIndex)
-	return p.arrays[resolved][index]
+	array := p.arrays[resolved]
+	v, ok := array[index]
+	if !ok {
+		// Strangely, per the POSIX spec, "Any other reference to a
+		// nonexistent array element [apart from "in" expressions]
+		// shall automatically create it."
+		array[index] = v
+	}
+	return v
 }
 
 // Set a value in given array by key (index)

interp/interp_test.go

@@ -135,6 +135,7 @@ BEGIN {
 		{`BEGIN { a["x"] = 3; print "x" in a, "y" in a }`, "", "1 0\n", "", ""},
 		{`BEGIN { a["x"] = 3; a["y"] = 4; delete a["x"]; for (k in a) print k, a[k] }`, "", "y 4\n", "", ""},
 		{`BEGIN { a["x"] = 3; a["y"] = 4; for (k in a) delete a[k]; for (k in a) print k, a[k] }`, "", "", "", ""},
+		{`BEGIN { a["x"]; "y" in a; for (k in a) print k, a[k] }`, "", "x \n", "", ""},
 		{`BEGIN { a[] }`, "", "", "parse error at 1:11: expected expression instead of ]", "syntax error"},
 		{`BEGIN { delete a[] }`, "", "", "parse error at 1:18: expected expression instead of ]", "syntax error"},
 

interp/io.go

@@ -347,10 +347,13 @@ func (p *interp) nextLine() (string, error) {
 					// Done with ARGV args, all done with input
 					return "", io.EOF
 				}
-				// Fetch next filename from ARGV
+				// Fetch next filename from ARGV. Can't use
+				// getArrayValue() here as it would set the value if
+				// not present
 				index := strconv.Itoa(p.filenameIndex)
 				argvIndex := p.program.Arrays["ARGV"]
-				filename := p.toString(p.getArrayValue(ScopeGlobal, argvIndex, index))
+				argvArray := p.arrays[p.getArrayIndex(ScopeGlobal, argvIndex)]
+				filename := p.toString(argvArray[index])
 				p.filenameIndex++
 
 				// Is it actually a var=value assignment?

testdata/gawk/delarpm2.awk

@@ -50,7 +50,7 @@
 	    clear_array(table)
 	    foo(table)
 	    for (key in table)
-		print key, table[k]
+		print key, table[key]
 	    clear_array(table)
 	    exit(0)
 	}

testdata/gawk/delarpm2.ok

@@ -1,2 +1,2 @@
-1 
-2 
+1 one
+2 two