How to use conditions with numbers and words - c

Question

I've been trying to make this simple program that basically a little conversation. The biggest problem I've run into is that whenever I wanted to ask for the age and if someone wrongly answered with something that it's not a number, but a phrase or a word, instead of it beeing impossible it just assume a value. What can I write so that if someone rights something that is not a number it just repeats the loop?
main (){
printf("Can you tell me your age\n");
scanf("%d",&age );
do{
if ( age < 16) {
printf("You're a child!\n");
}
else if ( age == 17){
printf("Me too, cool! Let's meet.\n");
}
else {
printf("Pretty old! You are dying.\n");
}
} while ((age<=0) && (age>130));
}

Answer 1

To check if a character represents a digit or not, you can use the standard function isdigit().
But in your case with scanf() you can just check for its return value.
You might want to "clean stdin" if you don't know what the user will type.
void clean_stdin(void)
{
int c = 0;
while (c != '\n' && c != EOF)
c = getchar();
}
int main(void)
{
int age;
int scanf_ret;
do
{
printf("Can you tell me your age\n");
scanf_ret = scanf("%d", &age);
clean_stdin();
} while (scanf_ret != 1);
/*
Now we know that age contains a valid number
*/
return (0);
}

Answer 2

On success, scanf() returns the number of items of the argument list successfully filled. We've got only one argument, so we can use this return value in the while loop to continually check it against 1 (a valid input):
#include <stdio.h>
int main() {
int age, temp, status;
printf("Can you tell me your age\n");
status = scanf("%d", &age);
while(status!=1) {
while( (temp=getchar()) != EOF && temp != '\n' ) {};
printf("Invalid input... please enter a number: ");
status = scanf("%d", &age);
}
if ( age < 16) {
printf("You're a child!\n");
}
else if ( age == 17) {
printf("Me too, cool! Let's meet.\n");
}
else {
printf("Pretty old! You are dying.\n");
}
}
Hope this helps.

Answer 3

You need to check what scanf is returning in the if statement,you can modify you code like this:
int main()
{
int age;
printf("Can you tell me your age\n");
scan:
if((scanf("%d",&age ))==1)
{
do{
if ( age < 16) {
printf("You're a child!\n");
}
else if ( age == 17){
printf("Me too, cool! Let's meet.\n");
}
else {
printf("Pretty old! You are dying.\n");
}
} while ((age<=0) && (age>130));
}
else
{
printf("Please enter a number\n");
fflush(stdin);
goto scan;
}
return 0;
}