sprintf to reverse an int and store as char - c

Question

I"m trying to store int array as a str and display it but in the reverse order.
Its only while printing the str that i get junk.
What is wrong in my code?
int main() {
int a[] = { 1, 2, 3 }; // Output should be 321 (char)
int size = sizeof(a) / sizeof(int);
char str[size + 1];
int i;
for (size = size - 1; size >= 0; size--) {
sprintf(&str[size], "%d", a[size]);
//printf("%c\n", str[size]);
}
printf("%s\n", str); // I get garbage.
}

Answer 1

I modified your solution with several bug fixes. For starters, you can't assume that your integer array will only hold single digit values.
And that for loop as you have it:
for(size=size-1;size >= 0;size--)
Is very suspicious looking. (the index variable is the thing its based off?)
Simple solution
This is likely what you meant:
for(i = 0; i < size; i++) {
sprintf(&str[i],"%d", a[size-1-i]);
}
str[size] = '\0';
Or this:
str[size] = '\0';
for(i = size-1; i <= 0; i--) {
sprintf(&str[i],"%d", a[size-1-i]);
}
Better solution
I'm not sure what you are expecting to do if an integer within the a array is negative. So the - sign will just get inserted into str inplace.
The solution I have will first count how many chars are needed for each integer in a. Then it will allocate the str buffer with that length (+1 for null char).
Then we make use of the return value from sprintf to figure out where to concatenate onto. We could use strcat, but this is likely faster.
int main() {
int j = 0;
int a[] = { 1,2,3 }; // Output should be 321 (char)
int size = sizeof(a) / sizeof(int);
int length = 1; // +1 for final null char
// Count the size of characters needed for each integer
// Do a dummy sprintf and use its return value to figure out how many chars are needed
for (int i = 0; i < size; i++) {
char tmp[sizeof(int) * 5]; // sizeof(int)*5 is big enough to hold any integer including a negative value
length += sprintf(tmp, "%d", a[i]); // utilize the return value from sprintf and add it to the running length
}
char str[length];
str[0] = '\0'; // initially null terminate our string
// reverse print chars from a into str
for (int i = 0; i < size; i++) { // use i as index variable, not size
j += sprintf(str + j, "%d", a[size - 1 - i]);
}
printf("%s\n", str);
}

Answer 2

Alternative solution, closer to original posts, and clearly not trying to address the general problem (assume values are single digit):
int a[]={1,2,3}; // Output should be 321 (char)
int size = sizeof(a)/sizeof(int);
char str[size+1];
for(int i=0; i<size ; i++) {
str[size-1-i] = ‘0’ + a[i];
}
str[size] = 0;
printf("%s\n", str); // I get garbage.
}
Taking advantage of the assumed input value, converting each int to character representation at the reverse position.