Increment variable by N inside array index

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Could someone please tell me whether or not such a construction is valid (i.e not an UB) in C++. I have some segfaults because of that and spent couple of days trying to figure out what is going on there.

// Synthetic example  
int main(int argc, char** argv)
{
    int array[2] = {99, 99};
    /*
      The point is here. Is it legal? Does it have defined behaviour? 
      Will it increment first and than access element or vise versa? 
    */
    std::cout << array[argc += 7]; // Use argc just to avoid some optimisations
}

So, of course I did some analysis, both GCC(5/7) and clang(3.8) generate same code. First add than access.

Clang(3.8):  clang++ -O3 -S test.cpp

    leal    7(%rdi), %ebx
    movl    .L_ZZ4mainE5array+28(,%rax,4), %esi
    movl    $_ZSt4cout, %edi
    callq   _ZNSolsEi
    movl    $.L.str, %esi
    movl    $1, %edx
    movq    %rax, %rdi
    callq   _ZSt16__ostream_insertIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_PKS3_l

GCC(5/7) g++-7 -O3 -S test.cpp

    leal    7(%rdi), %ebx
    movl    $_ZSt4cout, %edi
    subq    $16, %rsp
    .cfi_def_cfa_offset 32
    movq    %fs:40, %rax
    movq    %rax, 8(%rsp)
    xorl    %eax, %eax
    movabsq $425201762403, %rax
    movq    %rax, (%rsp)
    movslq  %ebx, %rax
    movl    (%rsp,%rax,4), %esi
    call    _ZNSolsEi
    movl    $.LC0, %esi
    movq    %rax, %rdi
    call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
    movl    %ebx, %esi

So, can I assume such a baheviour is a standard one?

By itself array[argc += 7] is OK, the result of argc + 7 will be used as an index to array.

However, in your example array has just 2 elements, and argc is never negative, so your code will always result in UB due to an out-of-bounds array access.

Index Array Variables, Index Array Variables.c Use a for loop to display all the elements in an array. i​++ increments the value of i, so the print statement becomes print("p[%d] = %d\n"​  In Bash, there are multiple ways to increment/decrement a variable. This article explains some of them. Using + and -Operators # The most simple way to increment/decrement a variable is by using the + and -operators.

In case of a[i+=N] the expression i += N will always be evaluated first before accessing the index. But the example that you provided invokes UB as your example array contains only two elements and thus you are accessing out of bounds of the array.

Why we can't increment array index by simple using ++ and , Array variable is supposed to point to the first element of the array or first How can I find the index of a number X from an array let's say A in O(log n) time? Sometimes, one may need (or want) a loop which its iterator (the index variable) is modified within the loop body in addition to the normal incrementation by the (do) loop structure index. Goal. Demonstrate the best way to accomplish this. Task. Write a loop which: starts the index (variable) at 42

Your case is clearly undefined behaviour, since you will exceed array bounds for the following reasons:

First, expression array[argc += 7] is equal to *((array)+(argc+=7)), and the values of the operands will be evaluated before + is evaluated (cf. here); Operator += is an assignment (and not a side effect), and the value of an assignment is the result of argc (in this case) after the assignment (cf. here). Hence, the +=7 gets effective for subscripting;

Second, argc is defined in C++ to be never negative (cf. here); So argc += 7 will always be >=7 (or a signed integer overflow in very unrealistic scenarious, but still UB then).

Hence, UB.

C Program to Increment every Element of the Array by one & Print , This is a C program which increments every element of the array by one & print array. Inside this function, using for loop, access each element of the array, add 1 to Next - C Program to Print Binary Equivalent of an Integer using Recursion. i++ (Post-increment): The operator will return the variable value first (i.e, i value) then only i value will incremented by 1. –i (Pre decrement): It decrements the value of i even before assigning it to the variable i. i– (Post decrement): The operator will return the variable value first (i.e., i value), then only i value decrements by 1.

It's normal behavior. Name of array actualy is a pointer to first element of array. And array[n] is the same as *(array+n)

Print modified array after multiple array range increment operations , Now, from the index i = 1 to n-1, accumulate the values in the sum[] array as: sum[​i] += sum[i-1]. Finally for the index i = 0 to n-1, perform the operation: arr[i] += sum[​  Every variable in MATLAB® is an array that can hold many numbers. When you want to access selected elements of an array, use indexing. For example, consider the 4-by-4 magic square A:

[Solved] increment value for the particular array index, I used an array counter as Count[15] = 0. I want to increment the counter for the particular index like Count[10]++. Is this valid statement or not? Yet in 'B' and 'C' PHP evaluates the index and saves it in a temporary variable. You can always force PHP to evaluate a variable without explicitly storing it as a named variable first, with a simple "+=0" like in example 'C'. Compared to 'A', 'C' gives the more logically expected result, when we expect evaluation occurs left to right.

C Program to Access arrays by incrementing an decrementing , printf("\n\n\t\tStudytonight - Best place to learn\n\n\n"); int var[] = {100, 200, 300}; int i, *ptr; /* storing address of the first element of the array in pointer variable  Various options to increment by 1, and performance analysis. Thanks to Radu Rădeanu's answer that provides the following ways to increment a variable in bash: var=$((var+1)) ((var=var+1)) ((var+=1)) ((var++)) let "var=var+1" let "var+=1" let "var++" There are other ways too. For example, look in the other answers on this question.

Learning the Korn Shell, $name reference to shell variable name ${name} use braces to delimit shell element n in array name ${name[word]} element word in associative array name ++ -- auto-increment, auto-decrement both prefix and postfix + unary +, no effect ! Especially in the case of arrays this may be more useful since you can index the array through the nameref and don't need to put the index in the variable used as the reference. arr1=(a b c) arr2=(x y z) typeset -n p=arr1 # or 'declare -n' echo "${p[1]}" # prints 'b' This doesn't work through the indirect access:

Comments
  • If you ask a question about C++ and UB, then don't tag other languages. C is a totally different language with other semantic rules and other points of UB.
  • As for your question, unless argc is -6 or -7 then you will use an out-of-bounds index and that is of course UB. The expression used for the index must be fully evaluated first, and in C++ all variants of assignment are simple expressions.
  • My question is if a[i+=N] will always increment i first.
  • @Dmitry Yes of course; problems occur only if you use argc elsewhere in the same expression because the order of the evaluation of sub-expressions within an expression (and hence the propagation of side-effects) is usually not defined.
  • @Dmitry Did you perhaps think of the semantics of argc++? That expression would have the original, pre- increment value as an index into argv; the new value would only be visible in the next statement. But the expression argc += 1 (or += 7) has the value of argc after the increment.
  • Provided argc is smaller than -7 or larger than -6 (but yes, that is likely the problem).
  • @PeterA.Schneider argc shouldn't be negative unless that is modified elsewhere inside main.
  • You got a point -- it is set by the runtime and I wouldn't know offhand how to make it appear negative to main. Perhaps by supplying INT_MAX+7 command line arguments, but I am afraid there is a limit to the command line length somewhere before that.