How to create a string of variable length with character X in C

Question

I am new to C and am having some troubles with strings. How do I create a string of variable length containing a specified character in C? This is what I have tried but I get a compiler error:

int  cLen     = 8    /* Specified Length    */ 
char chr      = 'a'; /* Specified Character */
char outStr[cLen];
int  tmp      = 0;
while (tmp < cLen-1)
  outStr[tmp++] = chr;

outStr[cLen-1] = '\0';

/* outStr = "aaaaaaaa" */

Answer 1

You can try:

char *str = malloc(cLen + 1);
memset(str, 'a', cLen);
str[cLen] = 0;

Answer 2

Strings in C might not be as flexible as you want, on the first look.

What you did with "char outStr[]" was to indicate you'd like a pointer to char, that can be iterated with array syntax... it creates no actual storage for the characters, because you never mentioned how many you would like to store.

In C you can have the storage decoupled from these special variables, called pointers. The example of wanting a variable length string is actually a good example of why would you want that: I want an entity that holds knowledge of where the storage is at; I want methods to allow me to change the storage size.

So, you prepare yourself to deal with dynamic memory allocation by including

#include <stdlib.h>

declare a pointer to chars by

char *cpString;

you ask for an allocation of "n" chars with

cpString=malloc(n*sizeof(char));

Now you can strcat, printf, whatever you want to do with a string that has n-1 charaters (because it must be null terminated). Specifically, you can now initialize your string with

memset(cpString,X,n-1);
cpString[n]=0;

which creates a XXXX...XXX\0 string, of n-1 characters.

When you want to change cpString storage size, here's the tricky part, you need to free the allocated memory before you request for a new storage allocation

if (cpString !=0)
{
 free(cpString);
 cpString=0;
}
cpString=malloc(n*sizeof(char));

otherwise the dynamic memory storage area (called a "heap") is left with an un-reclaimable piece of the old n size.

There are better allocators, that don't need free(), but I better leave you studying and practicing with malloc() free() usage.