Why array of struct lets me index to out of boundary array [duplicate]












-1
















This question already has an answer here:




  • How dangerous is it to access an array out of bounds?

    10 answers




Sorry for the bad title and i am new to the C but here is the part i don't understand.



I have a simple struct;



struct st{



        int a;

        int b;

};


and i am creating array of struct and indexing values to variable 'a' in main;



int main(){

         struct st st_arr[2];

         st_arr[0].a = 5;

         st_arr[1].a = 10;

         st_arr[4].a = 20;



         printf("%d %d %dn", st_arr[0].a, st_arr[1].a, st_arr[4].a);

}


i have assigned 2 array of structs but it lets me index 4th of the struct why is that? Isn't it suppose to give me an error?



the output is:



5 10 20






c arrays







share|improve this question













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 Nov 20 '18 at 10:28


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.











  • 4





    Because there is nobody checking that you go out of bounds,,,,

    – Paul Ogilvie
    Nov 20 '18 at 10:20






  • 2





    It is called undefined behavior meaning it can go wrong, it can go right, your program may abort or anything else can happen, either immediately or at a later stage in your program.

    – Paul Ogilvie
    Nov 20 '18 at 10:22








  • 1





    I know you wanted to be sure but it's important to understand that st_arr[2] is already out of bounds, despite its resemblance to the array definition.

    – Peter A. Schneider
    Nov 20 '18 at 10:31













  • thank you so much for your answers

    – M. O. Karaköz
    Nov 20 '18 at 10:44
















-1
















This question already has an answer here:




  • How dangerous is it to access an array out of bounds?

    10 answers




Sorry for the bad title and i am new to the C but here is the part i don't understand.



I have a simple struct;



struct st{



        int a;

        int b;

};


and i am creating array of struct and indexing values to variable 'a' in main;



int main(){

         struct st st_arr[2];

         st_arr[0].a = 5;

         st_arr[1].a = 10;

         st_arr[4].a = 20;



         printf("%d %d %dn", st_arr[0].a, st_arr[1].a, st_arr[4].a);

}


i have assigned 2 array of structs but it lets me index 4th of the struct why is that? Isn't it suppose to give me an error?



the output is:



5 10 20






c arrays







share|improve this question













marked as duplicate by Lundin arrays
Users with the  arrays badge can single-handedly close arrays questions as duplicates and reopen them as needed.

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 Nov 20 '18 at 10:28


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.











  • 4





    Because there is nobody checking that you go out of bounds,,,,

    – Paul Ogilvie
    Nov 20 '18 at 10:20






  • 2





    It is called undefined behavior meaning it can go wrong, it can go right, your program may abort or anything else can happen, either immediately or at a later stage in your program.

    – Paul Ogilvie
    Nov 20 '18 at 10:22








  • 1





    I know you wanted to be sure but it's important to understand that st_arr[2] is already out of bounds, despite its resemblance to the array definition.

    – Peter A. Schneider
    Nov 20 '18 at 10:31













  • thank you so much for your answers

    – M. O. Karaköz
    Nov 20 '18 at 10:44














-1












-1








-1









This question already has an answer here:




  • How dangerous is it to access an array out of bounds?

    10 answers




Sorry for the bad title and i am new to the C but here is the part i don't understand.



I have a simple struct;



struct st{



        int a;

        int b;

};


and i am creating array of struct and indexing values to variable 'a' in main;



int main(){

         struct st st_arr[2];

         st_arr[0].a = 5;

         st_arr[1].a = 10;

         st_arr[4].a = 20;



         printf("%d %d %dn", st_arr[0].a, st_arr[1].a, st_arr[4].a);

}


i have assigned 2 array of structs but it lets me index 4th of the struct why is that? Isn't it suppose to give me an error?



the output is:



5 10 20






c arrays







share|improve this question















This question already has an answer here:




  • How dangerous is it to access an array out of bounds?

    10 answers




Sorry for the bad title and i am new to the C but here is the part i don't understand.



I have a simple struct;



struct st{



        int a;

        int b;

};


and i am creating array of struct and indexing values to variable 'a' in main;



int main(){

         struct st st_arr[2];

         st_arr[0].a = 5;

         st_arr[1].a = 10;

         st_arr[4].a = 20;



         printf("%d %d %dn", st_arr[0].a, st_arr[1].a, st_arr[4].a);

}


i have assigned 2 array of structs but it lets me index 4th of the struct why is that? Isn't it suppose to give me an error?



the output is:



5 10 20





This question already has an answer here:




  • How dangerous is it to access an array out of bounds?

    10 answers







c arrays




c arrays






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked Nov 20 '18 at 10:20









M. O. KaraközM. O. Karaköz

44




44




marked as duplicate by Lundin arrays
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 Nov 20 '18 at 10:28


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.






marked as duplicate by Lundin arrays
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 Nov 20 '18 at 10:28


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.










  • 4





    Because there is nobody checking that you go out of bounds,,,,

    – Paul Ogilvie
    Nov 20 '18 at 10:20






  • 2





    It is called undefined behavior meaning it can go wrong, it can go right, your program may abort or anything else can happen, either immediately or at a later stage in your program.

