CODEWITHSUNDEEP
carraysscanfspace
Emilio Ferrucci
Emilio Ferrucci

Reputation: 627

How exactly does space work with scanf?

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.

Upvotes: 4

Views: 536

Answers (5)

Yevgeny Simkin
Yevgeny Simkin

Reputation: 28379

Improving on Mir Milad Hosseiny answer (I wrongly identified it as being an out of control infinite loop... it's actually exactly the infinite loop I describe in my comment)...

I would write a small function that has either a "white list" (things you want) or a "black list" things you don't want, and check each value to either belong or not (depending on the approach) to the list. That way you can keep a separate place where your store the values that you are willing to accept or the values you are not, so your primary function doesn't get really messy with exceptions or inclusions in the "if"

so your code would be 

if(isAllowed(a[i]){
   myList[j] = a[i]; //j is your alternate counter
}

Upvotes: 1

Mir Milad Hosseiny
Mir Milad Hosseiny

Reputation: 2857

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]);
    }
}

Upvotes: 1

asaelr
asaelr

Reputation: 5456

You could do something like this: 

int i,a[6];
for (int i=0;i<6;i++) {
 scan: scanf("%d",&a[i]);
}
for (int i=0;i<6;i++) if (a[i]==4) {
 printf("4 is not allowed. re-enter the last %d numbers\n",6-i);
 goto scan;
}

note that in most case, it's better to avoid using goto, but in this case I think that it's natural.

If you really want, you can print the first i numbers (before the goto), but it's complicated (and platform-depended) to let the user change those numbers.

Upvotes: 1

Ben Voigt
Ben Voigt

Reputation: 283733

This is related to your terminal being in "cooked" mode. Characters aren't even sent to the program until the user presses enter.

Upvotes: 1

Mir Milad Hosseiny
Mir Milad Hosseiny

Reputation: 2857

In for loop, after each iteration, the counter add by one automatically. If you get an invalid input, you should prevent the counter increasing. To do this, just add i--; to your code when you give an invalid input.

#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");
           i--;
        }
    }
    for (i = 0; i < 6; i++) {
        printf("%d ", a[i]);
    }
}

Upvotes: 2

Related Questions