Program skips if statements

Question

I've written a program with a lot of if sections.It´s written with Visual Studio 2013(scanf_s). It skips some if sections though these are met. Can you please tell me why? My suspicion: The first scanf command is executed cleanly. The other scanf commands don't work. I can't input anything. The program goes strictly on. When I insert fflush(stdin) between the scanf commands, it works. I heard bad things about fflsuh because of this I wanna ask: How can I solve it in another way?

Here is my code:

#include "stdafx.h"
#include <stdio.h>
#include <stdlib.h>

int _tmain(int argc, _TCHAR* argv[])
{
char versand='n', stammkunde='t';
double warenwert=1;
printf("Wieviel kostet die Ware?");
scanf_s("%lf", &warenwert);
fflush(stdin);
printf("Wird die Ware abgeholt?(y,n)");
scanf_s("%c", &versand);
if (versand == 'n')
{
    if (warenwert < 100)
    {
        warenwert = warenwert + 7;
    }
    printf("Expressversand?(y,n");
    scanf_s("%c", &versand);
        //fflush(stdin); 

    if (versand == 'y')
    {
        warenwert = warenwert + 10;
    }
}
printf("Stammkunde?(y,n)");
scanf_s("%c", &stammkunde);
if (stammkunde = 'y')
{
    warenwert = warenwert * 0, 97;
}
printf("Endpreis inkl. Versandkosten:%lf", warenwert);
getchar();
return 0;
}

P.S: Program output screenshot here: http://i.gyazo.com/01471ce3d563837f526fbcab8363e1f2.png

If the scope of the if statement isn't executed the conditions aren't met. Did you try and echo back what it received? — bzeaman, Sep 05 '14 at 11:18
Please compile with all warnings and debug info (e.g. `gcc -Wall -g`) then **use the debugger** (e.g. `gdb`) — Basile Starynkevitch, Sep 05 '14 at 11:18
`if (stammkunde = 'y')` is wrong. It should be `if (stammkunde == 'y')`. — Jayesh Bhoi, Sep 05 '14 at 11:20
Why aren't you checking the return value of `scanf_s`? Then you don't need to rely on your suspicions. — Jonathan Wakely, Sep 05 '14 at 11:20
output screenshot here: http://i.gyazo.com/01471ce3d563837f526fbcab8363e1f2.png — Harald Wiesinger, Sep 05 '14 at 11:23
you need to flush the stdin... check here for reference http://stackoverflow.com/questions/17248442/flushing-stdin-after-every-input-which-approach-is-not-buggy — madduci, Sep 05 '14 at 11:25

Sathish · Accepted Answer · 2014-09-06T05:35:37.887

3

printf("Wird die Ware abgeholt?(y,n)");
scanf_s("%c", &versand);

When you enter input and hit the ENTER key, a character and return key are placed in the input buffer, they are namely: the entered character and the newline character.The character gets consumed by the scanf_s but the newline remains in the input buffer.

Further,

printf("Expressversand?(y,n");
scanf_s("%c", &versand);

Your next scanf_s for reading the character just reads/consumes the newline and hence never waits for user input.

Way 1: Solution is to consume the extra newline by using:

scanf_s(" %c", &versand);
         ^  ---- note the space!

Way 2: You can try this also-

fflush(stdin); // flush the stdin before scanning input!
printf("Expressversand?(y,n");
scanf_s("%c", &versand);

Fix this following bugs also-

printf("Stammkunde?(y,n)");
scanf_s(" %c", &stammkunde); // give space before %c
if (stammkunde == 'y') // for comparison use == not =
{
    warenwert = warenwert * 0, 97;
}

Edit: In this equation

warenwert = warenwert * 0, 97;

warenwert * 0 have evaluated first, due to high priority. so

warenwert = 0 , 97;

Here = has high priority then , operator. so warenwert = 0 is assigned first. So you will get the result is 0 whenever this if (stammkunde = 'y') condition is true

Sample Run1:-

sathish@ubuntu:~/c/basics$ ./a.out 
Wieviel kostet die Ware?
2
Wird die Ware abgeholt?(y,n)
n
Expressversand?(y,n)
y
Stammkunde?(y,n)
n
Endpreis inkl. Versandkosten:19.000000

Run 2:-

sathish@ubuntu:~/c/basics$ ./a.out 
Wieviel kostet die Ware?
2
Wird die Ware abgeholt?(y,n)
n
Expressversand?(y,n)
y
Stammkunde?(y,n) // here your input value becomes 19, due to last condition it becomes zero!
y
Endpreis inkl. Versandkosten:0.000000

edited Sep 06 '14 at 05:35

answered Sep 05 '14 at 11:18

Sathish

3,740
1
17
28

`scanf_s` require the buffer size to be specified for parameters of `%c`, `%s`. http://msdn.microsoft.com/en-us/library/w40768et.aspx – BLUEPIXY Sep 05 '14 at 11:42
`fflush(stdin)` is not defined by the standards (although it might work as expected on most systems) – Ingo Leonhardt Sep 05 '14 at 11:46
@sathish thx for your answer! I changed all the things. The next problem is, that i always get the value back i´ve typed in at the beginning. in the if conditions are calculations which are not considered. :/ – Harald Wiesinger Sep 05 '14 at 11:59
1

@HaraldWiesinger what value you are getting? same as input or 97? – Sathish Sep 05 '14 at 12:08
2

The behavior of `fflush` is not defined for input streams; the operation is nonsensical. If it does clear the input stream, that's by accident, not design. – John Bode Sep 05 '14 at 12:11
@JohnBode It is not a standard but it is defined in MSVC. [fflush](http://msdn.microsoft.com/en-us/library/9yky46tz.aspx) – BLUEPIXY Sep 05 '14 at 12:18
this `warenwert = warenwert * 0, 97;`also won't compile, not even on a german machine. – ths Sep 05 '14 at 14:04
@Sathish and what would it mean? – ths Sep 05 '14 at 20:50
@speising See my New edits! it is working correctly! Executing all if conditions. i am using gcc on ubuntu 12.04 – Sathish Sep 06 '14 at 05:34
not so correctly if you arrive at a final price of 0.0 instead of 18.43. – ths Sep 06 '14 at 09:59

score 0 · Answer 2 · answered Sep 05 '14 at 12:49

And here comes Way 3 comes :

After scanf_s something it changes stdin->_base and stdin->_cnt and that cause thatproblem and if you want to solve it you can write std->_base="\0"; and std->_cnt=0; after every time you used scanf_s for something. But if you read chars from string it can be different situation i said that for read one variable value.

Program skips if statements

2 Answers2