Validation using scanf() to protect against string

Question

I have a code here that has a job to see if the user input is either string or integer from a range of 1-49. If I enter "asdas" it says invalid, and if I enter a integer from "1-49" it says valid. The problems I am having with this code is that if I enter "2 asda" it will it count it has valid, and invalid at the same time, and if I enter "2 2" It will consider that valid as well. Just found out it also accepts "2d" as a valid input to.

for (i = 0; i < 6; i++)
        {

            printf("\nPlease enter the %d winning ticket numbers!: ", i+1);
            if (scanf("%d", (&winningNumbers[i])) == 0 || (winningNumbers[i] <= 0) || (winningNumbers[i] >= 50))
            {
                    inputFlush();
                    printf("\nInvalid Input. Please re-enter.\n") ;
                    i = i - 1;
            }
       }

        for (i = 0; i < 6; i++)
        {
            printf("%d, ", winningNumbers[i]);
        }

Mihai Maruseac · Answer 1 · 2013-11-18T04:24:13.923

2

Read the entire line into a string (fgets, line 2 in the snippet). Read data from the string using sscanf: read the integer and one more character, after a space. Check whether sscanf returns something different than 1. If it does then you either have strings in the beginning (it returns 0 since it couldn't read an integer) or you have extra whitespace characters at the end (that is it also matched the %c format specifier). The space is needed to jump over whitespace until the end of the line (including the stored \n).

printf("\nPlease enter the %d winning ticket numbers!: ", i+1);
fgets(buffer, size, stdin);
if (sscanf(buffer, "%d %c", &winningNumbers[i], &c) != 1 || (winningNumbers[i] <= 0) || winningNumbers[i] >= 50))
{
    // inputFlush(); not needed now that you read the entire line
    printf("\nInvalid Input. Please re-enter.\n") ;
    i = i - 1;
}

edited Nov 18 '13 at 04:24

answered Nov 18 '13 at 03:17

Mihai Maruseac

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Another option is to use `strtol()` to do the conversion, rather carefully. Since the required range of valid inputs is only 1-49, the numeric range issues can be quietly ignored. But you probably do need to check that there are no characters other than white space after the number. – Jonathan Leffler Nov 18 '13 at 03:22
I have tried using fgets. The problem I was getting was that fgets wasn't storing numbers such as "45", but "4" – Nov 18 '13 at 03:24
Yes, `strtol` can be used as well. And it also gives a pointer to the extra characters, if you don't pass in `NULL` for the second argument. – Mihai Maruseac Nov 18 '13 at 03:24
@Leoc: I'm pretty sure you didn't use `fgets`. Maybe `fgetc` -- it has the behaviour you described but it is intended. – Mihai Maruseac Nov 18 '13 at 03:25
Yeah that's it fgetsc, but using fgets in the fashion of the array. My compiler made me change it so it accepts characters – Nov 18 '13 at 03:31
1

The code I posted should not require the compiler to make you change to characters. Please recheck. – Mihai Maruseac Nov 18 '13 at 03:33
I will once I get back. Which will be soon – Nov 18 '13 at 03:37
a5.c:89:33: warning: incompatible pointer types passing 'int *' to parameter of type 'const char *' [-Wincompatible-pointer-types] – Nov 18 '13 at 04:05
1

Consider `"%d %c"` instead of `"%d%c"`. That extra space will help when user enters "12". – chux - Reinstate Monica Nov 18 '13 at 04:07
? spacing it didn't help. Its coming from my integer type array. – Nov 18 '13 at 04:09
My bad, I forgot to add the string you're reading from (I feel stupid now :() – Mihai Maruseac Nov 18 '13 at 04:10
Does it work now? It was a stupid mistake from my part, I even told you that the code is perfect :-/ – Mihai Maruseac Nov 18 '13 at 04:12
Works beautifully now! Thank you. Can you maybe explain to me what you have done, and what you did to fix it! – Nov 18 '13 at 04:14
I've updated the text in front of it to contain all of the decisions. – Mihai Maruseac Nov 18 '13 at 04:17
@Mihai Maruseac A very _minor_, but useful idea: Since OP said "1-49", consider using "1" and "49" in your code as in `winningNumbers[i] < 1) || winningNumbers[i] > 49)`. Even better to document those _magic_ numbers: `int ValidMin = 1; int ValdMax = 49; `if (... (winningNumbers[i] < ValidMin) || winningNumbers[i] > ValidMax)`. – chux - Reinstate Monica Nov 18 '13 at 04:18
@chux: I would code them the same as you said but I took that part from his code and I want to make as minor changes as possible. – Mihai Maruseac Nov 18 '13 at 04:19
1

