Is it possible for an if statement to say if something is not entered?

Question

I'm doing a project and I'm just curious to know if it is possible to have a line that says "if something is not entered" and a prompt statement would be followed.
For example,

if(id_ == NULL){printf("John Doe is absent.")}.

Just a curious question because I want to explore C programming a bit more.

what is the the input type? `int`? `float`? `char`? `char` array? — Sourav Ghosh, Mar 20 '15 at 09:31
What does "something is not entered" mean? What kind of thing? Entered where? — interjay, Mar 20 '15 at 09:31

score 1 · Answer 1 · edited May 23 '17 at 12:28

1

You can do this with scanf (or similar functions: fscanf, sscanf...). Assuming id_ is an int:

if(scanf("%d",&id_)!=1){
      printf("John Doe is absent.");
}

These functions return the number of input items successfully matched and assigned.

see top voted answer here for more info.

edited May 23 '17 at 12:28

Community

1
1

answered Mar 20 '15 at 09:36

yiabiten

792
8
27

score 0 · Answer 2 · answered Mar 20 '15 at 09:46

#include <stdio.h>

void input_id(int **id){
    int num;
    printf("input id:");
    if(scanf("%d", &num)==1)
        **id = num;
    else
        *id = NULL;
}

int main(void){
    int id;
    int *id_ = &id;

    input_id(&id_);
    if(id_ == NULL){
        printf("John Doe is absent.\n");
    } else {
        printf("id : %d\n", id);
    }
    return 0;
}

Is it possible for an if statement to say if something is not entered?

2 Answers2