C/C++ for loop with existing start value

Question

Here's a C/C++ for loop:

int i;
for (i = myVar; i != someCondition(); i++)
  doSomething();
// i is now myVar plus the number of iterations until someCondition

I recently had to use a loop like this. I needed to keep the value of i because I wanted to know what i was when the return value of someCondition() became true. And the start value of i was myVar which had no further reason of existing. So what wanted to do was:

for (myVar; myVar != someCondition(); myVar++)
  doSomething();
// myVar is now myVar + the number of iterations.

This made a lot more sense to me. I didn't see why I had to use a whole new variable when myVar was just what I needed. But this is not valid code. Is there a way around creating a whole new variable for this situation?

Answer 1

What you need is,

for( ; myVar != someCondition(); myVar++)
       doSomething();

But you statement about the following loop being incorrect is wrong,

for (myVar; myVar != someCondition(); myVar++)
  doSomething();

The above code will also work fine in C.

Answer 2

I think this is what you're after:

for ( ; myVar != someCondition(); myVar++)
    doSomething();

Answer 3

I feel that a while loop more closely resembles your intentions. Indeed, you are doing something while someCondition() is true , and increasing myVar is a side effect.

while(myvar != someCondition()) {
 doSomething();
 myVar++;
}

To be clear: the statements are equivalent. I am just advocating for what I believe to be more idiomatic code.

You could even use a do / while loop, but it somewhat does not feel idiomatic to me. Below you will find three examples; they all do the same, but feel different. Take your pick!

#include<iostream>

int someCondition() {
  return 10;
}

void doSomething(int myVar) {
  std::cout<<"... I'm doing something with myVar = "<<myVar<<std::endl;
}

int using_for() {
  int myVar = 7;
  for( ; myVar!=someCondition(); myVar++) {
    doSomething(myVar);
  }
  return myVar;
}

int using_while() {
  int myVar = 7;
  while(myVar != someCondition()) {
    doSomething(myVar);
    myVar++;
  }
  return myVar;
}

int using_do() {
  int myVar = 7;
  do {
    doSomething(myVar);
  } while(++myVar != someCondition());
  return myVar;

}

int main() {
  std::cout<<"using for: "<<using_for()<<std::endl;
  std::cout<<"using while: "<<using_while()<<std::endl;
  std::cout<<"using do/while: "<<using_do()<<std::endl;

}

Output:

... I'm doing something with myVar = 7
... I'm doing something with myVar = 8
... I'm doing something with myVar = 9
using for: 10
... I'm doing something with myVar = 7
... I'm doing something with myVar = 8
... I'm doing something with myVar = 9
using while: 10
... I'm doing something with myVar = 7
... I'm doing something with myVar = 8
... I'm doing something with myVar = 9
using do/while: 10