Print with junk values,c - c

Question

I'm having a problem and I can not figure out why.
In main I create 2 arrays of strings.
The function accepts 2 strings and creates a new array of the appropriate size according to the following requirement:
For example:
Array 1: aviv ,
Array 2: 12 ,
The new array: a12v12i12v
The new array must be exactly the size!
And then send the new array to main and main to print it.
I also print junk values.
I checked the size of the new array and it is the right size.
My code:
char* CreateString(char* str1, char* str2)
{
int length1, length2, length3 = 0;
int i,j, index_help = 0;
char *str3 = NULL;
length1 = strlen(str1);
length2 = strlen(str2);
for (i = 0; i < length1; i++) // Check the size of the new array
{
length3++;
if (i == (length1 - 1))
{
break;
}
for (j = 0; j < length2; j++)
{
length3++;
}
}
str3 = (char*)malloc(length3+1 * sizeof(char));
if (str3 == NULL)
{
printf("There is not enough memory space\n");
return 0;
}
for (i = 0; i < length1; i++) //Copying data
{
str3[index_help] = str1[i];
if (i == (length1 - 1))
{
break;
}
for (j = 0; j < length2; j++)
{
index_help++;
str3[index_help] = str2[j];
}
index_help++;
}
return str3;
}
int main()
{
char *str1 = NULL, *str2 = NULL,*str4=NULL;
int size1, size2,i;
printf("enter the size of string number 1:\n");
scanf("%d", &size1);
printf("enter the size of string number2 :\n");
scanf("%d", &size2);
str1 = (char*)malloc((size1 + 1) * sizeof(char));
if (str1 == NULL)
{
printf("There is not enough memory space\n");
return 0;
}
str2 = (char*)malloc((size2 + 1) * sizeof(char));
if (str2 == NULL)
{
printf("There is not enough memory space\n");
return 0;
}
printf("Enter a value for the first string (the size is:%d):\n", size1);
scanf("%s", str1);
printf("Enter a value for the second string (the size is:%d):\n", size2);
scanf("%s", str2);
str4 = CreateString(str1, str2);
printf("%s",str4);
printf("\n");
return 0;
}

Answer 1

The function should be declared like
char* CreateString( const char* str1, const char* str2);
because neither string str1 nor string str2 are changed in the function.
You have to append the result string with a terminating zero.
The length of the resulted string can be calculated simpler without using loops.
As the function strlen has the return type size_t then variables that accept the result of a function call should be also declared as having the type size_t instead of the type int.
The function can be implemented the following way as it is shown in the demonstrative program.
#include <stdio.h>
#include <string.h>
char * CreateString(const char *s1, const char *s2)
{
size_t n1 = strlen(s1);
size_t n2 = strlen(s2);
size_t n3 = n1 == 0 ? 0 : n1 + (n1 - 1) * n2;
char *s3 = ( char * )malloc(n3 + 1);
if (s3)
{
if (n2 == 0)
{
strcpy(s3, s1);
}
else
{
char *p = s3;
while (*s1)
{
*p++ = *s1++;
if (*s1)
{
strcpy(p, s2);
p += n2;
}
}
*p = '\0';
}
}
return s3;
}
int main( void )
{
char *p = CreateString("aviv", "12");
printf("\"%s\"\n", p);
free(p);
p = CreateString("", "12");
printf("\"%s\"\n", p);
free(p);
p = CreateString("a", "12");
printf("\"%s\"\n", p);
free(p);
p = CreateString("av", "12");
printf("\"%s\"\n", p);
free(p);
p = CreateString("av", "");
printf("\"%s\"\n", p);
free(p);
return 0;
}
The program output is
"a12v12i12v"
""
"a"
"a12v"
"av"