Question

I am a math student, and I’m learning the very basics in programming in C. I need a program to read an input consisting in an array, the components of which must have certain requisites; I would like the program to ask the user for the components of the array. The user should then have to enter such components separating them with spaces. The details aren’t important to get the main question across; I’ll choose a simpler example then the one I am dealing with: let’s say I want an array with 6 components not to contain the number 4. So I tried:

#include <stdio.h>

int main(void) {
    int a[6];
    printf("enter components: ");
    int i;
    for (i = 0; i < 6; i++) {
        scanf("%d", &a[i]);
        if (a[i] == 4) printf(" \n\n4 is not allowed, try again\n\n");
    }
    for (i = 0; i < 6; i++) {
        printf("%d ", a[i]);
    }
}

If I compile this and run it, and for example enter:

1 2 3 4 5 6

I will get my error message, but only after having pressed enter, that is after having entered all six components (not straight after having pressed space for the fourth time). So here are my questions (I am looking for solutions which don’t make use of strings or pointers, unless it is impossible to do without them):

1. Is there a way to get the program to read a component (and to act accordingly) straight after its subsequent space has been entered? I’m guessing there isn’t because scanf only works after the user presses enter, and not space, correct?

2. If there isn’t, is there a way to get the program to read the components all at once after having pressed enter at the end, but letting the user pick up from the last right component? For example, with the above input, I would like the program to display something like this:

   4 is not allowed
   1 2 3 _
   

   so that the user can correct his/her input (possibly changing the first three digits as well).

Sorry if this question is too dumb! Thank you for your help!!

---

EDIT: Well, thanks for the great answers, you have all been very helpful! It’s a pity I can’t accept more than one.

Answer 1

Please see the bellow code:

#include <stdio.h>

int main(void) {
    int a[6];
    int i;
    bool hasError = false;
    int errorIndex = 0;
    do{
        hasError = false;
        printf("enter components: ");
        for (i = 0; i < errorIndex; i++)
            printf("%d ", a[i]);

        for (i = errorIndex; i < 6; i++) {
            scanf("%d", &a[i]);
            if (a[i] == 4 && hasError == false){
                printf(" \n\n4 is not allowed, try again\n\n");
                hasError = true;
                errorIndex = i;
            }
        }
    }while(hasError == true);
    for (i = 0; i < 6; i++) {
        printf("%d ", a[i]);
    }
}