Using isalpha()

I have tried & tried & tried to figure out what's wrong with my code but I can't . I's supposed to give an error if the user enters anything else but an integer . So something like : 34j , should be comming up as an error.
Thanks for any help you can give ! :)

Code:

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

#define MAXLINE 25 

int check(char *line);
char line[MAXLINE];
int error,n,digit,flag;
int i = 0;
int flag = 0;


void main(){
       
        do{
                printf("Input a posetive integer: ");
                if (fgets(line,MAXLINE,stdin) != NULL){
                digit = check(&line);
                error = ((sscanf(line,"/t%d/t",&n) != 1 || n <= 0) && (digit == 1)); 
                }
                if (error)
                        printf("\nERROR: Do it again .\n");
                else
                        printf("There was no error!");
        }while (error);
 
}

int check(char *line){
        while((i < MAXLINE) && (flag == 0)){
                if (isalpha((int)line[i]))
              flag = 1;
            ++i;
        }
        if (flag = 0)
                return 0;
        else
                return 1;
}

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Common problem, a swap of the assignment and the testing operator...

Code:

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          if (flag = 0)
                return 0;
        else
                return 1;

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notice you're assigning flag to 0, not testing if it EQUALS zero :D

Simple mistake that we all make every now and then, just change the = to a == and you'll be set to go

Thanks! Ive done that , but while debugging after i've entered the values 5t , t5, t, 5
the 5 still gives an error while it shouldn't ??

You're using a lot of global variables in this program that are causing you more of a headache then anything, heh.

What's happening is

1) not resetting the i to 0
2) not resetting the flag to 0
3) you should only test (i < strlen(line)) in order to only test the inputted data.

Code:

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int check(char *line){
        i=0;
        flag=0;
        while((i < strlen(line)) && (flag == 0))
        {
                if (isalpha((int)line[i]))
                  flag = 1;
                ++i;
        }
        if (flag == 0)
                return 0;
        else
                return 1;
 }

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Thank you soooo much it works now, it was such a pain in the butt this program lol . Thanks again! :)