Thread: Infinite Loiops

Originally Posted by deepdrive

Hi,

Apologies in advance this is a real newbie question. I tried the search and Youtube with no luck. I'm having a bit of a problem with infinite loops. My problem is what to do when an illegal character has been entered. For example if someone entered "a" in the code below, what is the best method to ask for a re-prompt. I tried doing a "do while" command but it just asked for "value for i" then displayed "press any key to continue". I tried break/continue also with similar results.

Any help would be appreciated thanks in advance.

Code:

#include<stdio.h>
int main (void)
{
    float number;
    int i;
        printf( "value for i \n" );
        i = scanf("%f", &number);
             while (i != 1)
                         {printf("Invalid input ");
                           i = scanf("%f", &number);
                          }


                    return 0; 
}

adding printf to your output to see what is going on with it is a good thing, adding exit(0); from stdlib.h is a good way to stop the loop so you can see that output. break works too.

Code:

 #include <stdio.h>
 #include <stdlib.h>
 
int main (void)
{
    float number;
    int i;
        printf( "value for i \n" );
        i = scanf("%f", &number);
        printf("I = %d\n", i);
    while (i != 1)
   {
        printf("Invalid input\n");
        i = scanf("%f", &number);
        printf("In loop I = %d\n",i);
    //    exit(0);
    }
return 0; 
}

From scanf: On success, the function returns the number of items successfully read.
This count can match the expected number of readings or fewer, even zero, if a matching failure happens.
In the case of an input failure before any data could be successfully read, EOF is returned
.

functions
isdigit()

for digits

isalpha()

for letters

Code:

//assignment
 if (i = 0)
//comparison
if ( i == 0)

Flowchart of continue Statement

I like this code:

Code:

#include <stdio.h>
 
#define SIZE 512
 
int main(void) {
    printf("%s\n", "Enter a float");
    char buffer[SIZE];
    fgets(buffer, SIZE, stdin);
 
    double value;
    while (sscanf(buffer, "%lf", &value) != 1) {
        puts("Invalid entry! Try Again!");
        fgets(buffer, SIZE, stdin);
    }
 
    printf("You entered %lf\n", value);
 
}

or a minus value was entered.

This code I wrote could be adapted to only take positive by changing value and sscanf to take unsigned instead of signed.

(There was three posts while I wrote this.)

Originally Posted by deepdrive

Thanks userxbw for taking the time to reply and explaining the difference between assignment and comparison. Unfortunately even changing the code to use comparison yields the same result. I was just wondering is their a "step command" to bring user back to the beginning of the code to input the parameters again in the event an illegal character or a minus value was entered.

I think you forgot to check that link on what the command "break" and "continue" does in a loop.

C Programming break and continue Statement

break kicks it out, and continue takes it back up to while ( ... )

Code:

 #include <stdio.h>
 #include <stdlib.h>
 

int main(void)
{
    float number;
    int i;
    printf("value for i \n");
    i = scanf("%f", &number);
    printf("I = %d\n", i);
    while (i != 1)
    {
        printf("Invalid input\n");
        i = scanf("%f", &number);
        printf("In loop I = %d\n", i);
      //  break;
 
 
        continue;
        if (i == 0)
        {
            printf("value for i \n");
            i = scanf("%f", &number);
        }
    }
        ;
    return 0;
}

gets a return of 0 so because of continue it moves it back up to while not 1 and keep looping because return is 0 (zero);

that scanf and end line no end line botch job they did on that function screws it up for using scanf in a loop. the post above this worked it out.

I believe scanf returns the number of values successfully matched, so:

Code:

while (scanf_s("%d", &integer1) < 0)

will never happen.
**Edit**
Okay, I see that EOF is often given the value -1, but I've read that this is implementation dependent. It seems it can't be relied upon. I think it would be safer to test it against the expected matches. In this case, that would be 1.

Also, this is the second successive occurance of scanf. When a scanf is put inside a condition, that scanf is executed. If you are not yet comfortable with this or you feel it to be aesthetically offensive, you could do as you and userxbw were doing above and assign the return value of scanf to an integer before testing the value of that integer in the while condition.

I used a buffer so that if the user enters in anything unexpected, it doesn't cause confusion in the stdin stream. It is just handled as buffer garbage instead.