argument passing

[paqueress]

What is meant by 'an argumentis passed by value'

[r035198x]

What is meant by 'an argumentis passed by value'

Only a copy of the object is passed not the object itself

www.cs.rutgers.edu/~borgida/314/4param.pdf.gz

[horace1]

What is meant by 'an argumentis passed by value'

C passes parameters into a function using pass by value:

1 copies of the actual parameters are made in temporary variables and these are passed;
2 the called function can access the copies of the values of the actual parameters via the names of the corresponding formal parameters.

e.g. consider a function to raise a number to an exponent

float power(float number, int exponent)                    /* function header */
{                                                                               
    float result;                              /* local variable holds result */
 
    for (result = 1; exponent >= 1; exponent--)    /* loop exponent down to 1 */
        result = result * number;                   /* number to the exponent */
    return result;                                  /* return function result */
}
 

if it is called

     float x=4.0;
     int exp = 3;
     y = power(x, exp);                             /* call function */
 

copies of x and exp are made and passesed into the function when the values are used in the calculation. Changes made (e.g. to exponent in the for() loop) only effect the copies.

[r035198x]

C passes parameters into a function using pass by value:

1 copies of the actual parameters are made in temporary variables and these are passed;
2 the called function can access the copies of the values of the actual parameters via the names of the corresponding formal parameters.

e.g. consider a function to raise a number to an exponent

float power(float number, int exponent) /* function header */
{ 
float result; /* local variable holds result */
 
for (result = 1; exponent >= 1; exponent--) /* loop exponent down to 1 */
result = result * number; /* number to the exponent */
return result; /* return function result */
}
 

if it is called

float x=4.0;
int exp = 3;
y = power(x, exp); /* call function */
 

copies of x and exp are made and passesed into the function when the values are used in the calculation. Changes made (e.g. to exponent in the for() loop) only effect the copies.

Are you trying to say that you cannot pass by reference in C?

[horace1]

Are you trying to say that you cannot pass by reference in C?

To pass an object into a function by reference the address of the object is passed as the actual parameter (using the address operator &) and the corresponding formal parameter is declared as a pointer. Inside the function the name of the formal parameter is prefixed with the indirection operator * which will then access the memory allocated to the original object.

e.g. to swap the values of two ints

void swap_int(int *p_int1, int *p_int2)                                 
{                                                                               
    int temporary = *p_int1;                     /* copy first variable into temporary */  
    *p_int1 = *p_int2;                          /* copy second variable into first */  
    *p_int2 = temporary;                     /* copy temporary variable into first */  
}                                                                               
 

called so

    int x=2, y=6;
    swap_int(&x, &y);
 

In C++ we can also use reference parameters

[r035198x]

To pass an object into a function by reference the address of the object is passed as the actual parameter (using the address operator &) and the corresponding formal parameter is declared as a pointer. Inside the function the name of the formal parameter is prefixed with the indirection operator * which will then access the memory allocated to the original object.

e.g. to swap the values of two ints

void swap_int(int *p_int1, int *p_int2) 
{ 
int temporary = *p_int1; /* copy first variable into temporary */ 
*p_int1 = *p_int2; /* copy second variable into first */ 
*p_int2 = temporary; /* copy temporary variable into first */ 
} 
 

called so

int x=2, y=6;
swap_int(&x, &y);
 

In C++ we can also use reference parameters

Your first statement

C passes parameters into a function using pass by value:

Seemed misleading

[horace1]

Your first statement

C passes parameters into a function using pass by value:

Seemed misleading

In C the actual parameters passed into the function when it is called are 'passed by value', consider

    int x=2, y=6;
    swap_int(&x, &y);
 

the parameters &x and &y are 'passed by value', i.e.
1. temporary variables are created which contain addresses of x and y
2. inside swap() these are used to access the memory locations allocated to x and y to swap the values

hence we effectively achieve 'pass by reference' by using the values of the addresses which were 'passed by value'

consider

void swap_int(int *p_int1, int *p_int2)                                 
{                                                                               
    int *temporary = p_int1;                      
    p_int1=p_int2;
    p_int2 = temporary;                      
    printf("swaped x %d y %d ???\n", *p_int1, *p_int2);
}      
 

we swap the pointers which appear to have swaped the values of x and y but this has no efecct on x and y in the main()

[r035198x]

In C the actual parameters passed into the function when it is called are 'passed by value', consider

int x=2, y=6;
swap_int(&x, &y);
 

the parameters &x and &y are 'passed by value', i.e.
1. temporary variables are created which contain addresses of x and y
2. inside swap() these are used to access the memory locations allocated to x and y to swap the values

hence we effectively achieve 'pass by reference' by using the values of the addresses which were 'passed by value'

consider

void swap_int(int *p_int1, int *p_int2) 
{ 
int *temporary = p_int1; 
p_int1=p_int2;
p_int2 = temporary; 
printf("swaped x %d y %d ???\n", *p_int1, *p_int2);
} 
 

we swap the pointers which appear to have swaped the values of x and y but this has no efecct on x and y in the main()

I suppose you've made your point