检查用户输入的值是否为数字的基本/手动方法

Question

I've searched in and out of these forums but am still having trouble. My understanding of C is still very basic. I'm creating a small program that takes 3 numerical values entered by the user and then calculates the highest. I nailed that.

I now want to ensure that the user enters only integer values. I managed to get the prog to keep prompting the user to re-enter the value until it is within the specified numerical range (for example, any number between 1 and 1000 inclusive, piece of cake) but that's not good enough. I used 3 while loops to test each value, but this only works as long as the input is of type integer.

The trick is I cant use built in functions. It needs to be manual (sorry, poor choice of words) I tried to use char variables and x = getchar(); to get the ASCII value and test it in a condition but I can't get it working in a loop. (while/ do-while)

I also tried using a "for loop" and array variables but once again am struggling to get it to keep prompting the user.

I've also tried to test the value returned by scanf to see if its integer but my knowledge level of correct C syntax is level: noob. Either my loops don't loop or they loop infinitely.

Here is some sample code:

int x, y, z =0;

printf("Enter the first number:\n");
scanf("d", &x);
while (condition) /* Here is where I need to determine that the entered val is false */
{
    printf("Wrong input. Re-enter a valid value.\n");
    x =0;
    scanf("%d", &x); /*user re-prompted */
}

I'm getting the idea that I'll have to use ASCII and a loop, but I just can't get to it. Also, the values entered get sent to a function for comparing and are then returned.

Could someone give me some advice and a few tips please?

Much thanks

Answer 1

You would have to use something like fgets , and strtol :

long someValue;

char *bufEnd = NULL;
char buf[128]; // max line size

do {
    printf("enter a value: ");
    fgets(buf, 128, stdin);
    someValue = strtol(buf, &bufEnd, 10); // base 10
} while (bufEnd == buf || *bufEnd != '\n');

printf("got value: %li", someValue);

What we are doing here is we are tapping into strtol 's capability to tell us where it stopped parsing, by passing in bufEnd .

Then, we are making sure that bufEnd doesn't point to the beginning of buf (in which case, it didn't start with a number), and also checking to make sure that bufEnd points to \\n , or the end of the line (making sure that the user didn't enter something like 123abc , which strtol would interpret as 123 ). You may wish to trim buf of whitespace characters first, however.

Answer 2

You're absolutely on the right track with "scanf()". Just check the return value. If you don't get the expected #/values, then you got invalid input:

char found = FALSE;
int ival;
double x;

while (!found)
{
    printf("Please enter a valid integer: ");
    if (scanf("%d", &ival) !=1) {
      printf ("Invalid!  Please re-enter!\n");
      continue;
    }
    printf("Please enter a valid floating point number: ");
    if (scanf("%lf", &x) !=1) {
      printf ("Invalid!  Please re-enter!\n");
      continue;
    }
    found = TRUE;
}

Answer 3

Here's my solution. It safe against buffer overflow and straightforward .

#include <stdio.h>
#define LEN 10

int main() {
    int a;
    char str[LEN];
    fgets( str, LEN, stdin );
    while ( !sscanf( str, "%d", &a ) )
        fgets( str, 10, stdin );
    printf("Num is : %d\n", a);
    return 0;
}

Answer 4

#include <stdio.h>
#include <limits.h>
#include <ctype.h>

int getInteger(int* err){
    int ch;
    int n;//int32
    int takeNum, sign;
    long long int wk;//long long int as int64

    wk=0LL;
    *err = 0;
    takeNum = 0;//flag
    sign = 1;//minus:-1, other:1
/*  //skip space character
    while(EOF!=(ch=getchar()) && (ch == ' ' || ch == '\t' || ch == '\n'));
    ungetc(ch, stdin);
*/
    while(EOF!=(ch=getchar())){
        if(ch == '-'){
            if(takeNum != 0){//in input number
                *err = 1;
                break;
            }
            if(sign == -1){//already sign
                *err = 2;
                break;
            }
            sign = -1;
            continue;
        }
        if(ch >= '0' && ch <= '9'){//isdigit(ch) in ctype.h
            if(takeNum == 0)
                takeNum = 1;
            wk = wk * 10 + (ch - '0')*sign;
            if(INT_MAX < wk || INT_MIN > wk){//overflow
                *err = 3;
                break;
            }
            continue;
        }
        if(ch != '\n'){//input other [-0-9]
            *err = 4;
        }
        break;
    }
    if(takeNum == 0){//not input number
        *err = 5;
    } else {
        n=wk;
    }
    while(ch != '\n' && EOF!=(ch=getchar()));//skip to newline
    return n;
}

int getValue(const char* redoprompt, int low, int high){
    int num, err=0;

    while(1){
        num = getInteger(&err);
        if(err || low > num || high < num)
            printf("%s", redoprompt);
        else
            break;
    }
    return num;
}

#define max(x,y) ((x)>(y))? (x) : (y)

int main(){
    const char *error_message = "Wrong input. Re-enter a valid value.\n";
    int x, y, z, max;

    x = getValue(error_message, 1, 1000);
    y = getValue(error_message, 1, 1000);
    z = getValue(error_message, 1, 1000);
    max = max(max(x,y), z);
    printf("max:%d\n", max);
    return 0;
}