我在 for 循环中声明了一个 i 变量。 但我无法在 for 循环之外打印它。 所以 i 就像 for 循环中的局部变量？

Question

#include<stdio.h>

void alldivisor(int n){

    for (int i=1; i*i<n; i++) {
       if (n%i==0) {
            printf("%d ",i);
       }
    }
          printf("\n%d",i);
}


int main(){
    int n;
    
    printf("Enter a num: ");
   
    scanf("%d",&n);
   
    alldivisor(n);
}

Answer 1

Yes, i is a local variable inside the loop.

Try:

#include <stdio.h>

void alldivisor(int n){
    int i = 1;
    for (; i*i<n; i++) {
       if ((n % i) == 0) {
            printf("%d ",i);
       }
    }
    printf("\n%d",i);
}


int main(){
    int n;
    
    printf("Enter a num: ");
   
    scanf("%d",&n);
   
    alldivisor(n);
}

A clear understanding in C should be that any variables declared inside the curly braces {} is inside the scope of the of braces. Accessing it somehow using memory address tricks after leaving the scope is UNDEFINED BEHAVIOUR .

Tricks like these is undefined, for example, so do not try them ever:

...
int *ptr = NULL;
{
  int a = 1;
  ptr = &a;
}
/* Accessing address of 'a' is undefined behaviour */
printf("%d\n", *ptr);
...

So, you must define the variable i inside the scope of the function alldivisor() ;

Also, you might see things like:

...
for (int i = 0; i < x; i++)
    /* one operation */
...

It is a shorthand when you only need to execute one operation for each time in the loop. It also creates a new scope even though curly braces are missing.

Important thing is that two scopes are created for for loops, one for the entire loop, hence you can access i , another for the body, hence you cannot access any declarations inside the body of the loop in the condition box.

Answer 2

You must define the variable i in the scope of the alldivisor() function:

#include<stdio.h>

void alldivisor(int n)
{
    /* The variable "i" must be defined in the global scope of the "alldivisor()"*/
    int i;
    
    for(i = 1 ; i * i < n; i++)
    {
       if(n % i == 0)
       {
            printf("%d ",i);
       }
    }
    printf("\nResult: %d",i);
}

int main()
{ 
    int n;
    printf("Enter a num: ");
    scanf("%d",&n);
    alldivisor(n); 
    return 0;
}

Answer 3

From for loop :

for ( init-clause; cond-expression; iteration-expression ) loop-statement

As with all other selection and iteration statements, the for statement establishes block scope: any identifier introduced in the init-clause, cond-expression, or iteration-expression goes out of scope after the loop-statement .