Concatenating two arrays

Question

I was trying to create a program that takes in two arrays and concatenates them to create a new string. These are the two strings.

 char a[8]={"hellostr"};
 char b[8]={"HELLOSTR"};

Can someone tell me how could I concatenate and display this concatenated string? I tried looking for it but could not understand much of it.

These are not strings. Note that in C, a sting is just a `char []` with a trailing `'\0'`. As you do not reserve enough space for that, it is no string. And: you cannot _declare_ a string - for much the same reason! — too honest for this site, Jul 08 '15 at 14:19

Sourav Ghosh · Accepted Answer · 2015-07-08T14:50:53.933

3

You need to make use of strcat() function from string.h.

A sample algo:

Define an array (say destarr[128], for example)large enough to hold the result (concatenated string).
memset()the destarr to 0.
use strcat(destarr, a) and strcat(destarr, b) to concatenate one after another.

That said,

 char a[ ]={"hellostr"};

is considered better and less error-prone over

char a[8]={"hellostr"};

as,

In the former case, compiler takes the charge to allocate the memory, as required, with the null-terminator in mind. The array can be used as a string.
In the later case, there is no room for the null-terminator and hence, the array can neither be considered nor be used as a string.

edited Jul 08 '15 at 14:50

answered Jul 08 '15 at 14:13

Sourav Ghosh

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Thank you for that example. I had one more question. Now if I want to display just one of the characters from this concatenated string how do I do that? – Anamika Ana Jul 08 '15 at 14:25
I don't agree that the former is less error prone. Because if you intended to have a string exactly x characters long, you don't want code with more characters than that to compile. For example the bugged code `char a[8]={"hellostr"};` should yield a compiler diagnostic, so that hopefully the programmer gets a slap: "hey, you didn't consider null terminarion! Go fix your code to a[8+1]." rather than "everything looks ok here, proceed even though there's potentially a bug here". – Lundin Jul 08 '15 at 14:27
@AnamikaAna `one of the characters` is that what you meant exactly? use index, naturally. – Sourav Ghosh Jul 08 '15 at 14:27
@SouravGhosh Actually gcc throws no warning for this with max warnings enabled. Very strange. – Lundin Jul 08 '15 at 14:32
@Lundin it should show something along the line of _"excess initializer for array....."_, right? – Sourav Ghosh Jul 08 '15 at 14:35
@SouravGhosh Indeed. Peculiar since it does so if you write the string as `{'h','e','l','l','o','s','t','r','\0'}`. – Lundin Jul 08 '15 at 15:02
1

@SouravGhosh I've posted a question [here](http://stackoverflow.com/questions/31296727/inconsistent-gcc-diagonistic-for-string-initialization), maybe someone out there knows more :) – Lundin Jul 08 '15 at 15:11

score 0 · Answer 2 · answered Jul 08 '15 at 14:26

strcat function can be used.

Header -string.h

If you want to do that without strcat function then you can write a simple function -

  void concatenate(char a[], char b[])
  {
     int c, d;

      c = 0;

     while (a[c] != '\0')
   {
        c++;    
   }

     d = 0;

   while (b[d] != '\0') 
    {
       a[c] = b[d];
       d++;
       c++; 
    }

       a[c] = '\0';
  }

Concatenating two arrays

2 Answers2