I want to read from file in c line by line and check each line if it is an integer

Question

how can i read from file line by line and check each line if it is an integer?

FILE *fp;
fp = fopen("users.txt", "r");


while(fscanf(fp, "%d", &IDRead)!=EOF)
{
  enter code here
}

fclose(fp);

Answer 1

You can use fgets() to read a line and isdigit() to check if each character in the string is a number.

First we can make an isnumber() function that checks if each character in a string is a number. To handle negative numbers we can check that the first character is either a digit or '-'.

bool isnumber(char* str) {
    int len = strlen(str);
    if (len <= 0) {
        return false;
    }

    // Check if first char is negative sign or digit
    if (str[0] != '-' && !isidigit(str[0])) {
        return false;
    }

    // Check that all remaining chars are digits
    for (int i = 1; i < len; i++) {
        if (!isdigit(str[i])) {
            return false;
        }
    }

    return true;
}

Our isnumber() function assumes the string has no leading or trailing white space, and a string retrieved from fgets() may have both. We'll need a function that strips whitespace from both ends of a string. You can read about how to do so in this answer.

Now we can use our isnumber() function inside of a while loop to check each line in a file with fgets().

FILE *fp = fopen("users.txt", "r");
if(!fp) {
    perror("Failed to open file");
    return -1;
}

const int MAX = 256;
char line[MAX];
while (fgets(line, MAX, fp) != NULL) {
    stripLeadingAndTrailingSpaces(line);
    printf("%s\t", line);

    if (isnumber(line)) {
        printf("is a number\n");
    }
    else {
        printf("is not a number\n");
    }
}

fclose(fp);