What is the purpose of the increment statement? - c++

Question

Why are increment statements a thing in for-loops in C++? To me it seems redundant, because you could simply put the increments inside the conditional code. Am I misunderstanding something important here?
To illustrate my question better, I'm including some pseudocode:
What is the difference between ->
for( int a = 10; a < 20; a = a + 1 )
{
cout << a << endl;
}
and
for( int a = 10; a < 20;)
{
a = a + 1
cout << a << endl;
}

Answer 1

It's more than mere convenience sometimes.
These are equivalent:
for (int a = 10; a < 20; a = a + 1) {
cout << a << endl;
}
for (int a = 10; a < 20; ) {
cout << a << endl;
a = a + 1;
}
But, these are not:
// this works ...
for (int a = 10; a < 20; a = a + 1) {
if (blah ...)
continue;
cout << a << endl;
}
// this doesn't
for (int a = 10; a < 20; ) {
if (blah ...)
continue;
cout << a << endl;
a = a + 1;
}
Since you're coming from python, an idiomatic for loop is like a python range, but much more powerful. Your C for loop, expressed in python would be:
for a in range(10,20,1)
It's more idiomatic to express this as:
for (a = 10; a < 20; a += 1)
Because the loop increment is 1, it's even more idiomatic to do this:
for (a = 10; a < 20; ++a)
But, for loops are:
for ([init_stmt]; [test_stmt]; [incr_stmt])
Where any of the *_stmt can be compound:
for (x = 0, y = 0; x < 10; ++x, y += 2)

Answer 2

Convenience.
However, your equivalent code should be:
for (int a = 10; a < 20;)
{
cout << a << endl;
a = a + 1;
}
It runs at the end of the loop body.

Answer 3

[ snips grumbling about quality of now deleted/ edited answers ;-) ]
This:
for (unsigned counter = 1; counter <= 10; ++counter) {
doStuff();
}
is largely equivalent to this:
unsigned counter = 1;
while (counter <= 10) {
doStuff();
++counter;
}
with the notable exception that, in the 1st case, you have the considerable benefit that counter is scoped only to within the for block and automatically goes out-of-scope as soon as it finishes - whereas with the latter, counter must remain in-scope after the loop, where it's potentially useless or even an obstacle.
(tangential: Note that C did not support within-for declaration, or any non-top-of-block declarations, until C99 - but barring extenuating circumstances, anyone not using at least C99 by now is making a questionable choice imho.)
edit: Craig also makes a very good point regarding continue - an oft-forgotten but certainly useful statement. I'm sure there are probably other differences we could conjure up.

Answer 4

for this example:
using namespace std;
int main(int argc, char** argv) {
for( int a = 10; a < 20;)
{
a = a + 1;
cout << a << endl;
}
return 0;
}
the output will be from 11-->20
the first example will be from 10-->19
your are putting the increment part outside the loop and this possible, but notice that the value 10 will not appear, because you are increment before printing the value of a
so in the 2nd example your printing the value and then increment, and at the end of the loop, you are quiting the loop without reaching 20, because the condition get you out the loop
executing code block before increment is the key for you, the for loop increment just after the code block is executed

Answer 5

Well it is not required, it is just for convenience.
In your second code, you made a little mistake which would make the code nonequivalent to the the first one.
Your increment should be at the end of loop in order to be equivalent to the first code
so it should rather be:
for( int a = 10; a < 20;)
{
cout << a << endl;
a = a + 1; //increment at the end of instructions
}
These little errors and also errors like forgetting to include your increment is why it is convenient to include the increment in the for loop.
Or you can use the while loop instead:
while (condition)
{//instructions here;}