CODEWITHSUNDEEP
carrays
jabe
jabe

Reputation: 834

Why is my array printing out a duplicate?

I want to copy an array in C to another array, and then print them both out. When I print out the second array, it prints out twice. What is causing the duplicate?

When I debug (Xcode), everything seems normal until after the final iteration of the for loop, and suddenly, the test2 array equals "testtest".

#include <stdio.h>

int main()
{
  char test1[4];
  char test2[4];

  test1[0] = 't';
  test1[1] = 'e';
  test1[2] = 's';
  test1[3] = 't';

  /* Copy the stuff from test1 into test2 */
  for(int j = 0; j<4; j++){
    test2[j] = test1[j];
  }

  printf("test1: %s\n", test1);
  printf("test2: %s\n", test2);
  printf("end testing");

  return 0;
}

My results are:

test1: test

test2: testtest

end testing

Upvotes: 1

Views: 929

Answers (3)

Rohit Manocha
Rohit Manocha

Reputation: 47

Try using the String copy function 

int main(){
char test1[4];
char test2[4];

test1[0] = 't';
test1[1] = 'e';
test1[2] = 's';
test1[3] = 't';
strcpy(test2, test1);
printf("test1: %s\n", test1);
printf("test2: %s\n", test2);
printf("end testing");
return 0;
}

Upvotes: 0

user2736738
user2736738

Reputation: 30926

Your code has undefined behavior. The char array is not null terminated.

Twice ouputted because it didn't get the \0 and then it outputted adjacent variables value test1 and test2.

%s expects a null terminated char array or string. You didn't provide it. And it seems that char array test1 and test2 are adjacent in memory. As it didn't know where to stop because it didn't get any \0 - it printed two arrays content. Then it somehow got \0 and it stopped. But in case there was not any \0 adjacent to the memory it will go on printing. You can't or should reply on this behavior.

You can do this by the way but again you would better use nul terminated char array. (Makes life easier).

printf("%.*s",4,test1);
printf("%.*s",4,test2);

You can initialize the string like this

char test1[]="test";

Then you can use strcpy to copy the string - though make sure you have destination with appropriate size (taking into account the nul terminating character also).

Don't use gets() - instead use fgets() or similar function.

Upvotes: 2

Najat
Najat

Reputation: 197

C strings are null terminated. If you don't put a null character ('\0') at the end of the string, it actually is not going to work properly (because of the mistake). Do this:

  char test1[5];           //****
  char test2[5];

  test1[0] = 't';
  test1[1] = 'e';
  test1[2] = 's';
  test1[3] = 't';
  test1[4] = '\0';         //****

  /* Copy the stuff from test1 into test2 */
  for(int j = 0; j<4; j++){
    test2[j] = test1[j];
  }
  test2[j] = '\0';         //****

But you don't need to put a '\0' when you take the entire string input from the user like: scanf("%s", s) or gets(s), because it's put then automatically.

Upvotes: 1

Related Questions