关于“ for（a，b && c，d）{…}”和“ for（a，b，＆d）if（c）{…}”之间的区别

Question

As a programmer of long standing, it's sobering to realise that even the humble for loop is not fully comprehended. Why does the following program print a single 1 to the console? I fully expect the first loop to also produce a 1 ! Compiled with -ansi switch.

/* gcc installed version: 4:4.4.4-1ubuntu2 */

#include <stdio.h>

#define SIZE 2

int main()
{
  int i;  
  int a[SIZE];

  a[0]=0; 
  a[1]=1;

  for(i=0; (i<SIZE) && (a[i]!=0); i++)        
    printf("%i\n",a[i]); 

  for(i=0; i<SIZE; i++)   
    if (a[i]!=0)  
      printf("%i\n",a[i]);      

  return 0;      
}

Answer 1

the first loop is equivalent to:

for(i=0; ; i++) {
    if( !((i<SIZE) && (a[i]!=0)))
        break;      
    printf("%i\n",a[i]); 
}

Answer 2

It's the difference between this:

for(i=0; i<SIZE; i++)   
  if (a[i]!=0)  
    printf("%i\n",a[i]);
  else
    continue; // implicit in your version with the if statement

and this:

for(i=0; i<SIZE; i++)   
  if (a[i]!=0)  
    printf("%i\n",a[i]);
  else
    break; // equivalent of what the non if statement version does.

(Credit goes to @ta.speot.is for actually spotting the problem, this is merely an explanation)