How to write a for statement that iterates through two arrays in c++

Question

I have completed an assignment, but Xcode is giving me a warning about the syntax of my for loop, and I'm not sure how to change it. I am new to programming. I am taking a course.

The assignment is to create two char arrays. One for month names, the other for total days in each month, then print both arrays combined sequentially (Jan 31 Feb 28 etc).

This is what I came up with:

f()
{
    std::string months[] = { "January", "February", "March" };
    std::string days[] = { "31", "28", "31" };

    stringstream ss;

    for (auto i = 0, j = 0; i < 3, j < 3; ++i, ++j)

    {
        ss << months[i] << " " << days[j] << " ";
    }

}

The compiler doesn't like the middle part of the for statement, the test to stop. The message I get says "Expression result unused".

I confess I'm mostly mimicking the samples I've seen from class, but we've never done a loop that evaluates two arrays. I tried a range for loop, but I can't get that to work at all.

Answer 1

Why would you use two indices? Just write a regular loop:

    for (int i = 0; i < 3; ++i) {
        ss << months[i] << " " << days[i] << " ";
    }

Answer 2

stringstream ss;

Replace with std::ostringstream ss;.

for (auto i = 0, j = 0; i < 3, j < 3; ++i, ++j)
{
    ss << months[i] << " " << days[j] << " ";
}

In i < 3, j < 3, the comma operator is used. As http://en.cppreference.com/w/cpp/language/operator_other#Built-in_comma_operator says:

In a comma expression E1, E2, the expression E1 is evaluated, its return value is discarded, and its side effects are completed before evaluation of the expression E2 begins [...]. The return type and value category of the comma operator are exactly the return type and the value category of the second operand, E2.

In other words, the i < 3 part is completely ignored (it does not have any side effects, either), and your code is equivalent to for (auto i = 0, j = 0; j < 3; ++i, ++j).

Here is a simpler example to show this behaviour:

#include <iostream>

int main()
{
    int i = 0;
    std::cout << (i != 0, i == 0);
}

This prints 1, because only the i == 0 is considered.

To me, the question is: What exactly do you want to do? Do you want to print both arrays? In that case, you do not need two different index variables at all:

for (auto i = 0; i < 3; ++i)
{
    ss << months[i] << " " << days[i] << " ";
}

There is still room for improvement, though. For example, you may want to assert that both arrays have the same size before iterating them, and you may want to get rid of the magic number 3 in the loop.

Answer 3

Change this:

i < 3, j < 3

to this:

i < 3 && j < 3

Answer 4

Use a single index, and use a single array while you're at it:

void f()
{
    struct
    {
        std::string month;
        std::string days;
    } calendar[] = {
        { "January",  "31" },
        { "February", "28" },
        { "March",    "31" }
    };

    for(auto i=0; i<ARRAY_SIZE(calendar); ++i)
    {
        ss << calendar[i].month << " " << caledar[i].days << " ";
    }
}