How do I fix the problem with the incorrect array value in for loop using C language?

Question

I am trying to understand how array boundaries work in C, so tried the following code:

#include <stdio.h>

int main(){
{
   int i, s[4], t[4], u=0;

// fisrt loop

    for (i=0; i<=4; i++)

{
    s[i] =i;
    t[i] =i;

    printf("s = %d\n", s[i]);
    printf("t = %d\n", t[i]);
}

 // second loop

    printf("s:t\n");

    for (i=0; i<=4; i++)

    printf("%d:%d\n", s[i], t[i]);
    printf("u = %d\n", u);

}
 return 0;
}

The output is as follows :

fisrt loop
s = 0
t = 0
s = 1
t = 1
s = 2
t = 2
s = 3
t = 3
s = 4
t = 4
second loop
s:t
4:0
1:1
2:2
3:3
4:4
u = 0

I am expecting both loops to print 5 elements from 0 to 4. As you can see, the first for loop's output looks ok, but in the second loop the value of s[0] is wrong and for some reason turned to 4. I am assuming that this is happening because of some problem in the array boundary, but I am not sure, please correct me if I am wrong. Is there any way to fix this, and make s[0] correct value? Cheers.

You are accessing out-of-the-bound of array in loop: `for (i=0; i<=4; i++)`. — Shubham, Jul 17 '20 at 09:57
You are doing out-of-bound access. Array indices range from 0 to N-1 (in this case 0 to 3 both inclusive). — P.P, Jul 17 '20 at 09:58
See this answer: https://stackoverflow.com/a/62951660/4386427 — Support Ukraine, Jul 17 '20 at 10:13
Your question missing to explain if you want to have the arrays of 5 or 4 `int` elements. The "*the first for loops output looks ok*" isn't sufficient for me to identify that you really want to have 5 elements. - What is your *expected* output? — RobertS supports Monica Cellio, Jul 17 '20 at 10:42

RobertS supports Monica Cellio · Answer 1 · 2020-07-17T10:53:04.580

0

If you want to have 5 elements for both arrays:

Change the declarations of s and t to s[5] and t[5]. Only at the declaration the 5 means 5 elements. Only at indexing and addressing a certain element, counting starts at 0, not 1.

If you want to have the arrays to be consisted of 4 elements:

for (i = 0; i <= 4; i++)

i <= 4 - With this condition and the initialization i = 0 you access memory beyond the bounds of the array at the last iteration, which invokes undefined behavior.

When i == 4, s[i] and t[i] is an out of bounds access as s and t are both arrays of 4 int, not 5.

It needs to be i < 4 or i <= 3 for the arrays of 4 int or (if you want to) or change the initialization to i = 1.

edited Jul 17 '20 at 10:53

answered Jul 17 '20 at 09:57

RobertS supports Monica Cellio

14,524
7
33
80

if I do this it returns this s:t 0:0 1:1 2:2 3:3 -13216:0 – dariahiguchi Jul 17 '20 at 10:02
@dariahiguchi You need to correct it for both loops. – RobertS supports Monica Cellio Jul 17 '20 at 10:03
@ RobertS supports Monica Cellio , hmmm but then it only counts until 3, so the only way is to decrease the value of i ? – dariahiguchi Jul 17 '20 at 10:08
@dariahiguchi `s[0]` is the first element and `s[3]` is the forth element. When the loop counts up to 4 you attempt to access a non-existing 5th element. – RobertS supports Monica Cellio Jul 17 '20 at 10:11
OP obviously wants 5 elements... From the question "..the first for loops output looks ok.." – Support Ukraine Jul 17 '20 at 10:11
@dariahiguchi Do you want the array to be consisted of 5 elements or 4? – RobertS supports Monica Cellio Jul 17 '20 at 10:14
@4386427 I'm not quite sure about this. This message also could just express that the output in its formally purpose and order is ok. I adjusted my answer for that case. I asked the OP to clarify that. – RobertS supports Monica Cellio Jul 17 '20 at 10:20
The comment from OP just above "... hmmm but then it only counts until 3,..." also clearly shows that OP wants the numbers from 0 to 4 (both included), so the correct answer is: Increase the array size – Support Ukraine Jul 17 '20 at 10:51

Jacopo Carlini · Answer 2 · 2020-07-17T10:12:27.823

0

Delete the equals in the for loops:

for (i = 0; i < 4; i++)

The length of the arrays is 4. From 0 to 4 (included) there are 5 positions.

When you declare int s[4]; you create an arrays of lenght 4: s[0], s[1], s[2], s[3]. You can't access s[4].

This is because the position starts to 0.

edited Jul 17 '20 at 10:12

answered Jul 17 '20 at 09:59

Jacopo Carlini

68
8

score 0 · Answer 3 · answered Jul 17 '20 at 10:00

0

for (i=0; i <4; i++)

in your code you access 5 elements of the array and your array has only 4 elements. It ans Undefined Behaviour.

Second loop the same error.

answered Jul 17 '20 at 10:00

0___________

60,014
4
34
74

score 0 · Answer 4 · answered Jul 17 '20 at 10:00

0

There is a problem with the size of the array, you need to modify your code of that line to given code:

int i, s[4], t[4], u=0;

to

int i, s[5], t[5], u=0;

answered Jul 17 '20 at 10:00

Ashish Karn

1,127
1
9
20

I wonder why this got a downvote - it's obviously the (first) correct answer. But @ashishkarn you could add a little more description to improve the answer. – Support Ukraine Jul 17 '20 at 10:10
@4386427 Thanks. description can be found on other answers which are also correct. – Ashish Karn Jul 17 '20 at 10:15
We don't know if that is exactly the solution to the problem. OP missed to give more information about that. – RobertS supports Monica Cellio Jul 17 '20 at 10:29

How do I fix the problem with the incorrect array value in for loop using C language?

4 Answers4