    – Paul Ogilvie
    Nov 20 '18 at 10:22








  • 1





    I know you wanted to be sure but it's important to understand that st_arr[2] is already out of bounds, despite its resemblance to the array definition.

    – Peter A. Schneider
    Nov 20 '18 at 10:31













  • thank you so much for your answers

    – M. O. Karaköz
    Nov 20 '18 at 10:44














  • 4





    Because there is nobody checking that you go out of bounds,,,,

    – Paul Ogilvie
    Nov 20 '18 at 10:20






  • 2





    It is called undefined behavior meaning it can go wrong, it can go right, your program may abort or anything else can happen, either immediately or at a later stage in your program.

    – Paul Ogilvie
    Nov 20 '18 at 10:22








  • 1





    I know you wanted to be sure but it's important to understand that st_arr[2] is already out of bounds, despite its resemblance to the array definition.

    – Peter A. Schneider
    Nov 20 '18 at 10:31













  • thank you so much for your answers

    – M. O. Karaköz
    Nov 20 '18 at 10:44








4




4





Because there is nobody checking that you go out of bounds,,,,

– Paul Ogilvie
Nov 20 '18 at 10:20





Because there is nobody checking that you go out of bounds,,,,

– Paul Ogilvie
Nov 20 '18 at 10:20




2




2





It is called undefined behavior meaning it can go wrong, it can go right, your program may abort or anything else can happen, either immediately or at a later stage in your program.

– Paul Ogilvie
Nov 20 '18 at 10:22







It is called undefined behavior meaning it can go wrong, it can go right, your program may abort or anything else can happen, either immediately or at a later stage in your program.

– Paul Ogilvie
Nov 20 '18 at 10:22






1




1





I know you wanted to be sure but it's important to understand that st_arr[2] is already out of bounds, despite its resemblance to the array definition.

– Peter A. Schneider
Nov 20 '18 at 10:31







I know you wanted to be sure but it's important to understand that st_arr[2] is already out of bounds, despite its resemblance to the array definition.

– Peter A. Schneider
Nov 20 '18 at 10:31















thank you so much for your answers

– M. O. Karaköz
Nov 20 '18 at 10:44





thank you so much for your answers

– M. O. Karaköz
Nov 20 '18 at 10:44












1 Answer
1






active

oldest

votes


















1














C does not provide a(ny) bound checking by default.



Accessing out of bounds (i.e., invalid memory) is defined to have undefined behaviour.






share|improve this answer
























  • Maybe you should change "defined" to "specified" so that I can get back out of the rabbit hole ;-)

    – Peter A. Schneider
    Nov 20 '18 at 10:28











  • @PeterA.Schneider That was intentional. :)

    – Sourav Ghosh
    Nov 20 '18 at 10:30


















1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









1














C does not provide a(ny) bound checking by default.



Accessing out of bounds (i.e., invalid memory) is defined to have undefined behaviour.






share|improve this answer
























  • Maybe you should change "defined" to "specified" so that I can get back out of the rabbit hole ;-)

    – Peter A. Schneider
    Nov 20 '18 at 10:28











  • @PeterA.Schneider That was intentional. :)

    – Sourav Ghosh
    Nov 20 '18 at 10:30
















1














C does not provide a(ny) bound checking by default.



Accessing out of bounds (i.e., invalid memory) is defined to have undefined behaviour.






share|improve this answer
























  • Maybe you should change "defined" to "specified" so that I can get back out of the rabbit hole ;-)

    – Peter A. Schneider
    Nov 20 '18 at 10:28











  • @PeterA.Schneider That was intentional. :)

    – Sourav Ghosh
    Nov 20 '18 at 10:30














1












1








1







C does not provide a(ny) bound checking by default.



Accessing out of bounds (i.e., invalid memory) is defined to have undefined behaviour.






share|improve this answer













C does not provide a(ny) bound checking by default.



Accessing out of bounds (i.e., invalid memory) is defined to have undefined behaviour.







share|improve this answer












share|improve this answer



share|improve this answer










answered Nov 20 '18 at 10:23









Sourav GhoshSourav Ghosh

109k14129188




109k14129188













  • Maybe you should change "defined" to "specified" so that I can get back out of the rabbit hole ;-)

    – Peter A. Schneider
    Nov 20 '18 at 10:28











  • @PeterA.Schneider That was intentional. :)

    – Sourav Ghosh
    Nov 20 '18 at 10:30



















  • Maybe you should change "defined" to "specified" so that I can get back out of the rabbit hole ;-)

    – Peter A. Schneider
    Nov 20 '18 at 10:28











  • @PeterA.Schneider That was intentional. :)

    – Sourav Ghosh
    Nov 20 '18 at 10:30

















Maybe you should change "defined" to "specified" so that I can get back out of the rabbit hole ;-)

– Peter A. Schneider
Nov 20 '18 at 10:28





Maybe you should change "defined" to "specified" so that I can get back out of the rabbit hole ;-)

– Peter A. Schneider
Nov 20 '18 at 10:28













@PeterA.Schneider That was intentional. :)

– Sourav Ghosh
Nov 20 '18 at 10:30





@PeterA.Schneider That was intentional. :)

– Sourav Ghosh
Nov 20 '18 at 10:30



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