How to scanf only integer and repeat reading if the user enters non-numeric characters

Question

Here is some C code trying simply to prevent the user from typing a character or an integer less than 0 or more than 23    include  lt stdio h gt   include  lt stdlib h gt   int main void        const char  input      char  iPtr      int count   0      int rows       printf  Enter an integer          scanf   s   input       rows   strtol input   amp iPtr  0       while   iPtr      0      Check if any character has been inserted               printf  Enter an integer between 1 and 23              scanf   s   input             while 0  lt  rows  amp  amp  rows  lt  24     check if the user input is within the boundaries               printf  Select an integer from 1 to 23              scanf   s   input               while  count    rows                       Do some stuff                  return 0        I made it halfway through and a small push up will be appreciated

User · Answer

You will need to repeat your call to strtol inside your loops where you are asking the user to try again  In fact  if you make the loop a do         while       instead of while  you don t get a the same sort of repeat things twice behaviour    You should also format your code so that it s possible to see where the code is inside a loop and not

User · Answer

char check1 10   check2 10   int foo   do    printf   gt  gt        scanf    s   check1     foo   strtol check1  NULL  10      convert the string to decimal number   sprintf check2    d   foo      re-convert  foo  to string for comparison   while    strcmp check1  check2     0  amp  amp  0  lt  foo  amp  amp  foo  lt  24       repeat if the input is not number   If the input is number  you can use foo as your input

User · Answer

You could create a function that reads an integer between 1 and 23 or returns 0 if non-int  e g   int getInt       int n   0    char buffer 128     fgets buffer sizeof buffer  stdin     n   atoi buffer      return   n  gt  23    n  lt  1     0   n

User · Answer

include  lt stdio h gt  main         char str 100       int num      while 1            printf  Enter a number              scanf     0-9  d  str  amp num           printf  You entered the number  d n  num             return 0         0-9  in scanf   gobbles up all that is not between 0 and 9  Basically it cleans the input stream of non-digits and puts it in str  Well  the length of non-digit sequence is limited to 100  The following  d selects only integers in the input stream and places it in num

User · Answer

Use scanf   d   amp rows  instead of scanf   s  input   This allow you to get direcly the integer value from stdin without need to convert to int   If the user enter a string containing a non numeric characters then you have to clean your stdin before the next scanf   d   amp rows    your code could look like this    include  lt stdio h gt     include  lt stdlib h gt    int clean stdin         while  getchar      n        return 1     int main void           int rows  0        char c      do                 printf   nEnter an integer from 1 to 23             while    scanf   d c    amp rows   amp c   2    c    n    amp  amp  clean stdin       rows lt 1    rows gt 23        return 0        Explanation  1   scanf   d c    amp rows   amp c    This means expecting from the user input an integer and close to it a non numeric character   Example1  If the user enter aaddk and then ENTER  the scanf will return 0  Nothing capted  Example2  If the user enter 45 and then ENTER  the scanf will return 2  2 elements are capted   Here  d is capting 45 and  c is capting  n  Example3  If the user enter 45aaadd and then ENTER  the scanf will return 2  2 elements are capted   Here  d is capting 45 and  c is capting a  2    scanf   d c    amp rows   amp c   2    c    n     In the example1  this condition is TRUE because scanf return 0    2   In the example2  this condition is FALSE because scanf return 2 and c      n   In the example3  this condition is TRUE because scanf return 2 and c     a       n    3     scanf   d c    amp rows   amp c   2    c    n    amp  amp  clean stdin      clean stdin   is always TRUE because the function return always 1  In the example1   The  scanf   d c    amp rows   amp c   2    c    n   is TRUE so the condition after the  amp  amp  should be checked so the clean stdin   will be executed and the whole condition is TRUE  In the example2  The  scanf   d c    amp rows   amp c   2    c    n   is FALSE so the condition after the  amp  amp  will not checked  because what ever its result is the whole condition will be FALSE   so the clean stdin   will not be executed and the whole condition is FALSE  In the example3   The  scanf   d c    amp rows   amp c   2    c    n   is TRUE so the condition after the  amp  amp  should be checked so the clean stdin   will be executed and the whole condition is TRUE  So you can remark that clean stdin   will be executed only if the user enter a string containing non numeric character   And this condition   scanf   d c    amp rows   amp c   2    c    n    amp  amp  clean stdin    will return FALSE only if the user enter an integer and nothing else  And if the condition   scanf   d c    amp rows   amp c   2    c    n    amp  amp  clean stdin    is FALSE and the integer is between and 1 and 23 then the while loop will break else the while loop will continue

[c] How to scanf only integer and repeat reading if the user enters non-numeric characters?

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