how to get program to accept only positive integer values in c

Question

writing a program that will be finding min, max, avg of values entered by user. Having trouble writing something that will check to make sure there are only postive integers entered and produce an error message. heres my for statement that is reading the input so far:

  for (int value = 0; value <= numofvals; ++value) {
      printf("Value %d: %f\n", value, val_input);
      scanf("%f", &val_input);
  }

mind you I've been learning code for about 3 weeks and was just introduced to loops this week so my understanding is rudimentary at best!

Answer 1

First, don't use scanf. If stdin doesn't match what it expects it will leave it in the buffer and just keep rereading the same wrong input. It's very frustrating to debug.

const int max_values = 10;

for (int i = 0; i <= max_values; i++) {
    int value;
    if( scanf("%d", &value) == 1 ) {
        printf("Got %d\n", value);
    }
    else {
        fprintf(stderr, "I don't recognize that as a number.\n");
    }
}

Watch what happens when you feed it something that isn't a number. It just keeps trying to read the bad line over and over again.

$ ./test
1
Got 1
2
Got 2
3
Got 3
foo
I don't recognize that as a number.
I don't recognize that as a number.
I don't recognize that as a number.
I don't recognize that as a number.
I don't recognize that as a number.
I don't recognize that as a number.
I don't recognize that as a number.
I don't recognize that as a number.

Instead, use fgets to reliably read the whole line and sscanf to parse it. %f is for floats, decimal numbers. Use %d to recognize only integers. Then check if it's positive.

#include <stdio.h>

int main() {
    const size_t max_values = 10;
    int values[max_values];
    char buf[1024];

    size_t i = 0;
    while(
        // Keep reading until we have enough values.
        (i < max_values) &&
        // Read the line, but stop if there's no more input.
        (fgets(buf, sizeof(buf), stdin) != NULL)
    ) {
        int value;

        // Parse the line as an integer.
        // If it doesn't parse, tell the user and skip to the next line.
        if( sscanf(buf, "%d", &value) != 1 ) {
            fprintf(stderr, "I don't recognize that as a number.\n");
            continue;
        }

        // Check if it's a positive integer.
        // If it isn't, tell the user and skip to the next line.
        if( value < 0 ) {
            fprintf(stderr, "Only positive integers, please.\n");
            continue;
        }

        // We got this far, it must be a positive integer!
        // Assign it and increment our position in the array.
        values[i] = value;
        i++;
    }

    // Print the array.
    for( i = 0; i < max_values; i++ ) {
        printf("%d\n", values[i]);
    }
}

Note that because the user might input bad values we can't use a simple for loop. Instead we loop until either we've read enough valid values, or there's no more input.

Answer 2

Something easy like this may work for you:

int n;
int ret;

for (;;) {
    ret = scanf("%d", &n);

    if (ret == EOF)
        break;

    if (ret != 1) {
        puts("Not an integer");
        for (;;)
            if (getchar() == '\n')
                break;
        continue;
    }

    if (n < 0) {
        puts("Not a positive integer");
        continue;
    }

    printf("Correct value %d\n", n);

    /* Do your min/max/avg calculation */
}

/* Print your results here */

This is just an example and assumes you do not need to read floating point numbers and then check if they are integers, as well as a few other things. But for starters, it is simple and you can work on top of it.

To break out of the loop, you need to pass EOF (typically Ctrl+D in Linux/macOS terminals, Ctrl+Z in Windows ones).

Answer 3

An easy and portable solution

#include <limits.h>
#include <stdio.h>
int get_positive_number() {
  char buff[1024];
  int value, ch;
  while (1) {
    printf("Enter positive number: ");
    if (fgets(buff, 1023, stdin) == NULL) {
      printf("Incorrect Input\n");
      // Portable way to empty input buffer
      while ((ch = getchar()) != '\n' && ch != EOF)
        ;
      continue;
    }
    if (sscanf(buff, "%d", &value) != 1 || value < 0) {
      printf("Please enter a valid input\n");
    } else {
      break;
    }
  }
  return value;
}
void solution() {
  // Handling malformed input
  // Memory Efficient (without using array to store values)
  int n;
  int min = INT_MAX;
  int max = INT_MIN;
  double avg = 0;
  printf("Enter number of elements: ");
  scanf("%d", &n);
  getc(stdin);
  int value;
  for (int i = 0; i < n; i++) {
    value = get_positive_number();
    if (value > 0) {
      if (min > value) {
        min = value;
      }
      if (max < value) {
        max = value;
      }
      avg += value;
    }
  }
  avg = avg / n;
  printf("Min = %d\nMax = %d\nAverage = %lf\n", min, max, avg);
}
int main() {
  solution();
  return 0;
}

• Output:

Enter number of elements: 3
Enter positive number: 1
Enter positive number: 2
Enter positive number: a
Please enter a valid input
Enter positive number: -1
Please enter a valid input
Enter positive number: 1
Min = 1
Max = 2
Average = 1.333333