Array initialization without comma

Question

In the following code I am having the output like(last one giving segmentation fault)

 U   
 s    
 HelloThisisatest    
 Segmentation fault (core dumped)

but i do not understand why. code is

int main()
{
   char *a[]={"Hello" "This" "is" "a" "test"};
   printf("%c\n",a[1][0]);
   printf("%c\n",a[0][8]);
   printf("%s\n",a[0]);
   printf("%s\n",a[3]);
   return 0;
}

Another question is that can we initialize an 2-D array without using comma? Another situation that I got that when i am replacing the \ns by \ts then output changes like

"U s HelloThisisatest (null)"

why?

phoxis · Accepted Answer · 2012-09-29 04:36:32Z

11

This is because in C, two adjacent strings are concatenated into one. So "Hello" "World" is actually a single string "HelloWorld".

In your case The array of pointer actually contains one single string "HelloThisisatest". In the first printf you are accessing the location 1, which is undefined behaviour.
The next printf accesses the character 8 of the string which is s .
The third printf prints the entire string stored on the location 0, which is "HelloThisisatest"
The last printf attempts to access the array location 3, which is undefined behavour, as it is not initialized.

EDIT

Refer C11/C99 standard Section 5.1.1.2 Paragraph 6, which tells:

Adjacent string literal tokens are concatenated.

edited Sep 29, 2012 at 4:36

answered Sep 29, 2012 at 4:31

phoxis

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Comments

Matthew Flaschen · Accepted Answer · 2012-09-29 04:33:02Z

1

You have an array of pointers to char. However, there is only one such pointer, so code like:

a[1][0]

overruns the array. The first index must always be 0 for this particular array.

answered Sep 29, 2012 at 4:33

Matthew Flaschen

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phoxis · Accepted Answer · 2012-09-29 15:39:57Z

1

you may want to do this:

char a[][20] = {"Hello", "This", "is", "a", "test"};

notice that the second dimension should have a size (20)

now you can print all the words:

printf("%s\n",a[0]);
printf("%s\n",a[1]);
printf("%s\n",a[2]);
printf("%s\n",a[3]);
printf("%s\n",a[4]);

edited Sep 29, 2012 at 15:39

phoxis

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answered Sep 29, 2012 at 7:53

arash kordi

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3 Comments

phoxis Over a year ago

Undefined behaviour, starts from a[1]

phoxis Over a year ago

a[1], [2], etc are not initialized.

arash kordi Over a year ago

they are! see, I've added comma after each word

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