Testing user input question?

I have a program that requires a user to input a number. The number determines how many random numbers(done by my program) will be displayed on the screen. The number input by the user will be of type int. Is it legal to use an isalpha() function to test and see if it actually is an int. Eg. if someone types in 46i instead of 461. Would this be a correct way to check it. I'm not at my PC with my compiler right now so I'm just trying to get close.

Code:

Code:

---

void get_info(char *filename, int *n_ptr)
{
    prinf("\n%S\n\n%s",
        "This progam creates a file of random numbers.",
        "Please enter in the number of random numbers
                  you would like to see as a positive integer.  "),
    if (scanf("%d", n_ptr)) != 1 || n_ptr <=0 || isalpha(n_ptr)
        {
            printf("\nERROR: Please enter positive integer.\n");
        }
   
    printf("\nIn what file would you like them?  ");
    scanf("%s", filename);
}

---

Thanks for any help.

No, that will not work as you expect it to.
If the user types 46i , the scanf will succesfully read 46 into n_ptr, and leave i and the newline character in the stdin buffer.
The isalpha test is useless as your != 1 test for scanf will always ensure n_ptr is an integer.
You'll have to read into a string and then test the input if you want to validate everything up to when the user presses <enter>

Also, make sure you dereference your pointers when using the value the pointer points to

Code:

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scanf("%d", n_ptr)) != 1 || *n_ptr <=0 || isalpha(*n_ptr)

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fgets() to read a line of input
strtol() to convert and validate the input.

Given this:

Code:

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#define  MAXLINE  100

---

and

Code:

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int main(void)
{
      int  line[MAXLINE];
  .....................

---

Would this be correct?

I'm not really familiar with strtol(). I looked it up, and gave this a try.

Code:

---

void get_info(char *filename, int *line)
{
    prinf("\n%S\n\n%s",
        "This progam creates a file of random numbers.",
        "Please enter in the number of random numbers
                  you would like to see as a positive integer.  "),
    fget( line, MAXLINE, stdin);
    if( strtol( line ) != 1)
        {
            printf("\nERROR: Please enter positive integer.\n");
        }
   
    printf("\nIn what file would you like them?  ");
    scanf("%s", filename);
}

---

Quote:

---

Originally Posted by Hoser83

Code:

---

    if( strtol( line ) != 1)
        {
            printf("\nERROR: Please enter positive integer.\n");
        }

---

---

Is 1 the only positive integer in the world?

...and yes, you can define an array size with a constant integer.

I'm not sure I totally understand. I thought that-

Code:

---

    if( strtol( line ) != 1)
        {
            printf("\nERROR: Please enter positive integer.\n");
        }

---

meant that if strtol returned a 1 or true, that the it would then make the if statment true.

strtol() returns the string converted to a long. Right now, the only number you're accepting is 1 because of that if statement. Whether that's what you want or not, I don't know, but your error message states "Please enter a positive integer". What would an end-user think if this happened:

Code:

---

Please enter in the number of random numbers you would like to see as a positive integer.    2

ERROR: Please enter positive integer.

---

...and for your edit, strtol() returns a 0 if no conversion was made. That doesn't mean it returns a 1 when it makes a conversion. It doesn't. It returns the value of the converted string.

If you think that this is the wrong application for strtol(), I won't disagree because I don't know for sure. I just want to make a check to see if the user inputs the right thing. for example: 461 is good where 46i is bad. I just want to check for this.

Did your manual page say strtol() only had one parameter?
http://www.mkssoftware.com/docs/man3/strtol.3.asp

no, it doesn't. It says that it take an nptr, endptr, and a base. I just need to figure out how to incorporate the pointers. Since it converts it to a long int, does this mean that if it encounters a character, it just returns a 0?

If it encounters any character before a numbers it returns 0. (failure)
If it encounters a number first it acts like scanf and returns upto what it could convert.
If you additionally pass pEnd you can check to see if extra characters were typed that could not be convereted.
Since you used fgets pEnd should equal '\n' if no extra characters were typed.

