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  1. /*
  2. Desc: Get a valid integer from keyboard.
  3. It will check if there are non digits or too many digits.
  4. A valid integer may include a sign and at most 9 digits.
  5.  
  6. Auth: Liutong XU
  7. Date: 2017.12.16
  8. */
  9.  
  10. #include<stdio.h>
  11. #include<stdlib.h>
  12. #define MAXCHARS 81
  13. #define TRUE 1
  14. #define FALSE 0
  15. int getanInt();
  16. int isValidInt(char val[MAXCHARS]);
  17. int main()
  18. {
  19.     int number;
  20.  
  21.     number = getanInt();
  22.  
  23.     printf("The number you entered is %d\n", number);
  24.     return 0;
  25. }
  26.  
  27. int getanInt()
  28. {
  29.     int isanInt = FALSE;
  30.     char value[MAXCHARS];
  31.  
  32.     do
  33.     {
  34.         printf("Enter an integer, at most 9 digits plus one sign: ");
  35.         fgets(value, MAXCHARS, stdin);
  36.  
  37.         int i = 0;
  38.         while (value[i]!='\n' && i < 11) i++;   //11th char is invalid if any
  39.         value[i] = '\0';
  40.  
  41.         isanInt = isValidInt(value);
  42.         if (!isanInt)
  43.         {
  44.             printf("Invalid integer\n");
  45.         }
  46.      } while (!isanInt);
  47.  
  48.     return (atoi(value));
  49. }
  50.  
  51. int isValidInt(char val[])
  52. {
  53.     int start = 0;
  54.     int i;
  55.     int valid = TRUE;
  56.     int sign = FALSE;
  57.  
  58.     if (val[0] == '\0') valid = FALSE;
  59.     if (val[0] == '-' || val[0] == '+')
  60.     {
  61.         sign = TRUE;
  62.         start = 1;
  63.     }
  64.     if (sign == TRUE && val[1] == '\0') valid = FALSE;
  65.  
  66.     i = start;
  67.     while (valid == TRUE && val[i] != '\0')
  68.     {
  69.         if (val[i] < '0' || val[i] > '9')
  70.            valid = FALSE;
  71.         i++;
  72.     }
  73.     if (i - sign > 9)
  74.     {
  75.         valid = FALSE;
  76.         printf("Too many digits...");
  77.     }
  78.  
  79.     return valid;
  80. }
Success #stdin #stdout 0s 4292KB
comments (?)
stdin 
+987654321
stdout 
Enter an integer, at most 9 digits plus one sign: The number you entered is 987654321
https://ideone.com/QewJdq
language:
C (gcc 8.3)
created:
7 years ago
visibility:
public

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