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/*
Desc: Get a valid integer from keyboard.
It will check if there are non digits or too many digits.
A valid integer may include a sign and at most 9 digits.
Auth: Liutong XU
*/
#include<stdio.h>
#include<stdlib.h>
#define MAXCHARS 81
#define TRUE 1
#define FALSE 0
int getanInt();
int isValidInt(char val[MAXCHARS]);
int main()
{
int number;
number = getanInt();
printf("The number you entered is %d\n", number);
return 0;
}
int getanInt()
{
char value[MAXCHARS];
// typical scenario to use do-while
do
{
printf("Enter an integer, at most 9 digits plus one sign: ");
fgets(value, MAXCHARS, stdin);
} while (!isValidInt(value)); // while NOT valid, re-input. checking with a function
return (atoi(value)); // convert a string into a number, or you can write a piece of code to convert
}
int isValidInt(char val[MAXCHARS])
{
int start = 0;
int pos;
int valid = TRUE;
int sign = FALSE;
// empty string is NOT valid
if (val[0] == '\0' || val[0] == '\n')
return FALSE;
// we have a sign
if (val[0] == '-' || val[0] == '+')
{
sign = TRUE;
start = 1;
}
// a sign without any digit, invalid
if (sign == TRUE && val[1] == '\0' && val[1] == '\n')
return FALSE;
// now start processing digits
pos = start;
while (valid == TRUE && val[pos] != '\0' && val[pos] != '\n')
{
if (val[pos] < '0' || val[pos] > '9') // non-digit
valid = FALSE;
pos++;
}
// too many digits CANNOT fit into an int type
if (pos - sign > 9)
{
valid = FALSE;
printf("Too many digits...");
}
// print a message
if (!valid)
{
printf("Invalid integer\n");
}
return valid;
}
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