Problems in the for loop in language C - c

Question

The program compiles perfectly, the problem is the cycle, it does not show me the position
here goes the include of library stdio.h
int main(void)
{ int x, i=0,j=0, a[100];
char sal;
printf("Este programa lee un arreglo de numeros enteros y un numero x, y muestra en pantalla los indices de las posiciones en donde se encuentra ese numero x\n");
do
{
printf("Ingrese un numero: ");
scanf("%i", &a[i]);
i++;
printf("Si desea salir presione s: ");
scanf(" %c", &sal);
}while(sal!='s');
printf("Ingrese el valor de la variable x: ");
scanf("%i", &x);
for (j; j<=i; j++)
{
if(a[i]==x)
printf("%i",i);
}
printf("\n");
}

Answer 1

You condition of the for loop should be j<i, not j<i+1. When the while
loop exits, i has the value for the next input, but it has not been set because
the loop exited.
Also you are using the index i instead of j in the for-loop. j is the
running index, not i:
for (j; j<i; j++)
{
if(a[j]==x)
printf("%i", j);
}
would be correct.
Your while loop is ok, but a bit clunky. First you don't check if you are
writing past the limit of a. The conditions should be
while(sal!='s' && i < (sizeof a / sizeof *a));
so that the user cannot input more values than a can hold.
The way you exit the loop is also awkward, the user has to type something
different to s to continue and it can only be one character. This would be
better:
int c;
char line[100] = { 0 };
do
{
printf("Ingrese un numero: ");
scanf("%i", &a[i]);
while((c = getchar()) != '\n' && c != EOF); // to clear the input buffer
i++;
printf("Si desea salir ingrese SALIR. Para continuar presione ENTER: ");
fgets(line, sizeof line, stdin);
} while(strcmp(line, "SALIR\n") && strcmp(line, "salir\n") && i < (sizeof a / sizeof *a));
Note that strcmp returns 0 when the strings are equal, a non-zero otherwise.

Answer 2

A tip, you should always set the loop counter to 0 either right before the loop, or in the loop. And it's always a good practice to initialize all variables during declarations. In your case, you did not initialize integer array. Below is a modified version of your code.
int main(void)
{
int x, i, j;
x = i = j = 0;
int a[100] = { 0 };
char sal = 0;
printf ("Este programa lee un arreglo de numeros enteros y un numero x, "
"y muestra en pantalla los indices de las posiciones en donde se "
"encuentra ese numero x\n");
while(1)
{
printf("Ingrese un numero: ");
scanf("%d", &a[i]);
i++;
printf("Si desea salir presione s: ");
scanf(" %c", &sal);
if (sal == 's')
break;
}
printf("Ingrese el valor de la variable x: ");
scanf("%d", &x);
for (j = 0; j <= i; j++)
{
if (a[i] == x)
printf("%d", i);
}
printf("\n");
}