Check if input is a string (4 characters only) and if not return to input again

Question

My aim is to accept 4-digit numbers, and 4-character strings (string should not contain digits or special characters)

If an invalid input is given the program should not terminate and it must allow the user to enter the details and continue until he wish to terminate.

I am able to find whether the input is a digit.

if(scanf("%d",&input)!=1)
{
    printf("enter the number please");
    ... // I have option to re enter using while and flags
}
else
{
    // I continue my work
    ...
}

To check it is four digits I have tried using the commands

i=0;
num = input;
while(num>0)
{
    i = i+1;
    num = num/10;
}
if(i==4){
    ...//I continue
}
else
    printf("please enter four digit");

I have no idea of checking the same for characters. (I know how to check its length using strlen())

Please help me with the code in C. (Also help me to reduce/optimize the above logic to check whether the input is a 4-digit number)

You can check `num >= 1000 && num <=9999` to check 4-digit numbers. — Rohan, Jan 17 '14 at 05:56
@Rohan that's helpful. I could avoid the loop. thank you. please help me to find the solution in string. — MELWIN, Jan 17 '14 at 06:01

score 2 · Accepted Answer · answered Jan 17 '14 at 06:11

2

I believe you want 2 inputs a number and a string. You can do that as

int number= 0;
char string[10] = { 0 };

do {
    printf("please enter four digit");
    scanf("%d", &number);
    if(number >=1000 && number<= 9999)
       break;
} while(1);

do {
    printf("please enter four character string");
    fgets(string, sizeof(string), stdin);
    if(strlen(string) == 4)
       break;
} while(1);

answered Jan 17 '14 at 06:11

Rohan

52,392
12
90
87

for string its not working. can you show a complete prgm. the prgm should display "invalid please re enter" if its not correct. else let it continue till i press x. the code above never terminates. even if i enter a vaild or invalid input. please edit the answer or give new answer. thank you – MELWIN Jan 17 '14 at 06:41

score 0 · Answer 2 · answered Jan 17 '14 at 05:59

0

To check it is four digit number you can simply put a check whether the number lies between 1000 and 9999. (I am assuming you don't want the number to start with 0.)

answered Jan 17 '14 at 05:59

Sarwan

595
3
8
21

Bogdan · Answer 3 · 2014-01-17T17:11:00.807

0

char input[16];
int ok = 1, k = 0;
if (scanf("%s", input) > 0 && strlen(input) == 4) {
    // check if it's a word
    for (; k < 4; k++)
    if (!isalpha(input[k])) { 
        // check if it's a number
        for (int k = 0; k < 4; k++)
           if (!isdigit(input[k])) 
               ok = 0;
        break;
    }
}
else ok = 0;
if (!ok) 
    printf("invalid input, please enter a 4-digit number or 4-letter word");
else {
    printf("valid input");
    ...
}

edited Jan 17 '14 at 17:11

answered Jan 17 '14 at 06:00

Bogdan

51
4

Because you want your input number to have 4 digits. – Bogdan Jan 17 '14 at 06:08
this code also dont accept a 4 letter string. please correct it. :( – MELWIN Jan 17 '14 at 10:56

phuclv · Answer 4 · 2019-07-02T06:55:31.640

0

You can use ~~gets()~~¹ fgets() to get the whole line and check line length. If the first character is between '0' and '9' then check the remaining if they are 3 numbers too. If the first character is a valid character in string then check the 3 remaining chars if it's also valid in string.

¹See Why is the gets function so dangerous that it should not be used?

edited Jul 02 '19 at 06:55

answered Jan 17 '14 at 06:13

phuclv

37,963
15
156
475

1

`gets()` is dangerous and deprecated – David Ranieri Jan 17 '14 at 07:23

score 0 · Answer 5 · answered Jan 17 '14 at 07:09

strtol can help:

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

int main(void)
{
    char s[32], *p;
    int x;

    fgets(s, sizeof(s), stdin);
    if ((p = strchr(s, '\n')) != NULL)
        *p = '\0';
    x = (int)strtol(s, &p, 10);
    if ((p - s) == 4) {
        printf("%d\n", x);  
    } else {
        printf("Please enter four digit\n");
    }
    return 0;
}

Check if input is a string (4 characters only) and if not return to input again

5 Answers5