C++ For-Loop – Structure with Multiple Variable Initialization

c++for-loop

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] << ' ';
  }
}

Best Answer

Try without the int before col.

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

Related Solutions

C++ – Declaring and Initializing Variable in a For Loop

It's valid in C++.

It was not legal in the original version of C.
But was adopted as part of C in C99 (when some C++ features were sort of back ported to C)
Using gcc

gcc -std=c99 <file>.c

The variable is valid inside the for statement and the statement that is looped over. If this is a block statement then it is valid for the whole of the block.

for(int loop = 0; loop < 10; ++loop)
{
    // loop valid in here aswell
}

// loop NOT valid here.

C++ Nested Loops – How to Use Variable Nested For Loops

For efficiency, I've avoided recursion. Also, it doesn't use any specific c++ stuff - it will work fine on C as well.

We're trying to create N nested "for" loops. Instead of using

for(int i = 0; i<max; i++)
  for (int j = 0; j<max; j++)
    ...

I'll be replacing i, j, ... with an array: i[0], i[1], ..., i[n-1].

Here's my solution:

const int n = /*Insert N here: how many loops do you need?*/;
int i[n+1]; // if "n" is not known before hand, then this array will need to be created dynamically.
//Note: there is an extra element at the end of the array, in order to keep track of whether to exit the array.

for (int a=0; a<n+1; a++) {
  i[a]=0;
}

int MAX = 79; //That's just an example, if all of the loops are identical: e.g. "for(int i=0; i<79; i++)". If the value of MAX changes for each loop, then make MAX an array instead: (new) int MAX [n]; MAX[0]=10; MAX[1]=20;...;MAX[n-1]=whatever.

int p = 0; //Used to increment all of the indicies correctly, at the end of each loop.
while (i[n]==0) {//Remember, you're only using indicies i[0], ..., i[n-1]. The (n+1)th index, i[n], is just to check whether to the nested loop stuff has finished.

  //DO STUFF HERE. Pretend you're inside your nested for loops. The more usual i,j,k,... have been replaced here with i[0], i[1], ..., i[n-1].


  //Now, after you've done your stuff, we need to increment all of the indicies correctly.
  i[0]++;
  // p = 0;//Commented out, because it's replaced by a more efficient alternative below.
  while(i[p]==MAX) {//(or "MAX[p]" if each "for" loop is different. Note that from an English point of view, this is more like "if(i[p]==MAX". (Initially i[0]) If this is true, then i[p] is reset to 0, and i[p+1] is incremented.
    i[p]=0;
    i[++p]++; //increase p by 1, and increase the next (p+1)th index
    if(i[p]!=MAX)
      p=0;//Alternatively, "p=0" can be inserted above (currently commented-out). This one's more efficient though, since it only resets p when it actually needs to be reset!
  }
}

There, that's all. Hopefully the comments make it clear what it's meant to be doing. I think it should be pretty efficient - almost as much as real nested for-loops. Most of the overhead is a one-off at the beginning, so this should be more efficient that using recursive functions etc

Best Answer

Related Solutions

C++ – Declaring and Initializing Variable in a For Loop

C++ Nested Loops – How to Use Variable Nested For Loops

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