Cap requires names; panics with only default-name

[zacharysyoung]

The bigger logic of Cap (implicitly) allows for the user to leave the --names flag empty and specify only --default-name:

If names happens to be an empty slice, then numNames is 0. The first half of the predicate is true, but if defaultName is not empty then Cap doesn't error-out:

gocsv/cmd/cap.go

Lines 49 to 52 in f9d4372

	if numColumns > numNames && defaultName == "" {
	fmt.Fprintf(os.Stderr, "Must specify --default-name if there are more columns than column names provided")
	os.Exit(1)
	}

Then... with nunNames being 0, the code flows through the else statement and builds the header exclusively off of defaultName:

gocsv/cmd/cap.go

Lines 60 to 71 in f9d4372

	for i := range firstRow {
	if i < numNames {
	newHeader[i] = names[i]
	} else {
	if j == 0 {
	newHeader[i] = defaultName
	} else {
	newHeader[i] = fmt.Sprintf("%s %d", defaultName, j)
	}
	j++
	}
	}

And I would like this behavior: if I have a headerless CSV with any number of columns that I want to feed into another GoCSV command I want to easily cap it with --default-name=Col.

But, prior to that, Cap tries to create the slice of strings, names, based on the assumption that the --names flag is not an empty string:

gocsv/cmd/cap.go

Lines 37 to 40 in f9d4372

	func (sub *CapSubcommand) RunCap(inputCsv *InputCsv, outputCsvWriter OutputCsvWriter) {
	names := GetArrayFromCsvString(sub.namesString)
	Cap(inputCsv, outputCsvWriter, names, sub.truncateNames, sub.defaultName)
	}

The call to GetArrayFromCsvString(...) will panic if the passed string is empty.

I propose:

1. marking the --names flag optional in documentation
2. adding a check in RunCap to make sure at least one the --names or --default-name flags were set on the command line
3. guarding against passing an empty string to GetArrayFromCsvString() in RunCap