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The [ ] operator

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• Another syntactical sugar in C
  • Recall the very nicely looking operator ->:

    If p points to a struct typed variable, then: p->member ≡ (*p).member

    that we learned in this webpage: click here

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  • We will now learned an even more nicely looking operator used to simplify pointer arithmetic !!!

    And the result will shock you.....

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• The [ ] (array selection/subscripting) operator
  • Fact:

    [ ] is an operator !!!          The [ ] operator is called the array selection/subscripting operator

    Evidence that [ ] is an operator:

    Take a look at any table that list the precedence of operators in C/C++ Example: click here You will find that [ ] is among the higher priority operators

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  • As I have said so many times:

    An operator (in any programming language) will return something !!!         (It can be an value (e.g.: 3 + 4) and it can be a variable (e.g.: *p)

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• The effect of the [ ] operator
  • The [   ] operator:

    The [   ] operator is a binary operator I.e.: [   ] has 2 operands (Let's call these 2 operands: p and i)          The [   ] operator must be applied as follows: p [ i ] where: p is a reference typed expression/variable i is an integer typed expression/variable The meaning of the [ ] operator expression is a short hand (or syntactical sugar) for:             p [ i ] <===> *(p + i)

    Note:

    You have already learned/seen about pointer arithmentic, so you should know what *(p + i) mean !!!

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  • Note:

    The [ ] operator can work with a constant of a reference type. Example: double x[10]; // Defines an array x == &x[0] // x is a constant reference (address) x[i] == *(x + i) // x[i] selects the i-th element in array x The [ ] operator can also be applied to a reference typed variable p !!! The C compiler will translate: translates to p[ index ] ====> *(p + index) Example: double x[10]; // Defines an array double *p; // p is a reference variable p == &x[2] // p is a variable of a reference (address) type p[i] == *(p + i) // x[i] selects the (i+2)-th element in array x !!

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  • Common use of the [ ] operator:

    Instead of using (*p + i) to access the ith array element in a array, we can now use: p[i] (This looks a whole lot nicer and a lot more like an ordinary array access !!!!)

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• Array access using pointer arithment revisted
  • Recall that previously, we used the ugly *(p + i) to access array elements:

    #include <stdio.h> int main(int argc, char *argv[]) { int a[10] = { 11, 22, 33, 44, 55, 66, 77, 88, 99, 777 }; int* p; p = &a[0]; // p points to variable a[0] printf("*(p + 0) = %d, a[0] = %d\n", *(p+0), a[0] ); printf("*(p + 1) = %d, a[1] = %d\n", *(p+1), a[1] ); printf("*(p + 2) = %d, a[2] = %d\n", *(p+2), a[2] ); printf("*(p + 3) = %d, a[3] = %d\n", *(p+3), a[3] ); }

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  • We can now re-write the same program using the nicer looking [ ] operator:

    #include <stdio.h> int main(int argc, char *argv[]) { int a[10] = { 11, 22, 33, 44, 55, 66, 77, 88, 99, 777 }; int* p; p = &a[0]; // p points to variable a[0] printf("p[0] = %d, a[0] = %d\n", p[0], a[0] ); printf("p[1] = %d, a[1] = %d\n", p[1], a[1] ); printf("p[2] = %d, a[2] = %d\n", p[2], a[2] ); printf("p[3] = %d, a[3] = %d\n", p[3], a[3] ); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set2/pointer-arithm2a.c

    How to run the program:

    To compile:   gcc pointer-arithm2a.c          To run: ./a.out