I always use it since it clearly describes what you are expecting to read. – Mihai Maruseac Nov 18 '13 at 04:22
1

@Mihai Maruseac (Sorry to pester) "or you have extra **non-whitespace** characters at the end (that is it also matched the %c format specifier)." Drop the "The space is needed for platforms which implement newlines as CR-LF (\r\n) pair of characters." as that is incorrect. The space will scan any number of space, tab, \r, \n, whitespace. – chux - Reinstate Monica Nov 18 '13 at 04:22
@mihaiMaruseac with the same coding except for strings what if I wanted it to do the opposite how would that look like? – Nov 18 '13 at 04:29
I'm afraid I didn't understand. What exactly do you want to read? – Mihai Maruseac Nov 18 '13 at 04:37
@MihaiMaruseac Sorry for being unclear. Is there a way I can PM you a code? or no. What I want is to enter a name, and protect the input against integers thats all :) – Nov 18 '13 at 04:43
You cannot use the same trick. But you can read the string representing the name and loop through it and check each character if it is a digit or not (`isdigit`) – Mihai Maruseac Nov 18 '13 at 04:54
@MihaiMaruseac Can you give me an example? – Nov 18 '13 at 04:59
1

Please move extended discussion to [chat], thanks :) – Tim Post Nov 18 '13 at 07:27
I tried doing this but he doesn't have enough reputation for that. Anyway, now the question is answered so no need for further discussion. – Mihai Maruseac Nov 18 '13 at 13:09

Deck · Answer 2 · 2013-11-18T04:07:32.650

0

Look at this example.

#define MAX_LINE_SIZE 500

int main(int argc, char** argv)
{

    char line[MAX_LINE_SIZE];
    unsigned int num;
    char *ptr;

    while (fgets(line, MAX_LINE_SIZE, stdin) != NULL){
        num = strtol(line, &ptr, 10);
        if (line[0] != '\n' && (*ptr == '\n' || *ptr == '\0')) {
            printf("Your num: %u\n", num); // check num if you need
        } else {
            printf("Error\n");
        }
    }

    return 0;
}

The output:

12
Your num: 12
45
Your num: 45
34 2 
Error
ads
Error

Here fgets function read data from stdin. strtol parse read string and assign an address of a char after parsed number to ptr pointer. Assuming that user has to input only one number (without any character after) we have to check whether *ptr is new line or end of line.

line[0] != '\n' prevents empty string.

edited Nov 18 '13 at 04:07

answered Nov 18 '13 at 03:44

Deck

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Works amazing so far. Can you please explain this to me:) – Nov 18 '13 at 03:53
I understand the strtol part, but if you can explain to me the if statement you did with the null, and new line – Nov 18 '13 at 03:57
One problem is that you need to enter another input till you get those messages. – Nov 18 '13 at 04:03
Ahh makes sense. Thats cleaver, but how do you fix the part where you need to input again in order to have it working as intended ? – Nov 18 '13 at 04:10
@Leoc I would use counter variable, increment it in "good" part of the loop and check whether it equals 6 (like you did). If yes then break. – Deck Nov 18 '13 at 04:16

score -1 · Answer 3 · edited May 23 '17 at 11:56

-1

Check the next character using peek, as detailed here. You can tell if it's good or not that way.

Another SO question that's pretty much the same.

edited May 23 '17 at 11:56

Community

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answered Nov 18 '13 at 03:16

kmort

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Thank you for the fast comment. Ill try it as soon as I get home. – Nov 18 '13 at 03:17

Validation using scanf() to protect against string

3 Answers3