Member Avatar for hawita

Hi, what does this mean in c++?

sum += c
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As , and already covered the basic idea, the following adds a few practical notes about compound assignments and a compact C++ routine to check palindromes (since asked about that as well).

Compound-assignment notes (practical):

  • Common compound forms include +=, -=, *=, /=, %=, <<=, >>=, &=, |=, ^=. The right-hand side is converted to the left-hand type as part of the operation, so watch signed/unsigned or narrowing conversions.
  • For user-defined types implement operator+= as a member that returns *this by reference; this allows chaining and avoids unnecessary temporaries. Example pattern:
struct Vec {
    int x, y;
    Vec& operator+=(const Vec& o) {
        x += o.x;
        y += o.y;
        return *this;
    }
};
  • Using += on standard types like std::string typically modifies in-place and is more efficient than forming a = a + b (fewer temporaries).
  • Avoid expressions that modify the same object more than once in a single full-expression (for example, a += ++a): such patterns are not safe and lead to undefined behavior.

Palindrome check (robust, O(n), O(1) extra space):

  • A palindrome reads the same forward and backward (examples: racecar, level, phrases like A man, a plan, a canal, Panama when non-alphanumerics and case are ignored). The two-pointer method scans from both ends and skips non-alphanumeric characters.
#include <string>
#include <cctype>

bool is_palindrome(const std::string& s) {
    size_t i = 0, j = s.size();
    if (j == 0) return true;
    --j;
    while (i < j) {
        while (i < j && !std::isalnum(static_cast<unsigned char>(s[i]))) ++i;
        while (i < j && !std::isalnum(static_cast<unsigned char>(s[j]))) --j;
        if (std::tolower(static_cast<unsigned char>(s[i])) != std::tolower(static_cast<unsigned char>(s[j])))
            return false;
        ++i; --j;
    }
    return true;
}

The cast to unsigned char before std::isalnum/std::tolower avoids locale/negative-char issues.

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Member Avatar for hag++

That is the addition assignment operator... You are taking the value of c and adding it to the current value of sum. So if sum is 5 and c is 2 then after that operation sum will equal 7.

Edited by hag++ because: n/a
Member Avatar for Narue

It's functionally equivalent to sum = sum + c .

Member Avatar for Fbody

The C++ language has "compound" operators. The '+=' operator is used to increase the current value of the Left-Hand operand by the value of the Right-Hand operand.

Thus, the statements:

int a = 10;
a += 10;

Would cause an integer variable, called 'a', to be declared and initialized to (10), and subsequently be increased by (10) to a value of (20). It produces the same net result as:

int a = 10;
a = (a + 10);

EDIT:
Oops, Narue snuck in there.

Edited by Fbody because: n/a
Member Avatar for hawita

Oh okey thank you and it will be the same with any other operator right?

Member Avatar for Fbody

Not all, but most of the arithmetic operators.

Have a look at this.

Member Avatar for hawita

oh okey thank you a lot. Also do you think you can help me understand what a palindrome is?

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