Printing an array from a struct

Question

When I try to print an array contained in an instance of a struct part of the result is what I'm expecting and other parts seem to be gibberish. What is going on here?

Example output:

$./makevector test

NAME: test 16481592918288327671592918096327670000000000100011809530144490000159291832032767

My code is as follows:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>

static int vec_length = 30;

typedef struct {
  char* name;
  int* vector;
} word_entry;

static word_entry entry_one = {NULL,NULL}; 

void MakeEntry(char* word, word_entry* entry){
 entry->name = word;
 int i;
 int this_vector[vec_length];
 srand(time(NULL));
 for(i=0;i<vec_length;i++){
   this_vector[i] = rand()%2;
 } 
entry->vector = this_vector;
}

int main(int argc, char* argv[]){
  int i;
  MakeEntry(argv[1], &entry_one);
  printf("NAME: %s\n", entry_one.name); 
  for (i=0;i<vec_length;i++){
    printf("%d",entry_one.vector[i]);
  }
  printf("\n");
}

chrisaycock · Accepted Answer · 2012-04-30 16:24:05Z

3

this_vector is a local array to MakeEntry(). When that function ends, that array goes out of scope. So entry->vector is pointing to something invalid and you get undefined behavior.

answered Apr 30, 2012 at 16:24

chrisaycock

38.1k15 gold badges94 silver badges128 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Jeff · Accepted Answer · 2012-04-30 16:23:56Z

2

Mulitple issues here - "this_vector" is local to MakeEntry, and goes out of scope when MakeEntry returns.

Also - I think you want some \n in your printfs

Finally - what output were you expecting?

answered Apr 30, 2012 at 16:23

Jeff

2,0293 gold badges25 silver badges36 bronze badges

2 Comments

PrckPgn Over a year ago

Thank you, that was it. I guess I have to declare a global array specifically for my struct instance and then modify it? I was looking for a random binary string as the output to associate with the word.

Jeff Over a year ago

Sure a global would work, or you can use the calloc() functions that others describe. And that should give you a string of 1's and 0's, but I'm not sure how that is useful? Good luck!

twitch · Accepted Answer · 2012-04-30 18:27:47Z

2

Try something along these lines:

void MakeEntry(char* word, word_entry* entry)
{

      int i = 0;
      int *this_vector = calloc(vec_length+1, sizeof(int)); //Allocate an array of             Nelements x sizeof(int)

      entry->name = word;

      srand(time(NULL));

     for(i = 0; i < vec_length; i++)
       this_vector[i] = rand()%2;

      entry->vector = this_vector;
}

answered Apr 30, 2012 at 18:27

twitch

1838 bronze badges

Comments

abelenky · Accepted Answer · 2012-04-30 16:25:10Z

1

You create this_vector on the stack. But then assign it to entry->vector, for use outside the function MakeEntry.

Once MakeEntry returns, this_vector is no longer valid, and is likely the source of your garbage.

void MakeEntry(char* word, word_entry* entry){
    int this_vector[vec_length];
    [...]
    entry->vector = this_vector;
}

answered Apr 30, 2012 at 16:25

abelenky

65k24 gold badges115 silver badges163 bronze badges

Comments

Jack · Accepted Answer · 2012-04-30 16:21:38Z

0

In my opinion the problem is that you are printing numbers without any spacing, try with

printf("%d ",entry_one.vector[i]);

so that you can identify all the single ints in your array.

Oh well, read chrisaycock answer, I didn't notice you were assigning the whole vector from a stack allocated variabile. It goes out of scope after the local scope, you should dynamically allocate it:

entry->vector = calloc(vec_length,sizeof(int));
for(i=0;i<vec_length;i++){
  entry->vector[i] = rand()%2;
}

answered Apr 30, 2012 at 16:21

Jack

134k33 gold badges252 silver badges349 bronze badges

1 Comment

PrckPgn Over a year ago

Here's what that did: NAME: test 16 48 60530528 32767 60530336 32767 0 0 0 0 0 0 0 0 1 0 1 0 0 0 1 0 809530144 49 1 0 0 0 60530560 32767

Mahesh · Accepted Answer · 2012-04-30 16:24:06Z

0

entry->vector = this_vector;

this_vector is a local array type variable and is lost as soon as the function returns. But still the struct member hold reference to it.

Instead of allocating the array on stack, use malloc or any other dynamic memory allocater. Remember to free it when no longer needed.

answered Apr 30, 2012 at 16:24

Mahesh

34.7k23 gold badges93 silver badges116 bronze badges

Collectives™ on Stack Overflow

Printing an array from a struct

6 Answers 6

Comments

2 Comments

Comments

Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

Comments

2 Comments

Comments

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related