The child should have bought 3 bottles of milk.

I always think this joke is more of a linguistics/grammar joke than programming. The kid resolves the ambiguity in the ellipsis incorrectly, but why is this a programmer joke?

For a serious reoly, I think the expression "If they have eggs, buy two" is redundant. If they didn't have eggs, the kid just can't and won't buy any eggs.

I think the proper command would be, "Please buy 1 bottle of milk and two eggs." That way, the kid won't be confused and it's still a proper valid command.

Unfortunately though, the sentence is ambiguous even to non-programmers. It is unknown whether the if condition applies to

• buying two eggs (buy two eggs)
• buying two bottles of milk (buy two bottles instead)
• or buying a bottle of milk (buy another bottle)

Simply because they didn't specify which to buy.

For a non-serious reply,

cart.add(supermarket.takeProduct(ProductType.milk, 1));

if (supermarket.getProduct(type: ProductType.eggs).length > 0) {
    cart.add(supermarket.takeProduct(Product type.milk, 2));
}

cart.checkout();

The kid should have bought a total of 3 bottles.