validating input

**c-rookie** · 08-21-2003

hello,

i've tried looking at the FAQs etc. on this topic, but my knowledge of C is very, very small. My problem is that i'm trying to make a program that requests the user to put in a positive integer, but anything other than a positive integer asks the user to try again.

This is what i've done so far:

Code:

#include <stdio.h> 

void main()
{
int i, j;
printf("Please enter a positive integer:");
scanf("%d", &i);

while (i<=0){printf("\nYou have not entered a positive integer.\n\n");
             printf("Please enter a positive integer:");
             scanf("%d", &i);}
j=i;
for (i=0; i<j; i++){printf("It Worked!\n");}

}

So far it won't take negative numbers, but if i put in a character it loops the for statement (sorry if thats the wrong terminology)

How do i get it to request the user to re-enter a positive integer if they had input a character the first time?

thanks for any help

**Markallen85** · 08-21-2003

I'd go with something like:

Code:

int i=-1;
char string[256];
while (i<=0)
  {
  printf("please enter a positive interger\n");
  scanf("%s",&string);
  
  i=atoi(string);
  }

//i is now your integer

that'll not count 0 as a valid value, if you do want 0 to be valid, use while (1<0)

and you'll probably pick up a few semi-flames for using a void main, it's much better to use:

int main ()
{
return 0;
}

-mark

**c-rookie** · 08-21-2003

thanks Mark

i tried using it, but when i try to compile i get an error saying:

implicit declaration of function `int atoi(...)'

and unfortunately i have no idea what that means.

**Markallen85** · 08-21-2003

I think atoi is part of stdlib.h, it's not in stdio.h
put the line

#include <stdlib.h>

so the whole code should be:

Code:

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

int main(int argc, char *argv[])
{
  int i=-1;
  char string[256];
  while (i<=0)
    {
    printf("please enter a positive interger\n");
    scanf("%s",&string);
    
    i=atoi(string);
    }
  //i is now your integer
  return 0;
}

just tested that and it compiles fine on dev-c++

**jetfreggel** · 08-21-2003

include #include<stdlib.h> at the top of the program

**Hammer** · 08-21-2003

>>scanf("%s",&string);
This is wrong, it should be
>>scanf("%s",string);

atoi() isn't a good function to use, due to its lack of error checking and undefined behaviour.

>>void main()
This is also wrong, and is covered in the FAQ.

Read
http://faq.cprogramming.com/cgi-bin/...&id=1043284385
http://faq.cprogramming.com/cgi-bin/...&id=1043284376

**char** · 08-21-2003

Code:

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

int main(void)
{
    int i, ch;

    printf("Enter a positive intiger\n");

    while(scanf("%d", &i) != 1 || i < 0)
    {
	printf("That was not a positive intiger. Try again!\n");

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

    return EXIT_SUCCESS;
}

**XSquared** · 08-21-2003

Use fgets( ) and sscanf( ).

**Hammer** · 08-21-2003

Neither sscanf() or scanf() will help the OP achieve their goal. Look:

make a program that requests the user to put in a positive integer, but anything other than a positive integer asks the user to try again.

Using scanf() will not let you see the error if the user enters a number such as 999hhh, as the 999 will be placed into the int variable, and hhh will be left in the input stream, unseen by the program. Again, all this is covered in the FAQ.

**WaltP** · 08-22-2003

Originally posted by Hammer
Read
http://faq.cprogramming.com/cgi-bin/...&id=1043284385
http://faq.cprogramming.com/cgi-bin/...&id=1043284376

Hammer, every time I use your links to look at a FAQ I get a 'table of contents' for the FAQs -- no matter what link you give. Links given to eskimo work fine.

Any ideas why?

**cc0d3r** · 08-22-2003

Faq links work fine for me (in Mozilla/IE6). What browser are you using?

**Hammer** · 08-22-2003

Originally posted by WaltP
Hammer, every time I use your links to look at a FAQ I get a 'table of contents' for the FAQs -- no matter what link you give. Links given to eskimo work fine.

Any ideas why?

Strange, I don't get that, and I haven't had any other complaints. Maybe its the Anti-FAQ Movement trying to passing on another subtle message

PM with the details of the problem if you still have it.

**XSquared** · 08-22-2003

Works fine for me (IE6, Opera 7).

**WaltP** · 08-23-2003

Originally posted by XSquared
Works fine for me (IE6, Opera 7).

AHaaaa, in IE it does work (unfortunately). NS7 it is not working.

the link locations does say http://faq.cprogramming.com/cgi-bin/...&id=1043284385
but the index is displayed. Looks like the smartfaq.cgi program is a little dumb when in comes to NS/Mozilla browsers.

And before it's suggested, no, using IE is not an appropriate option... Only NS and Mozilla has the "open in new window" with one click of the mouse wheel. IE/Opera won't open new window's in one click, which is important to me.

**quzah** · 08-23-2003

Originally posted by WaltP
Only NS and Mozilla has the "open in new window" with one click of the mouse wheel. IE/Opera won't open new window's in one click, which is important to me.

Why don't you just use tabs in Mozilla? Who uses new windows? That's so 1995.

Quzah.

Thread: validating input

Thread Tools

Search Thread

Display

validating input

thanks for the reply

Similar Threads

Input class project (again)

About aes

Structure and Linked List User Input Question

Validating Monetary Input

need help with some input