Best way to check if a character array is empty

Question

Which is the most reliable way to check if a character array is empty?

char text[50];

if(strlen(text) == 0) {}

or

if(text[0] == '\0') {}

or do i need to do

 memset(text, 0, sizeof(text));
 if(strlen(text) == 0) {}

Whats the most efficient way to go about this?

So you're wondering if you need to test for null or if you need to set the string to all 0s and then test for 0? — Jeremy L, Nov 25 '09 at 00:52
Yes, I wasn't sure if I could test a string to be NULL if I just declared it and did not do anything with it first. — ZPS, Nov 28 '09 at 05:45

score 91 · Accepted Answer · edited Dec 22 '15 at 12:07

91

Given this code:

char text[50];
if(strlen(text) == 0) {}

Followed by a question about this code:

 memset(text, 0, sizeof(text));
 if(strlen(text) == 0) {}

I smell confusion. Specifically, in this case:

char text[50];
if(strlen(text) == 0) {}

... the contents of text[] will be uninitialized and undefined. Thus, strlen(text) will return an undefined result.

The easiest/fastest way to ensure that a C string is initialized to the empty string is to simply set the first byte to 0.

char text[50];
text[0] = 0;

From then, both strlen(text) and the very-fast-but-not-as-straightforward (text[0] == 0) tests will both detect the empty string.

edited Dec 22 '15 at 12:07

waldyrious

3,683
4
33
41

answered Nov 25 '09 at 00:19

bbum

162,346
23
271
359

1

I structured that badly, I meant for the memset(text, 0, sizeof(text)); to come immediately after char text[50]; because I was not sure if it was bad practice to strlen a char array before it was assigned anything. – ZPS Nov 25 '09 at 00:24
1

It certainly is a bad idea to strlen an array before it is assigned - strlen will advance through memory until it reaches a 0 byte, which may be well beyond the end of the array. – Graeme Perrow Nov 25 '09 at 01:22

score 22 · Answer 2 · answered Nov 25 '09 at 00:14

22

Depends on whether or not your array is holding a null-terminated string. If so, then

if(text[0] == '\0') {}

should be sufficient.

Edit: Another method would be...

if (strcmp(text, "") == 0)

which is potentially less efficient but clearly expresses your intent.

answered Nov 25 '09 at 00:14

Parappa

7,566
3
34
38

for me following is working if(text[0]==''){return false;} return true; – Amandeep Rohila Dec 09 '21 at 12:27

score 8 · Answer 3 · answered Nov 25 '09 at 00:15

8

This will work to find if a character array is empty. It probably is also the fastest.

if(text[0] == '\0') {}

This will also be fast if the text array is empty. If it contains characters it needs to count all the characters in it first.

if(strlen(text) == 0) {}

answered Nov 25 '09 at 00:15

Peter Stuifzand

5,084
1
23
28

2

Don't use strlen to test if a cstring is empty. It is not a constant time operation. – Thomas Eding Dec 13 '12 at 22:34
1

Which is what I explain in the answer. – Peter Stuifzand Dec 24 '12 at 14:04

score 4 · Answer 4 · answered Nov 25 '09 at 00:14

The second method would almost certainly be the fastest way to test whether a null-terminated string is empty, since it involves one read and one comparison. There's certainly nothing wrong with this approach in this case, so you may as well use it.

The third method doesn't check whether a character array is empty; it ensures that a character array is empty.

score 4 · Answer 5 · answered Nov 25 '09 at 00:18

The second one is fastest. Using strlen will be close if the string is indeed empty, but strlen will always iterate through every character of the string, so if it is not empty, it will do much more work than you need it to.

As James mentioned, the third option wipes the string out before checking, so the check will always succeed but it will be meaningless.

score 2 · Answer 6 · edited Nov 13 '18 at 16:19

2

if (text[0] == '\0')
{
    /* Code... */
}

Use this if you're coding for micro-controllers with little space on flash and/or RAM. You will waste a lot more flash using strlen than checking the first byte.

The above example is the fastest and less computation is required.

edited Nov 13 '18 at 16:19

Ed The ''Pro''

875
10
22

answered Apr 10 '16 at 13:03

Tony

21
1

score 0 · Answer 7 · answered Sep 10 '19 at 20:45

0

if (!*text) {}

The above dereferences the pointer 'text' and checks to see if it's zero. alternatively:

if (*text == 0) {}

answered Sep 10 '19 at 20:45

fja0568

53
4

Best way to check if a character array is empty

7 Answers7

Linked