Thread: Rejecting float and string with int on the front

So here's my code, with already implemented check for input data.
However, it doesn't reject float and eg. 3table as input for the size of a matrix.
How to fix that?

Code:

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

float determinant (float * data, int num);

int error = 0;

int main ()
{
    int num, i;
    char temp[BUFSIZ];
    float *data;
    float *counter;
    float value;
    char *p; /*used in function strtol in order to find the end of the input number*/
    
    printf("Enter the size of matrix: ");
    
    do
    {
    scanf("%s", temp);
    num = strtol(temp, &p, 10);
    if (num > 10 || num < 1)
        {
        printf("matrix size must be integer from interval 1-10\n");
        }    
    }while(num > 10 || num < 1 || !(*p == '\n' || *p == '\0'));    
    
    
   printf ("Size of the matrix %d by %d\n", num, num); 
                    
          
    if (1) 
    {
        data = malloc(sizeof(*data) * (num * num));

        if (data != NULL) 
        {
                for (counter = data, i = 0; counter < data + (num * num); counter++, i++) {
                printf("Enter the %d element: ", (i + 1));
                scanf("%s", temp);
                *counter = atof(temp); 
        }
            
            value = determinant (data, num);
            free (data);
            
            if (!error)
                printf("Value = %f\n", value);
        }
        else 
            printf("Memory allocation failed!\n\n");
    }    
    else
        printf("Invalid size of matrix\n");
    
    return 0;
}

float determinant (float * data, int num) {
    float result = 0;
    int i, j, place;
    float value = 1;
    float * counter, * new_data, * new_counter;
    
    if (num > 1) {
        for (place = 1; place < num + 1; place++) {
            new_data = malloc(sizeof(*new_data) * (num - 1) * (num - 1));
            if (new_data != NULL) {
                for (counter = data, i = 1, j = 1, new_counter = new_data; counter < data + (num * num); counter++, i++) {
                    if (i % (num + 1) == 0) {
                        j++;
                        i = 1;
                    }
                    if (i == place || j == 1)
                        continue;
                    *new_counter = *counter;
                    new_counter++;
                }
                
                value = (*(data + (place - 1))) * determinant(new_data, num - 1);
                
                if (error) {
                    free(new_data);
                    return 0;
                }
                
                if (place % 2 == 0)
                    value *= -1;
                
                result += value;
                free(new_data);
            }
            else {
                printf("\nMemory allocation failed!\n");
                error = 1;
                return 0;
            }
        }
        return result;
    }
    return *data;
}

To do that you'll pretty much have to read the line in as a string, and parse it yourself. There's no easy way to do it unfortunately.

one trick might be to use sscanf with %f instead of strtol to read the input. then if the number has a fraction, you can detect that and reject the number. detect it by casting to int then back to float and compare it to the original. if they aren't equal you got a value with a fractional part.

Robustly checking input is quite hard. strtod() is good enough, however. You pass an end pointer. If the end pointer doesn't point to the nul character on return, then there must have been trailing non-digits in your string.