Project: an argument explorer

We have learned the main building blocks of a small Pop program. Let us combine them into a command-line tool that counts its arguments and prints each one.

Create explore.pop with the code in Listing 2-1:

explore.pop

namespace Explorer

function showArguments(arguments: Array<String>)
    local index = 1

    while Array.length(arguments) > index - 1 do
        print(Array.get(arguments, index))
        index = index + 1
    end
end

function main(arguments: Array<String>)
    local count = Array.length(arguments)

    print("Argument count")
    print(count)

    if count > 0 then
        print("Arguments")
        showArguments(arguments)
    else
        print("No arguments were supplied")
    end
end

Listing 2-1: Counting and displaying command-line arguments

Run it with a few values:

pop run explore.pop -- Pop "two words" 42

The result is:

Argument count
3
Arguments
Pop
two words
42

The final 42 is still a string because every command-line argument arrives as text. print chooses its string overload from that type.

Following the program

Execution begins in main. It asks the array for its length and stores that Int in count. The if then chooses between calling showArguments and printing the empty case.

showArguments starts at position 1, because Pop array positions are one-based. Its loop condition is another way to say that the current position is no greater than the array length: while length > index - 1, an element exists at index. After printing that element, assignment advances the position.

The length does not change during the loop because arrays are fixed-length. That makes the condition stable and ensures the loop stops after the last item.

Try it yourself

Change the program to print First argument and only the first item before it prints the complete list. Remember to do this only inside the count > 0 branch, where position 1 is known to exist.

Then add a local named remaining initialized to count - 1 and print it. These small changes practice reading a value, performing checked integer arithmetic, and keeping an array access inside the condition that makes it safe.