When Iodine and sodium hydroxide are added to a compound that contains either a methyl ketone or a secondary alcohol with a methyl group in the alpha position, a pale yellow precipitate of iodoform or triiodomethane is formed.

Which will generate an iodoform?

Ethanol is the only primary alcohol to give the triiodomethane (iodoform) reaction.

Which will give iodoform reaction?

Ethanal is the only aldehyde to give the triiodomethane (iodoform) reaction. If "R"is a hydrocarbon group, then you have a ketone. Lots of ketones give this reaction, but those that do all have a methyl group on one side of the carbon-oxygen double bond. These are known as methyl ketones.