find middle point in c array

Question

I am studying C programming. I want to find a middle point of array without knowing about length of it. If length of array is 6, middle point must be 4th, but I have an error and I don't know what it is. Here is my code:

    int *a =(int *) malloc(sizeof(int) * n); 

    int i;
    for( i = 0; i < sizeof(a); i++)
        scanf("%d",&a[i]);

Anyone can help, thank you.

[Please don't cast the return value of `malloc()` in C](http://stackoverflow.com/a/605858/28169). — unwind, Apr 22 '14 at 12:38
"so how do i know length of array?" You store it in a separate variable. Believe it or not, that's the only way to do it in C when you allocate a chunk of memory dynamically. — Sergey Kalinichenko, Apr 22 '14 at 12:40
@Deduplicator Right, there are at least two very simple errors here. There must be at least 100500 answers covering each one of them, making this question not particularly attractive in terms of keeping it open. — Sergey Kalinichenko, Apr 22 '14 at 13:08
"so how do i know length of array? " - you have your `n`, and you even pass it to your `find_middle`. It is not clear for me what else you need — Dabo, Apr 22 '14 at 13:08
@dasblinkenlight: That's true for sure. Still, it should be closed as duplicate for an array-passing question, not for a typo. — Deduplicator, Apr 22 '14 at 13:12

Ferenc Deak · Answer 1 · 2014-04-30T18:32:24.267

10

scanf is reading into an address, so:

scanf("%d",a[i])

should really be:

scanf("%d", &a[i])

And do not forget to free the memory when you're done. (For tiny applications like this it does not matter, but it is a good habit if you get used to it when you are stepping out from the learning phase and getting into the real life programming)

And also: sizeof(a) is the size of the pointer, not the size of the memory allocated the pointer points to (as correctly pointed out in the other answer).

edited Apr 30 '14 at 18:32

answered Apr 22 '14 at 12:39

Ferenc Deak

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Also, `` should be included; and if the `scanf` fails then the code may try to print uninitialized memory, so it'd be a bit safer to use `calloc` (in lieu of doing proper error checking) – M.M Apr 22 '14 at 12:50
i just modified it and still get the wrong result , please help – user3376115 Apr 22 '14 at 12:56
@fritzone: Add something about the allocation and element_count errors from the comments? Because this way it does not actually even try to answer the question. – Deduplicator Apr 22 '14 at 13:00

score 3 · Answer 2 · answered Apr 22 '14 at 13:01

3

This is wrong: for( i = 0; i < sizeof(a); i++). Here sizeof(a) will return size of the int pointer, likely 4 or 8 on your system. In general, don't use sizeof to determine the element count of an array, use separate variable instead.

You'll want to use for( i = 0; i < n; i++) instead.

answered Apr 22 '14 at 13:01

user694733

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Using sizeof to determine the element count of an ***array*** is good practice, it allows dropping magic constants. You meant *pointer to array*. – Deduplicator Apr 22 '14 at 13:04
@Deduplicator In my experience using regular constants is always better than `((sizeof(a)/sizeof(a[0]))` -hack, especially for beginners that do not grasp difference between the array/pointer. Also, I'm not sure what you mean with "pointer to array". – user694733 Apr 22 '14 at 13:08
Using regular constants is ok, unless you are talking about arrays sized to fit their initialisers. To make my other point clearer, change that to pointer to multiple `int`, so `int*`. (Bad idea of me to mix datastructures and C types without explicitly differentiating) – Deduplicator Apr 22 '14 at 13:15

find middle point in c array

2 Answers2