Question

Look at this example:

int i;
for (i=1;i.......

and this:

for (int i=1;i........

What’s the difference between them?

Answer 1

The first one declares the variable in the scope outside of the loop; after the loop has ended, the variable will still exist and be usable. The second one declares the variable such that it belongs to the scope of the loop; after the loop, the variable ceases to exist, preventing the variable from being inadvertantly/mistakenly used.

In C99, C++, Java, and other similar languages, you will find mostly the second syntax as it is safer — the loop index belongs to the loop and isn’t modified / shared elsewhere. However, you will see a lot of the former in older C code, as ANSI C did not allow the loop variable to be declared in the loop like that.

To give an example:

int i;
// ... lots of stuff
for ( i = 0; i < 5; i++ ){
    printf("%d\n",i); // can access i; prints value of i
}
printf("%d\n",i); // can access i; prints 5

By contrast:

for (int i = 0; i < 5; i++ ){
    std::cout << i << std::endl; // can access i; prints value of i
}
std::cout << i << std::endl; // compiler error... i not in this scope