How do you check for valid input?

Hi, I am trying to make a program that asks a user to enter a speed. The speed shoud be a positive int. How do I make it if a user enters say xyz, a message will say invalid input?

eg
int speed = 0;

printf("Please enter a speed: "); --------user inputs xyz
scanf("%d%", &speed);
printf("Speed equals %d%", speed); ----the output would be 0

is there a way to make it recognize xyz or whatever?

Thanks heaps fo any info
:)

You can't with scanf(). You should get input using fgets() and then use functions from ctype.h in your validation check. Look at functions like isdigit().

Thanks itsme,

I found a different solution before you post that seems to work

int speed = -1000;

printf("Please enter a speed: ");
scanf("%d", &speed);

if(speed == -1000)
printf("Invalid input!\n");
else
if(speed < 0)
printf("Invalid input!\n");

i noticed speed remains the same when invalid input is used.

Thanks again :)

The only problem with that is it leaves "xyz" in the input buffer, waiting for the next scanf() call. So if you loop back to the printf() when there's an invalid input you'll just get stuck in an infinite loop unless you go through the trouble of clearing the input buffer.

I still think fgets() would be a much better choice. You could do something like:

Code:

---

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

int main(void)
{
  int speed;
  char buf[100];

  for(;;)
  {
    printf("Please enter a speed: ");
    fflush(stdout);  // Needed for non-newline'd printf()

    fgets(buf, sizeof(buf), stdin);
    speed = atoi(buf);
    if(!isdigit(*buf) || speed < 0)
      puts("Invalid input!");
    else
      break;
  }

  printf("You entered %d for the speed.\n", speed);
  return 0;
}

---

you can try this bit of code. which helps you in validating the input

Code:

---

#include<stdio.h>
void flush(void);
int main()
{
        int speed;
       
        printf("Enter a integer value for speed\n?");
        while(!scanf("%d",&speed))  // on invalid conversion the scanf return 0
        {
            printf("Inlvalid data\n");
            printf("Please enter a integer value\n");
            flush();
    }
    fflush(stdin);
        printf("you entered %d",speed);
        getchar();
  }     
 
void flush()
{ 
    char c;
    while((c=getchar())!='\n');
    return;
}

---

s.s.harish

Code:

---

    fflush(stdin);

---

Argh. Evil.

ohh ya

thats should't be used. any way no use of that in that prog.

Code:

---


#include<stdio.h>
void flush(void);
int main()
{
        int speed;
       
        printf("Enter a integer value for speed\n?");
        while(!scanf("%d",&speed))
        {
            printf("Inlvalid data\n");
            printf("Please enter a integer value\n");
            flush();
    }
        printf("you entered %d",speed);
        getchar();
  }     
 
void flush()
{ 
    char c;
    while((c=getchar())!='\n');
    return;
}

---

thax itsme86

s.s.harish

Code:

---

void flush()
{ 
    char c;
    while((c=getchar())!='\n');
    return;
}

---

One more time?

very sorry

getchar returns int right

thats should be

Code:

---


int c;

while((c=getchar())!='\n');
return;

---

You may want to handle EOF, too.

ok fine, need to check EOF as well

Code:

---


while((c=getchar())!='\n && ch!=EO);

---

right

Close,

Current:

Code:

---

while((c=getchar())!='\n && ch!=EO);

---

Correct:

Code:

---

while((c=getchar())!='\n' && c!=EOF);

---

- Stack Overflow

> right
Still no - the idea is broken before it's begun, so there's really no point in trying to fix it.
See, when you've fixed the flush, you've got the scanf to fix as well.

> while(!scanf("%d",&speed))
User signals EOF, the while loop exits (because scanf returns EOF) and your printf ends up printing some undefined value of speed.

@itsme86
> speed = atoi(buf);
strtol() is better, since it also detects integer overflow as well as conversion errors.

Quote:

---

Originally Posted by Salem

@itsme86
> speed = atoi(buf);
strtol() is better, since it also detects integer overflow as well as conversion errors.

---

I can see that being true if speed is type long. But if, on a given architecture, ints and longs weren't the same size, couldn't it introduce its own problems?

Worst case scenario it gets truncated. (If you're passing a long, and assigning it to an int.) And if that's the case, they've entered something too big anyway.

Quzah.