higher/lower game - how do I check if an input is an integer or not in a while loop

I tried out your code and indeed, the program did get stuck in an endless loop if some characters were entered as data. In viewing your code, it looks like it's time for you to familiarize yourself with conventional character and string functionality (e.g. checking if a character is a digit, converting strings to integers, and so forth).

With that in mind, following is a refactored version of your code.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <math.h>
#include <ctype.h>

int main() {
    // This function allows the user to guess a number
    // and the program decides if the user is correct

    int i;
    unsigned int seed = time(NULL);
    int randomNumber = rand_r(&seed) % 9;
    char num[5];

    while (1)
    {
        printf("Guess a number between 0-9: ");
        i = scanf("%s", num);                                   /* Input user entry into a character array                          */

        for (i = 0; i < strlen(num); i++)                       /* Ensure no non-digits are entered in the for loop test            */
        {
            if ((isdigit(num[i]) == 0))
                break;
        }

        if ((isdigit(num[i]) == 0) && (i < strlen(num))) {
            printf("Error, %s is not a number\n", num);       /* Move non-digit test as the first test                            */
            continue;
        }

        i = atoi(num);                                          /* Use conventional string function to convert string to an integer */

        if ((i < 0) || (i > 9)) {
            printf("%d is not between 0 - 9\n", i);
        } else if (i == randomNumber) {
            printf("You guessed correctly!\n");
            break;
        } else if (i > randomNumber) {
            printf("You guessed too high!\n");
        } else {
            printf("You guessed too low!\n");
        }
    }

    printf("Done.\n");

    return 0;
}

Comments have been added, but here are some key points.

• the "string.h" and "ctype.h" include files have been added to utilize standard character manipulation.
• a character array is utilized in lieu of an integer for data entry in the "scanf" function (note that there is no ampersand).
• the character array entry is then validated, making sure, only digits were entered, that the value entered is within the guess range, and then where the guess falls in relation to the random number.

Following was a test run at the terminal.

craig@Vera:~/C_Programs/Console/Guess/bin/Release$ ./Guess 
Guess a number between 0-9: huh
Error, huh is not a number
Guess a number between 0-9: 11
11 is not between 0 - 9
Guess a number between 0-9: 5
You guessed too high!
Guess a number between 0-9: 2
You guessed too low!
Guess a number between 0-9: 3
You guessed correctly!
Done.

The take away from this is probably reference some good tutorial literature on the usage of string and character functions.

You are getting garbage values when you try and enter a value that is not 0 to 9 (so characters like a, b , c etc) You need to get it as a char and convert it to the asci ii value by applying an offset.

int main() {  
unsigned int seed = time(NULL);
int randomNumber = rand_r(&seed) % 9;
int scanerrorcode = 0;

while (1)
{
    int num = 0; //need to reset ever time we loop
    char ch;
    printf("\nGuess a number between 0-9: ");  
    scanf(" %c", &ch);  

    //ascii 2 char for 0 - 9 is 48 - 57! https://www.ascii-code.com/
    printf("The ASCII value of %c is %d \n", ch, ch);

    // To make a digit
    num = num * 10 + (ch - 48);

    //Hey look it's valid!
    if (ch >= 48 && ch <= 57) {
        printf("\n%d is valid and between 0-9", num);
    }

    //not valid!
    if (ch> 57 || ch < 48){
        printf("\n%d is NOT between 0-9!", num);
    } else if (num == randomNumber) {
        printf("\nYou guessed correctly!");
        break;
    } else if (num > randomNumber) {
        printf("\nYou guessed too high!");
    } else if (num < randomNumber) {
        printf("\nYou guessed too low!");
    } else {
        printf("\nError, %d is not a number", num);
    }
}  

printf("\nDone.");

}