Lesson 9.5. Arrays and functions

Passing arrays as arguments

As for variables, it is possible to pass an array as an argument to a function. The syntax consists in adding empty square brackets in the arguments of the function to specify that it is an array and not a variable:

void init(int tab[], int size) {
  int i=0;
  for (i=0; i<size; i++) tab[i]=0;
}

However, passing arrays as parameters to functions has some differences with variables. The function does not receive a copy of the array (as for variables). It receives the address of the first cell of the array. This has two major consequences:

When an array is passed as a parameter to a function, its content can be modified by the function code. In the example above, we can see that the function returns nothing (void init()). It will directly modify the content of the array passed to it as a parameter.
The size of the array is not known to the function. It is therefore necessary to inform the function about the size of the array. In the example above, a second parameter size is passed to the function. size informs the function about the number of cells in the array.

Keyword const

The keyword const placed before an argument tells the compiler and the user of the of the function that this argument will not be modified by the function. For example, the following function displays an array without modifying its content:

void display(const int tab[]);

Specifying that an array cannot be modified by a function is a good programming habit. This allows some compilers to perform additional checks at compile time.

Exercises

In the following exercises, we will consider the declaration of the constant NB_GRADES and and the following 3 arrays:

#define NB_GRADES 6

float grades1[NB_GRADES] = {10.2, 12.5, 18.5, 9.8, 13.2, 12.1};
float grades2[NB_GRADES];
float averages[NB_GRADES];

Exercise 1

Write a function void display(const float grades[], int size) which displays the values of the array passed as parameter. Test your function with the array grades1 :

grade[0]    = 10.20
grade[1]    = 12.50
grade[2]    = 18.50
grade[3]    = 9.80
grade[4]    = 13.20
grade[5]    = 12.10

Do the exercise

Solution

Exercise 2

Write a function int init(float class[], int size, float value) that fills the array class with the note grade. Initialize each cell of grades2 array to 10 thanks to the function before displaying it:

grade[0]    = 10.00
grade[1]    = 10.00
grade[2]    = 10.00
grade[3]    = 10.00
grade[4]    = 10.00
grade[5]    = 10.00

Do the exercise

Solution

Quiz

In C, an array passed as an argument to a function ...

is copied when the function is called

operates as a variable

loses information about its size

is converted to a one-dimensional array

Check

The tab array passed as argument ...

void fct(int tab[]);

is neither readable nor writable in the function

is only available for reading

is only accessible in writing

is readable and writable

Check

The tab array passed as argument ...

void fct(const int tab[]);

is neither readable nor writable in the function

is only available for reading

eis only accessible in writing

is readable and writable

Check Bravo! The const keyword specifies that the array should not be modified by the function.

How can a function know the size of an array?

by adding a second argument to the function for the size

by using the sizeof() function

by adding a special character in the array that indicates the end of the data

if the size is fixed, it can be hard-coded in the function

Check

The keyword const ...

is strongly discouraged

is mandatory for arrays

is recommended once its use is adequate

indicates that the argument can not be modified by the funcion

Check Bravo! The const keyword gives additional information to the user of a function.

Lesson 9.4 Summary Lesson 9.6

Lesson 9.5. Arrays and functions

Passing arrays as arguments

Keyword const

Exercises

Exercise 1

Exercise 2

Quiz

See also