Code to change an array element changes a different variable

Question

I'm quite puzzled by why my variable NumberOfArrays changes the second time through the for loop in my code. Can anyone help me out?

#include <stdio.h>
#include <cs50.h>

int main(int argc, string argv[])

{
//variable declarations
int NumberOfArrays = 0;
int arrayRack[0];

//Get number of arrays
printf("Key in the number of arrays you'd like to have\n");
NumberOfArrays = GetInt();



//Get number for each element in arrayRack[]
for(int i = 0; i < NumberOfArrays; i++)
    {
        printf("give me an int for the %i th array\n", i + 1);
        arrayRack[i] = GetInt();
        // *** on the second pass, my "NumberOfArrays" gets adjusted to my GetInt number here.  Why?
    }


//print out numbers stored in respective arrays
for(int j = 0; j < NumberOfArrays; j++)
    {
        printf("{%i}<-- number in %ith array\n", arrayRack[j], j + 1);
    }

    return 0;

}

Can we all just appreciate how this is an actual stack overflow on Stack Overflow? — Charles
– Charles, Commented Oct 4, 2016 at 18:25
for ease of readability and understandability: 1) please consistently indent the code. Indent after every opening brace '{'. unindent before every closing brace '}'. — user3629249
– user3629249, Commented Oct 5, 2016 at 4:28
the compiler will output a number of warnings regarding the posted code. Amongst those warnings. warning: unused parameter 'argc' warning: unused parameter: 'argv'. When parameters are not used in main() you can use the prototype: int main( void ) for other functions, follow the function signature by a statement similar to: (void)parmName; — user3629249
– user3629249, Commented Oct 5, 2016 at 4:31
this line: int arrayRack[0]; does not make any room for anything. The posted code does not get, from the user, the number of entries in each array. So getting that value needs to be added to the code. Then declare the arrays as: int arrayRack[ NumberOfArrays ][ NumberOfArrayEntries ]; Then modify the rest of the code to actually input that number of entries for each array, Then modify the rest of the code to output all the entries in each array. — user3629249
– user3629249, Commented Oct 5, 2016 at 4:36

Community · Accepted Answer · 2017-05-23 12:00:37Z

Because you declared arrayRack as an empty array ([0]). Try int arrayRack[100]; or some other number, and make sure that NumberOfArrays is less than that number before you use it.

Explanation: (edit note that this may vary by compiler) your variables are most likely stored on the stack in nearby memory addresses. So arrayRack points somewhere close to NumberOfArrays in memory. C doesn't generally check if you've run off the end of an array, so accessing arrayRack[1] doesn't cause a compiler error in this situation. However, arrayRack[1] isn't part of your array, so accessing it actually accesses something else — in this situation, NumberOfArrays.

Edit gcc permits length-0 arrays but does not allocate space for them per this. However, length-0 arrays are prohibited by the C standard (e.g., see this, the answers to this, and this). Given the behaviour you've seen, it looks to me like the compiler is allocating one int's worth of space on the stack, pointing arrayRack to that space, and packing that space right next to NumberOfArrays. As a result, &(arrayRack[1]) == &NumberOfArrays. In any event, using variable-length arrays as suggested by @dasblinkenlight is a cleaner way to handle this situation.

In general, given int arrayRack[N];, you can only safely access arrayRack[0] through arrayRack[N-1].

Sergey Kalinichenko · Accepted Answer · 2016-10-04 18:09:01Z

3

You declared the array too early. Move the declaration to after the call of GetInt(), like this:

printf("Key in the number of arrays you'd like to have\n");
int NumberOfArrays = GetInt();
int arrayRack[NumberOfArrays];

Note: NumberOfArrays is not an ideal name for the variable, because it denotes the number of array elements, not the number of arrays; your code has only one array.

answered Oct 4, 2016 at 18:09

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

2 Comments

Charles Over a year ago

I'm pretty sure that you can't do int arrayRack[NumberOfArrays] in C. Or, at least, it will trigger a warning... A more proper way, imho, is to do the standard malloc and free.

Sergey Kalinichenko Over a year ago

@c650 VLAs are allowed starting with C99.

Collectives™ on Stack Overflow

Code to change an array element changes a different variable

2 Answers 2

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related