Code:

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char *pEnd;
strtol(...,&pEnd,..);
if(*pEnd != '\n')
  bad

---

Okay, let me try that:

The way I have this now. It takes the string line, and the pointer pEnd, and also the base address.
If it comes to something other that a character before it reaches pEnd, it returns a 0, which in turn then you get the error message? Is this right?

Code:

---

void get_info(char *filename, int *line)
{
    char *pEnd;

    prinf("\n%S\n\n%s",
        "This progam creates a file of random numbers.",
        "Please enter in the number of random numbers
                  you would like to see as a positive integer.  "),
    fget( line, MAXLINE, stdin);
    strtol( line, &pEnd, line[0] )
        if(*pEnd != '\n')
        {
            printf("\nERROR: Please enter positive integer.\n");
        }
   
    printf("\nIn what file would you like them?  ");
    scanf("%s", filename);
}

---

You're close.
I guess you are not actually testing this as you go.

The 3rd paramater should be the number base.

Quote:

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The Third argument (base) can have a value of 0 or 2-32.

0 - strtol will attempt to pick the base. Only Dec, Oct Hex supported.
2-31 - The base to use.

---

You've also removed the check for positive number, and after your call to strtol you don't store the value to use anywhere.

consider this

Code:

---

    char *pEnd; long l; char line[BUFSIZ];

    printf("Enter a positive int: ");
    fgets(line, BUFSIZ, stdin);

    if((l = strtol(line, &pEnd, 0)) <= 0 || *pEnd != '\n') {
        printf("Bad\n");
    }else{
        printf("Your number: %ld\n", l);
    }

---

This may be a dumb question, but what does the BUFSIZ mean. It that a default thing?

Why do you have a character being passed as your radix arguement? That's supposed to be what the base of the string it's trying to convert is. In your case it would be base 10.

Quote:

---

Originally Posted by Hoser83

This may be a dumb question, but what does the BUFSIZ mean. It that a default thing?

---

No, BUFSIZ is a user-defined constant. He wrote the code that way to tell you, the one taking the example, it's variable to whatever you want it to be.

so base 10 equals a decimal integer?

Yes. Do you understand the concept of bases, though? It's a basic computer concept that will be very essential in your progamming future.

Here is my entire code:

I'm not able to use my compiler right now so I'm just trying to get close:

My book has not gone over bases yet. That is why I'm sort of lost here.

If this does work, how do I use what is in the string to get the correct number of random numbers printed. Right now, I use a for loop, but I don't know for sure how to use the for loop with the string.

Code:

---

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

#define MAXLINE  100

FILE *cfopen(char *filename, char *mode);

FILE *gfopen(char *filename, char *mode);

int main(void)
{
    FILE  *ofp;
    int    i, line[MAXLINE];
    char  filename[120];
    char  *pEnd;
    long  1;   

    prinf("\n%S\n\n%s",
        "This progam creates a file of random numbers.",
        "Please enter in the number of random numbers
                  you would like to see as a positive integer.  "),
    fget( line, MAXLINE, stdin);
   
    if((l = strtol(line, &pEnd, 10)) <= 0 || *pEnd != '\n')
        {
            printf("\nERROR: Please enter positive integer.\n");
        }
   
    printf("\nIn what file would you like them?  ");
    scanf("%s", filename);

    ofp = cfopen(filename, "r+");

     
       
        for(i=1; i<=line; ++i){
          fprintf(ofp, "%12d", rand());
          if(i%6 == 0 || i == n)
              fprintf(ofp, "\n");
    }
}





FILE *cfopen(char *filename, char *mode)
{
    char  reply[2];
    FILE  *fp;
 
    if(strcmp(mode, "w") == 0
      && (fp = fopen(filename, "r")) != NULL){
        fclose(fp);
        printf("\nFile exists.  Overwrite it? ");
        scanf("%1s", reply);
        if(*reply != 'y' && *reply != 'Y') {
        printf("\nBye!\n\n");
        exit(1);
        }
      fp = gfopen(filename, mode);
      return fp;
}



FILE *gfopen(char *filename, char *mode)
{
    FILE *fp;
   
    if((fp = fopen(filename,mode)) == NULL){
    fprintf(stderr, "Cannot open %s - bye!\n", filename); 
    exit(1);
    }
    return fp;
}

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