How to check whether a variable is of integer type

Question

I simply want to check whether a value which is present in a variable is integer or not.

My program below reads two values from "myfile.txt" and performs addition if and only if they are integers otherwise it returns "Invalid Output". I have successfully read values from "myfile.txt" but i have no way of checking whether the two values are integer or not.How to perform this test?

#include <stdio.h>
#include <conio.h>

int main()
{
    FILE *fp;
    int i, mkji, num, num1, num2;
    int s = 0;
    int a[2];
    char term, term2;
    clrscr();
    fp = fopen("myfile.txt", "r+");
    if (fp != 0)
    {
        for(i = 0; i < 2; i++)
        {
            fscanf(fp, "%d", &a[i]);
        }
        for(i = 0; i < 2; i++)
        {
            printf("%d\n", a[i]);
        }
        num = a[0];
        num1 = a[1];
        num2 = num + num1;
        printf("%d", num2);
        mkji = fclose(fp);
    }
    else
        printf("FILE CANNOT BE OPEN");
    getch();
    return 0;
}

If you really want to know for c++ as tagged, one duplicate is How to test whether stringstream operator>> has parsed a bad type and skip it. For c you may check the return value of fscanf(). — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Feb 20, 2016 at 12:32
fscanf returns an error if it cannot read an integer from the input file. So, always check the return value. I think, it must also read num2 = num+num1 or just num2 = a [0] + a [1]. — Martin Zabel
– Martin Zabel, Commented Feb 20, 2016 at 12:35
Possible duplicate of Check if a value from scanf is a number? — fuz
– fuz, Commented Feb 20, 2016 at 12:40
C does not support dynamic typing. So the type of a variable is known at compile time. — too honest for this site
– too honest for this site, Commented Feb 20, 2016 at 12:59

fuz · Accepted Answer · 2016-02-20 12:31:33Z

1

Check the return value of fscanf. fscanf returns the number of succesfully scanned items or -1 if there was an IO error. Thus, if the number was malformed, fscanf("%d",&a[i]); should return 0.

answered Feb 20, 2016 at 12:31

fuz

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4 Comments

fuz Over a year ago

@AnidhSingh No. It returns 0 as outlined in the last sentence. The best way is to check if the return value is equal to 1. If it is, you successfully scanned a number. If not, some sort of error occured or there was no number.

Anidh Singh Over a year ago

oh.. so if fscanf reads a NUMBER it returns 1 else some error occured?

fuz Over a year ago

@AnidhSingh Yes. fscanf returns the number of items successfully scanned. %d is a directive. If a number was found, the item for %d was successfully scanned, thus one item was successfully scanned, thus 1 is returned. Please read the manual page for fscanf for more details.

Anidh Singh Over a year ago

Thanks. It was very helpful

Martin Gardener · Accepted Answer · 2016-02-20 12:43:26Z

0

Read the file as ASCII character and check for their ASCII Codes. If the ASCII lies in the range of a number 0 number has ASCII 48 and 9 number has ASCII 57. so each number lies between ASCII 48 to 57.

you can concatenate them in a string since you are reading characters until a space appears and once you get 2 numbers just add them :)

code can be in the form of

char[] number1;
char numRead;
keep reading the number using fscanf until a non-number appears.
if ( numRead >= '0' && num <= '9' ) // checking for if it is a number.
   number1 += numRead; // atleast thats how you do it in C++

edited Feb 20, 2016 at 12:43

answered Feb 20, 2016 at 12:33

Martin Gardener

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2 Comments

fuz Over a year ago

I'm not sure what this is supposed to do. If I use fscanf to scan the number 100, surely your check would give an incorrect indication?

Martin Gardener Over a year ago

thats what i said, if there are a series of numbers keep concatenating them until a space appears, 1st iteration '1' read. 2nd iteration '0' read. 3rd iteration '0' read. 4rth iteration ' ' read. concatenate them into 1 result '100'. If this was C++ i would show you how to code it since i haven't touched C for like 4/5 years i forgot how to deal with this type of problems.

Danyal Imran · Accepted Answer · 2016-02-20 12:47:00Z

0

check the return value from fscanf()

it can return -1 for a non number
and 0 or 1 for a number.

answered Feb 20, 2016 at 12:47

Danyal Imran

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