Hamza Azam - 1 month ago 14
C Question

# Re-prompting a user until he/she enters a positive integer value greater than 1

I'm solving CS50 (problemset 1) i.e water.c. It asks user to write a program that prompts the user for the length of his or her shower in minutes (as a positive integer) and then prints the equivalent number of bottles of water (as an integer).
1 min of shower = 12 bottles consumed
MAIN PROBLEM: The problem is that we have to ensure that the user inputs a positive number of minutes otherwise it keeps on re-prompting his back to input/scanf statement. As long as he enters he enters length<=0, I can re-prompt him back using while(length<=0) condition but as he enters a character i.e abc123 in input my code keeps on executing. Any solutions??

>

`````` #include <stdio.h>
int main()
{   int length=0;
int min=12;
int bottle=0;
printf("Enter length of his or her shower in minutes");
scanf("%d", &length);
while (length <= 0){
printf("Enter length of his or her shower in minutes");
scanf("%d", &length);
}
bottle= (min*length);
printf("%d", bottle);

return 0;
}
``````

Answer

You can solve this by reading a string first, and then extracting any number:

``````#include <stdio.h>

int main(void)
{
int length = 0;
char input[100];
while(length <= 0) {
printf("Enter length: ");
fflush(stdout);
if(fgets(input, sizeof input, stdin) != NULL) {
if(sscanf(input, "%d", &length) != 1) {
length = 0;
}
}
}

printf("length = %d\n", length);
return 0;
}
``````

Program session:

``````Enter length: 0
Enter length: -1
Enter length: abd3
Enter length: 4
length = 4
``````

Crucially, I always check the return value from `scanf`, the number of items successfully converted.

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