Trying to remove all numbers from a string in C - c

Question

I'm trying to take all of the numbers out of a string (char*)...
Here's what I have right now:
// Take numbers out of username if they exist - don't care about these
char * newStr;
strtoul(user, &newStr, 10);
user = newStr;
My understanding is that strtoul is supposed to convert a string to an unsigned long. The characters that are not numbers are put into the passed in pointer (the 2nd arg). When i reassign user to newStr and print it, the string remains unchanged. Why is this? Does anyone know of a better method?
From the documentation example:
#include <stdio.h>
#include <stdlib.h>
int main()
{
char str[30] = "2030300 This is test";
char *ptr;
long ret;
ret = strtoul(str, &ptr, 10);
printf("The number(unsigned long integer) is %lu\n", ret);
printf("String part is |%s|", ptr);
return(0);
}
Let us compile and run the above program, this will produce the following result:
The number(unsigned long integer) is 2030300
String part is | This is test|

Answer 1

char* RemoveDigits(char* input)
{
char* dest = input;
char* src = input;
while(*src)
{
if (isdigit(*src)) { src++; continue; }
*dest++ = *src++;
}
*dest = '\0';
return input;
}
Test:
int main(void)
{
char inText[] = "123 Mickey 456";
printf("The result is %s\n", RemoveDigits(inText));
// Expected Output: " Mickey "
}
The numbers were removed.

Answer 2

Here is a C program to remove digits from a string without using inbuilt functions. The string is shifted left to overwrite the digits:
#include <stdio.h>
int main(void) {
char a[] = "stack123overflow";
int i, j;
for (i = 0; a[i] != '\0'; i ++) {
if (a[i] == '0' || a[i] == '1' || a[i] == '2' || a[i] == '3' || a[i] == '4' || a[i] == '5' || a[i] == '6' || a[i] == '7' || a[i] == '8' || a[i] == '9') {
for (j = i; a[j] != '\0'; j ++)
a[j] = a[j + 1];
i--;
}
}
printf("%s", a);
return 0;
}
Example of execution:
$ gcc shift_str.c -o shift_str
$ ./shift_str
stackoverflow

Answer 3

strtoul() does not extract all numbers from string, it just trying to covert string to number and convertion stops when non digit is find. So if your string starts from number strtoul() works as you expect, but if string starts from letters, strtoul() stops at the first symbol. To solve your task in simple way you should copy all non-digits to other string, that will be a result.

Answer 4

The problem you are having is that strtoul is converting characters at the beginning of the string into an unsigned long. Once it encounters non-numeric digits, it stops.
The second parameter is a pointer into the original character buffer, pointing at the first non-numeric character.
http://www.cplusplus.com/reference/cstdlib/strtoul/
Parameter 2 : Reference to an object of type char*, whose value is set by the function to the next character in str after the numerical value.
So, if you tried to run the function on "123abc567efg" the returned value would be 123. The original string buffer would still be "123abc567efg" with the second parameter now pointing at the character 'a' in that buffer. That is, the pointer (ptr) will have a value 3 greater than original buffer pointer (str). Printing the string ptr, would give you "abc567efg" as it simply points back into the original buffer.
To actually remove ALL the digits from the string in C you would need to do something similar to this answer : Removing spaces and special characters from string
You build your allowable function to return false on 0-9 and true otherwise. Loop through and copy out digits to a new buffer.