Why printf() is not used when scanf returns negative number?

Question

I have written a code which checks when scanf() returns a negative number, for example if I press "Ctrl+Z" it should get out of the while loop and printf("Finish!") , but instead it is not printing the "finish!" can anybody take a look?

#include <stdio.h>
#include <conio.h>
#include <math.h>

#define G 9.81

float Height(float speed, float angle, float time);
float Horizontal(float speed, float angle, float time);


void main()
{
    float speed;
    float angle;
    float time = 0.1;
    float height = 0;
    float horizontal = 0;
    int res = 0;

    printf("Enter v <0.0 - 100.0 m/s> and a <0-90 degrees>: ");
    res = scanf_s("%f %f", &speed, &angle);
    while (res != -1)
    {
        for (time = 0.1; height >= 0; time += 0.1)
        {
            printf("Time: %.1f .... H = %.2f  S = %.2f \n", time, horizontal = Horizontal(speed, angle, time), height = Height(speed, angle, time));
        }
        height = 0;
        printf("Fallen!\n");
        printf("Enter v <0.0 - 100.0 m/s> and a <0-90 degrees>: ");
        res = scanf_s("%f %f", &speed, &angle);
    }
    printf("\nFinish!\n");
    getch();
}

float Height(float speed, float angle, float time)
{
    float height;
    angle = ((3.14 / 180) * angle);
    height = (speed * sin(angle) * time) - ((G*time*time) / 2);

    return height;
}

float Horizontal(float speed, float angle, float time)
{
    float horizontal;
    angle = ((3.14 / 180) * angle);
    horizontal = (speed * cos(angle)) * time;

    return horizontal;
}

I'm using Visual Studio (C language) on Windows.

Why would scanf() return -1. ( not a rhetorical question, I'm really interested ) ? — Transcendental
return value of the scanf function is number of successful conversions, ie. arguments , input conversion. upon failure it returns -1 — Fawzan
@IlanAizelmanWS Yes, but why would it fail if you pressed Ctrl + Z? — Transcendental
You mention scanf in your question, but your code uses scanf_s, which is not a standard function (I think it's a Microsoft thing?). I think you're also assuming that EOF is -1, which is not guaranteed to be true in general (though it may be in the MS case, I don't know and don't have a compiler handy to check!). The scanf function returns the number of fields assigned - not characters written - or EOF on error, or if when reading the first character it gets end-of-file. I think to force an end-of-file condition you have to press Enter-Ctrl+Z-Enter, so that might be the issue? — psmears
@Transcendental Because scanf should get 2 floats. if it gets (Ctrl+Z) it is not a float. which means it will return -1 — Ilan Aizelman WS

Christophe Christophe · Accepted Answer · 2015-11-28T16:00:39

Here the reference for scanf(): http://www.cplusplus.com/reference/cstdio/scanf/

When scanf() encounters end of file (that's the usual meaning of ctrl+z on the console), it is not a matching error: it's only that a part of the expected items could be read.

Make your while use >0 instead of >=0.

Why printf() is not used when scanf returns negative number?

3 Answers