how to compare 2 input with if else in c? and how to make yes or no selection in c?

Question

may i have some help? Here's my script.

#include <stdio.h>

void main(void) {
    int u;
    char ktp;
    
    printf("Masukkan umur anda : ", &u);
    scanf ("%i ", &u);
    
    printf("Apakah anda memiliki ktp (y atau n): ", &ktp);
    scanf ("%c", &ktp);
    

    
    if (u>=17 && ktp=='y') {
        printf("Selamat, anda layak untuk memilih.");
    } else if(u<17) {
        printf("Mohon maaf, anda belum layak untuk memilih.");
    } else {
        printf("Tolong masukan input yang benar.");
    }

}

1.First of all, why i cant input the second input?? when I inputed the variable u, the value is immediately displayed without inputting the variable ktp. 2.how to make selection with yes or no and scan it to compare to the if else statement.

Please see [scanf() leaves the newline char in the buffer](https://stackoverflow.com/questions/5240789/scanf-leaves-the-new-line-char-in-the-buffer). — Weather Vane, Nov 01 '21 at 09:11
Also unless you want to input other number bases don't use `%i` use `%d`. With `%i` an input of say `0123` is taken as octal. — Weather Vane, Nov 01 '21 at 09:12
@WeatherVane: [That question](https://stackoverflow.com/questions/5240789/scanf-leaves-the-new-line-char-in-the-buffer) does not explain the issue here. The question here is somewhat opposite. The first `scanf` does not leave the new-line in the buffer. Instead, because of its trailing space, it consumes the new-line, and further white space and new-lines, which results in it not terminating when the user presses enter, and so the second prompt does not appear. — Eric Postpischil, Nov 01 '21 at 10:20
@EricPostpischil thank you, I didn't notice the trailing space. — Weather Vane, Nov 01 '21 at 10:21

Asaf Itach · Answer 1 · 2021-11-01T11:43:36.307

0

#include <stdio.h>

void main(void) {
    int u;
    char ktp;
    
    printf("Masukkan umur anda : ");/*", &u" - this is meaningless*/
    scanf ("%d", &u);\\%*c
    
    printf("Apakah anda memiliki ktp (y atau n): "); /*, &ktp); - this is meaningless*/
    scanf (" %c", &ktp);
    

    
    if (u>=17 && ktp=='y') {
        printf("Selamat, anda layak untuk memilih.");
    } else if(u<17) {
        printf("Mohon maaf, anda belum layak untuk memilih.");
    } else {
        printf("Tolong masukan input yang benar.");
    }

}

The problem as @weather-vane write is that scanf takes your new-line into the buffer and not as an ending input.

also, you tried to print &u while you didn't used the printf %d to print it. anyway, it is undefined variable so you shouldn't do that.

edited Nov 01 '21 at 11:43

answered Nov 01 '21 at 09:50

Asaf Itach

300
6
13

There is no need to kludge whitespace from the inputs to `scanf`, just use it the way it was designed. Place a space in front of `%c` to filter the previous newline (or any amount of whitespace in the input). That's more robust than your kludge which *must* find *one* whitespace character. – Weather Vane Nov 01 '21 at 10:06
This does not fix the problem the OP reports, which is that the program does not print the second prompt after a line is entered. That is because the code in this answer still contains a space at the end of the first `scanf`, which causes `scanf` to continue reading white space instead of returning. Also backslashes not mark the start of a comment; it should be `//`. – Eric Postpischil Nov 01 '21 at 10:27
@weather-vane does this is the right way? Tnx – Asaf Itach Nov 01 '21 at 11:45

score 0 · Answer 2 · answered Nov 01 '21 at 10:25

1.First of all, why i cant input the second input?? when I inputed the variable u, the value is immediately displayed without inputting the variable ktp.

In scanf ("%i ", &u);, the space in the format string tells scanf to consume white space characters, which includes the new-line character. Thus, when you enter a number and press enter, which sends your input and a new-line character to the program, scanf reads the new-line character and continues trying to read. It waits for more input, so your program does not proceed to the second prompt.

To fix this, remove the space, changing the call to scanf("%i", &u);. Then, when scanf finishes reading the number and reads the next character, which is not part of the number, it will put that next character back in the input stream and return. Then your program will continue to the next prompt and the next scanf.

Change that second scanf to scanf(" %c", &ktp);. Here, the space will also tell scanf to consume white space characters, which you want it to do to consume the new-line that is still in the stream. After it consumes the white space characters, it will put the next character in ktp.

2.how to make selection with yes or no and scan it to compare to the if else statement.

Use strcmp to compare strings:

if (u >= 17 && strcmp(ktp, "y") == 0)

strcmp returns 0 the two strings passed to it are the same, a negative number if the first string is earlier in sort order, and a positive number if the second string is later.

Vlad from Moscow · Answer 3 · 2021-11-01T12:05:40.767

In these two calls of printf

printf("Masukkan umur anda : ", &u);
printf("Apakah anda memiliki ktp (y atau n): ", &ktp);

the argument expressions &u and &ktp are not used.

So rewrite them like

printf("Masukkan umur anda : " );
printf("Apakah anda memiliki ktp (y atau n): " );

Remove the space character from the format string in this call of scanf

scanf ("%i ", &u);

That is write

scanf ("%i", &u);

Also as @Weather Vane wrote in a comment if you want the entered number is interpreted as a decimal constant even if it is preceded by the 0 then use the conversion specifier %d instead of %i.

scanf ("%d", &u);

After this call of scanf the input buffer contains the new line character '\n' that corresponds to the pressed Enter key.

So the following call of scanf

scanf ("%c", &ktp);

reads this new line character in the variable ktp. You need to skip it. To do that prepend the format string with a space character like

scanf ( " %c", &ktp);
        ^^^^^

From the C Standard (7.21.6.2 The fscanf function)

8 Input white-space characters (as specified by the function) are skipped, unless the specification includes a [, c, or n specifier.

and

5 A directive composed of white-space character(s) is executed by reading input up to the first nonwhite-space character (which remains unread), or until no more characters can be read. The directive never fails.

Pay attention to that according to the C standard the function main without parameters shall be declared like

int main( void )

how to compare 2 input with if else in c? and how to make yes or no selection in c?

3 Answers3