Call by Value

When you pass an argument to a function, the function receives a copy of the argument. After the function call, the passed argument remains unchanged, even if the called function modifies the received argument.

session03/page05/ex4.abc 
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@ <stdio.hdr>                                                                    
                                                                                 
fn foo(n: int)                                                                   
{                                                                                
    printf("foo entered: n = %d\n", n);                                          
    n = 42;                                                                      
    printf("foo leaving: n = %d\n", n);                                          
}                                                                                
                                                                                 
fn main()                                                                        
{                                                                                
    local n: int = 123;                                                          
                                                                                 
    printf("main: n = %d\n", n);                                                 
    printf("calling foo:\n");                                                    
    foo(n);                                                                      
    printf("returned from foo\n");                                               
    printf("main: n = %d\n", n);                                                 
}

Task

  • Translate the program and execute it. Go through the program line by line again to understand which output is generated in which line. Confirm the claim that a function only receives a copy (and not the original) of the argument. On my computer, I get the following output: 

    theon$ abc ex4.abc
theon$ ./a.out
main: n = 123
calling foo:
foo entered: n = 123
foo leaving: n = 42
returned from foo
main: n = 123
theon$ 

  • To clarify that the caller of a function can name the argument differently than the callee, make the following change: Rename the local variable in function main to x.