printf in a while loop is printing twice instead of once

Question

In this code it need to calculate the number of times the operators appear. It doesn't calculate it and while running the code it prints "Please enter a note. to finish press Q:" twice and i don't know whats wrong.

#include <stdio.h>

int main(void)
{

    char note;
    int result1 = 0, result2 = 0, result3 = 0, result4 = 0;


    do
    {
        printf("Please enter a note. to finish press Q:\n");
        scanf("%c", &note);
        switch (note)
        {
        case'+':
            result1 = result1 + 1;
            break;
        case'-':
            result2 = result2 + 1;
            break;
        case'*':
            result3 = result3 + 1;
            break;
        case'/':
            result4 = result4 + 1;
            break;
        }

    } while (note != 'Q');

    printf("+ appears %d times\n", result1);
    printf("- appears %d times\n", result2);
    printf("* appears %d times\n", result3);
    printf("/ appears %d times\n", result4);

    system("pause");
}

Next time you copy & paste the code from your IDE to your question exactly how it is. — Swordfish, Nov 20 '18 at 18:16
Not identical question - but worth reading: https://stackoverflow.com/questions/13542055/how-to-do-scanf-for-single-char-in-c — Igal S., Nov 20 '18 at 18:21
You are reading the character (ex. `+`) and then reading a newline (you pressed enter). You can not print the `Plese enter a note ..` if the last character is newline or if you receive a newline scanf again or similar. — KamilCuk, Nov 20 '18 at 18:24
Tip: **always** add a `default` case to a `switch` statement. This helps you detect unexpected events. — Tim Randall, Nov 20 '18 at 20:01

score 0 · Answer 1 · answered Nov 20 '18 at 19:04

0

To debug this, insert

  printf("note='%c'\n", note);

immediately after the scanf. You'll see the problem immediately.

answered Nov 20 '18 at 19:04

Alcamtar

1,478
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19

static_cast · Answer 2 · 2018-11-20T21:25:13.503

Using scanf("%c", %c), you will be reading one character at a time. Which is fine, but..

scanf will not "re-scan" for input, until the buffer is clear.

So, if the user were to input something longer than 1 character, your loop will iterate for every character the string is long + 1 (\n counts as a character).

Example (note it loops 3 times +, +, \n):

Please enter a note. to finish press Q:
++
Please enter a note. to finish press Q:
Please enter a note. to finish press Q:
Please enter a note. to finish press Q:

That's the trick. You need to change your code to accommodate this (i.e. hide the please enter message until empty or read a char array note[128]... scanf('%s', note) vs. single character with a sub loop to iterate the over the input, etc. or some other way).

printf in a while loop is printing twice instead of once

2 Answers2