Unexpected behaviour in a for loop

Question

This compiles and runs, but produces garbage values for "a". Why doesn't "a" increment like "b"? Why is it producing garbage?

for(a,b=0; a,b != 55; a,b++)
{
    //outputs garbage
    std::cout << "a = " << a << std::endl;
    //outputs expected results 
    std::cout << "b = " << b << std::endl;
}

you need to study c++ syntax -- in particular the comma operator doesn't do what you think it does. — Dale Wilson, Nov 02 '16 at 21:17
Of course it works! However, it may work in a way different from what you expected. How do you expect the program to work? — CiaPan, Nov 02 '16 at 21:18
@DaveB To be pedantic, it doesn't work for you since as written it contains undefined behavior. the `a` variable is uninitialized. — Dale Wilson, Nov 02 '16 at 21:22
@DaleWilson good point, I should have said I get the output I expected. — DaveB, Nov 02 '16 at 21:27
Seems the OP needs to define "doesn't work" -- not really clear what is expected here; are they expecting the `a` variable to magically define itself? -- to magically increment itself? -- (Because as defined, the program says that `a` outputs garbage, and that's exactly what it does, and it's exactly what you would expect it to do -- so, the OP will need to define their expected behavior a little better.) — BrainSlugs83, Nov 02 '16 at 21:33
@BrainSlugs83 Respectively, If I understood it, I wouldn't need to have asked it, the previous comments covered what needed to be asked and to me it seems like that was a bit much. — Semaphore, Nov 02 '16 at 21:57

Dale Wilson · Accepted Answer · 2016-11-03T14:37:48.593

The comma operator says execute the expression on the left then execute the expression on the right:

  a, b=0

first executes a which does nothing, then it executes b=0 which assigns zero to b.

Why does the comma operator exist? The comma operator can be useful when the expressions have side effects.

It also serves a sequence point which tell the compiler "everything on the left must be complete before anything on the right happens. This constrains the optimizations allowed by the compiler, so for example a += 1, b = a + c[a] will always add one to a before using it as an index. Something like b = ++a + c[a] is undefined because the compiler can increment a before or after it uses it as an index.

Unexpected behaviour in a for loop

1 Answers1