Adding elements to an array in C for loop incrementing forever

Question

I'm trying to add elements to an array with a for loop in C, however something strange is happening. The i variable is effected by the numbers input using scanf.

  int intArray[4];
  int i;
  printf("Input 5 numbers\n");
    for(i=0;i<5;i++){
    scanf("%d", &intArray[i]);
    printf("i: %d\n",i);
    }

Examples of outputs:

And any number greater than 3 input constantly works as intended or any number inserted greater than 3 when i = 3

I don't understand why i changes in this for loop in this way.

Any help would be appreciated.

John3136 · Accepted Answer · 2015-11-08 02:51:48Z

1

intArray[4] has indexes 0, 1, 2, 3. Your for loop's end condition is i<5, so it uses index 4, which is past the end of the array, and probably coincides with the variable i.

answered Nov 8, 2015 at 2:51

John3136

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2 Comments

PlasmaticDW Over a year ago

Ahh I see the error and how to fix it now. But why does this change the "i" variable within the loop? Likewise, why does inputting numbers greater than 3 overcome this problem too?

John3136 Over a year ago

Short answer: Undefined behavior is undefined. Longer answer: intArray[4] is probably the same piece of memory as i, so you're overwriting i. As for numbers > 3, interesting. >4 I'd (since i wouldn't be < 5 the loop would stop).

Tarek Abdel Maqsoud · Accepted Answer · 2015-11-08 02:55:32Z

0

Well, arrays are zero based, so in your case i < 5 will result (i from 0 to 4) which is invalid and out of index, cuz it is supposed to be (i from 0 to 3 - 4 elements)

answered Nov 8, 2015 at 2:55

Tarek Abdel Maqsoud

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