I keep getting an error when declaring a variable inside a loop. what's the problem?? (C)

Question

#include <stdio.h>
#include <string.h>

int main()
{
    char input[100], output[100];
    gets(input);

    for(int i=0, a=0; i<strlen(input); a++)
    {
        char word = input[i];
        output[a] = word;
        if(word == 'a' || word == 'e' || word == 'i' || word == 'o' || word == 'u') i+=3;
        else i+=1;  
    }

    output[a] = '\0';    
    puts(output);
}

It says a is not declared, but didn't i declare it in the loop? How to declare a variable inside a loop without getting an error?

Stop using `gets` immediately, it has been removed from the language because it has no bounds checking. — Barmar, Jan 19 '23 at 18:35
`a` (and `i`) goes out of scope when the `for` loop ends. Declare `int a = 0;` _before_ the `for` loop if you need it _after_ the `for` loop — Ted Lyngmo, Jan 19 '23 at 18:35
The scope of the variable is the loop body. You can't access it outside. — Barmar, Jan 19 '23 at 18:35
... and as @Barmar mentioned: Never use `gets`. Use `fgets(input, sizeof input, stdin);` instead (and check the return value). — Ted Lyngmo, Jan 19 '23 at 18:37
I suggest that you read this: [Why is the gets function so dangerous that it should not be used?](https://stackoverflow.com/q/1694036/12149471) — Andreas Wenzel, Jan 19 '23 at 20:55
`if (!fgets (input, sizeof input, stdin) { return 1; } input[strcspn (input, "\n")] = 0;` is your replacement for `gets()` that also trims the `'\n'` from the end of `input` by overwriting with the null-byte. — David C. Rankin, Jan 20 '23 at 06:16

Harith · Answer 1 · 2023-01-19T18:48:58.900

Local scope :

In

for(int i=0, a=0; i<strlen(input); a++)

a goes out of scope, or is destroyed, once the loop exits.

From C11:

For such an object that does not have a variable length array type, its lifetime extends from entry into the block with which it is associated until execution of that block ends in any way.

Fix:

Declare a outside the loop.

Using gets():

The function gets() is inherently dangerous, and has been removed from the C standard. It provides no bounds checking and can potentially cause a buffer overflow. Instead, use fgets.

See: Why is the gets function so dangerous that it should not be used?

[Demo](https://godbolt.org/z/8fa198fcM) with the issues fixed — Ted Lyngmo, Jan 19 '23 at 18:42

score 1 · Answer 2 · answered Jan 19 '23 at 19:17

You're getting that error because you're using a outside the body of the loop:

output[a] = '\0';

a is only visible within the body of the for loop, and once the loop exits it no longer exists.

The easy way to deal with it is to declare a before the loop:

int a = 0;
for ( int i = 0; i < strlen( input ); a++ )
  ...

I keep getting an error when declaring a variable inside a loop. what's the problem?? (C)

2 Answers2

Local scope :

Using gets():