CODEWITHSUNDEEP
c
MetaCoin
MetaCoin

Reputation: 67

what is the length of this array? c language

char msg[100] = {’C’,’P’,’R’,‘E’,‘\0’,‘2’,‘8’, ‘8’,‘\0’};
int my_length = 0xFFFFFFFF;
my_length = strlen(msg);

I thought it is nine, however, the answer is 4. anyone can explain? thanks

Upvotes: 0

Views: 209

Answers (5)

John Bode
John Bode

Reputation: 123578

Remember that in C, a string is simply a sequence of character values followed by a zero-valued terminator. Strings are stored in arrays of char (or wchar_t for wide strings), but not every array of char (or wchar_t) is a string. To store a string that's N characters long, you need an array with at least N + 1 elements to account for the terminator.

strlen returns the number of characters in the string starting at the specified address up to the zero terminator.

To get the size (in bytes) of the msg array, use the sizeof operator:

char msg[100] = {'C','P','R','E','\0','2','8','8','\0'};    
size_t my_length = strlen( msg );
size_t my_size = sizeof msg;

if ( my_length >= my_size )
  // whoopsie

In this case, you're actually storing two strings in one array ("CPRE" and "288").

The size of the msg array is 100 (as given by the declaration).

The length of the string "CPRE" starting at msg[0] is 4, since you have a zero terminator in the fifth element of the array ('\0' == 0).

The length of the string "288" starting at msg[5] is 3 since you have another zero terminator in the ninth element of the array.

Upvotes: 2

Govind Parmar
Govind Parmar

Reputation: 21572

You cannot assume that the return value of strlen represents the size of an array.

strlen will take a pointer to the start of a string and increment the pointer while looking for a null terminator; once it finds that, it returns the counter (i.e. number of increments before the null was found).

You declared msg to be of length 100, but only populated 9 elements in the array. sizeof(msg) will be 100.

Are you actually asking "how can I find out how many values are initialized in an array"? There's really no answer to that.

Upvotes: 1

Kninnug
Kninnug

Reputation: 8053

strlen returns 4 because the (first) string in msg is terminated by the \0 at msg[4]. However, the array msg has a length of 100 chars because it was declared as such.

Upvotes: 4

MarianD
MarianD

Reputation: 14191

Maybe it is typo in your

char msg[100] = {’C’,’P’,’R’,‘E’,‘\0’,‘2’,‘8’, ‘8’,‘\0’};

and you wanted

char msg[100] = {’C’,’P’,’R’,‘E’,‘0’,‘2’,‘8’, ‘8’,‘\0’};

(plainly: CPRE0288), so binary 0 (instead of the character representation of 0 , i. e. '0') prematurely finishes your string.

Upvotes: 1

Justin Niessner
Justin Niessner

Reputation: 245479

strlen will stop counting as soon as it hits a null terminator (as C uses null terminated strings and expects to only find them at the end of a string).

You have four characters before your first null terminator, therefore the length is 4.

Upvotes: 4

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