C++ for-loop structure with multiple variable initialization

Question

On the 2nd for-loop, I get the following error from gcc:

error: expected unqualified-id before 'int'

I'm not sure what I'm missing. I've looked over documentation for how a for-loop should look and I'm still confused. What's wrong here?

#include <iostream>
#include <vector>

int main() { 
std::vector<int> values; 

for (int i = 0; i < 20; i++) { 
  values.push_back(i); 
}   

std::cout << "Reading values from 'std::vector values'" << std::endl;
for (int i = 0, int col = 0; i < values.size(); i++, col++) {
  if (col > 10) { std::cout << std::endl; col == 0; }
  std::endl << values[i] << ' ';
  }
}

score 16 · Accepted Answer · answered Jan 09 '12 at 04:22

16

Try without the int before col.

for (int i = 0, col = 0; i < values.size(); i++, col++)

answered Jan 09 '12 at 04:22

jweyrich

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Thank you - I didn't notice this at all. – jdphenix Jan 09 '12 at 04:40
Can we perform logical operation between the two conditions pertaining to the variables `i` and `j` in placeholder for condition in the `for` loop? – Chaitanya May 07 '13 at 10:55

score 7 · Answer 2 · answered Jan 09 '12 at 04:25

7

Others have already told you how to fix the problem you've noticed. On a rather different note, in this:

if (col > 10) { std::cout << std::endl; col == 0; }

It seems nearly certain that the last statement here: col==0; is really intended to be col=0;.

answered Jan 09 '12 at 04:25

Jerry Coffin

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That would be true - this was a simple test case I put together for the question. Good catch – jdphenix Jan 09 '12 at 04:42

score 3 · Answer 3 · answered Jan 09 '12 at 04:23

3

This should fix it

for (int i = 0, col = 0; i < values.size(); i++, col++) {
  if (col > 10) { std::cout << std::endl; col == 0; }
  std::endl << values[i] << ' ';
  }
}

A variable definition goes like this

datatype variable_name[=init_value][,variable_name[=init_value]]*;

answered Jan 09 '12 at 04:23

Chethan Ravindranath

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1

The last sentence is not really true. For example, the following is valid: `int w, *x, (*y)(int, int), z;` – James McNellis Jan 09 '12 at 04:29
I agree! I couldn't find the c++ standards' definition of variable declaration. Hence have given an abstract representation of a var declaration. here variable name doesn't strictly confirm to [a-zA-Z][a-zA-Z0-9]*. It can also mean *x, e.g. int w = 10, *x = NULL; Thanks for clarifying ! :) – Chethan Ravindranath Jan 09 '12 at 04:39

score 2 · Answer 4 · answered Jan 09 '12 at 04:32

2

Don't declare int after comma use,

for (int i = 0,col = 0; i < values.size(); i++, col++) {
  if (col > 10) { std::cout << std::endl; col == 0; }
  std::endl << values[i] << ' ';
  }
}

answered Jan 09 '12 at 04:32

Kalpesh Patel

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score 1 · Answer 5 · answered Feb 10 '17 at 17:44

1

This is akin to a regular multiple variables declaration/initialization in one line using a comma operator. You can do this:

int a = 1, b = 2;

declaring 2 ints. But not this:

int a = 1, int b = 2;   //ERROR

answered Feb 10 '17 at 17:44

def

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C++ for-loop structure with multiple variable initialization

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