Cairo’s Function Exercise II

Use the information given in the last section, in order to write a piece of code that when executed puts the current values of ap, fp and pc in memory (say, write ap into [ap], fp into [ap + 1] and pc into [ap + 2]).

Solution

A very simple program useful just to prove a point.

func main():
    [ap] = ap; ap++
    [ap] = fp; ap++
    [ap] = pc; ap++
    ret
end

Trying to execute the program we get the following error:

$ cairo-compile exercise.cairo --output exercise.json
>>>
exercise.cairo:4:12: Unknown identifier 'pc'.
    [ap] = pc; ap++
           ^^

The pc keyword is not recognized by the compiler. Let’s remove that line and compile again.

func main():
    [ap] = ap; ap++
    [ap] = fp; ap++
    ret
end

$ cairo-compile exercise.cairo --output exercise.json
>>>
exercise.cairo:2:12: Invalid RHS expression.
    [ap] = ap; ap++
           ^^
Preprocessed instruction:
[ap] = ap; ap++

Another error, we can’t access the ap pointer directly. Let’s remove that line from the code and just try to access the fp pointer instead.

func main():
    [ap] = fp; ap++
    ret
end

$ cairo-compile exercise.cairo --output exercise.json
>>>
exercise.cairo:2:12: Invalid RHS expression.
    [ap] = fp; ap++
           ^^
Preprocessed instruction:
[ap] = fp; ap++

Failure again. This exercise just helps to prove that we can’t directly access any of the underlying pointers and that we need to use a library instead.

Reference

The exercise in this article can be found in this section of the Cairo docs.