Difference between ++*p, *p++ and *++p

Predict the output of following C programs.

filter_none

edit
close

play_arrow

link
brightness_4
code

// PROGRAM 1
#include <stdio.h>
int main(void)
{
    int arr[] = {10, 20};
    int *p = arr;
    ++*p;
    printf("arr[0] = %d, arr[1] = %d, *p = %d",
                          arr[0], arr[1], *p);
    return 0;
}

chevron_right






                        filter_none

















                                    filter_none




                                    edit

                                    close




                                    play_arrow




                                    link

                                    brightness_4

                                    code
                                




















// PROGRAM 2 


#include <stdio.h> 


int main(void) 


{ 


    int arr[] = {10, 20}; 


    int *p = arr; 


    *p++; 


    printf("arr[0] = %d, arr[1] = %d, *p = %d", 


                          arr[0], arr[1], *p); 


    return 0; 


} 



















                        chevron_right







                        filter_none

















                                    filter_none




                                    edit

                                    close




                                    play_arrow




                                    link

                                    brightness_4

                                    code
                                




















// PROGRAM 3 


#include <stdio.h> 


int main(void) 


{ 


    int arr[] = {10, 20}; 


    int *p = arr; 


    *++p; 


    printf("arr[0] = %d, arr[1] = %d, *p = %d",  


                          arr[0], arr[1], *p); 


    return 0; 


} 



















                        chevron_right







                        filter_none








The output of above programs and all such programs can be easily guessed by remembering following simple rules about postfix ++, prefix ++ and * (dereference) operators

1) Precedence of prefix ++ and * is same. Associativity of both is right to left.

2) Precedence of postfix ++ is higher than both * and prefix ++.  Associativity of postfix ++ is left to right.


(Refer: Precedence Table)


The expression ++*p has two operators of same precedence, so compiler looks for assoiativity. Associativity of operators is right to left. Therefore the expression is treated as ++(*p). Therefore the output of first program is “arr[0] = 11, arr[1] = 20, *p = 11“.


The expression *p++ is treated as *(p++) as the precedence of postfix ++ is higher than *. Therefore the output of second program is “arr[0] = 10, arr[1] = 20, *p = 20“.




The expression *++p has two operators of same precedence, so compiler looks for assoiativity. Associativity of operators is right to left. Therefore the expression is treated as *(++p). Therefore the output of third program is “arr[0] = 10, arr[1] = 20, *p = 20“.


Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.


Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.





    

        My Personal Notes
        arrow_drop_up
    

    

        
        

            
            
        

    


Recommended Posts:

Improved By :  srinam