Is it possible to use a variable from if block outside of that if block?

Question

I was wondering if there's a easy way to define a variable inside an if block and then use it out of that if statement (like you can in Python).

Like that:

    int a = 2;

    if (a <= 1) {
        char string[10] = "foo";
    }
    else {
        char string[10] = "bar";
    }

    printf("%s", string);

Output

bar

Answer 1

You can not declare a variable inside an if statement (or other compound block) and use it outside of it's scope.

If you need the value outside, then you have to declare an appropriate variable and assign it in the if statement.

In your example this would looke like this:

char string[10];

if (a <= 1)
    strcpy(string, "Jahr");
else
    strcpy(string, "Jahren");

printf("%s\n", string);

Answer 2

You can do that by declaring the value to use as static and taking out a pointer to that. Marking as static prevents the variables from vanishing on exiting from the scope.

    int a = 2;
    char* pString;

    if (a <= 1) {
        static char string[10] = "Jahr";
        pString = string;
    }
    else {
        static char string[10] = "Jahren";
        pString = string;
    }

    printf("%s", pString);

I think it is better to declare a variable outside the if statement and copying data into that inside the if statement.

    int a = 2;
    char string[10];

    if (a <= 1) {
        strcpy(string, "Jahr");
    }
    else {
        strcpy(string, "Jahren");
    }

    printf("%s", string);

Add #include <string.h> (if it doesn't already exists) at the top of your code to use strcpy().

Answer 3

The easy solution would be this:

int a = 2;
const char* str;

if (a <= 1) {
    str = "Jahr";
}
else {
    str = "Jahren";
}

puts(str);

Optionally, in case you need to write to the string later:

const char* STR_TABLE[] =
{
  "Jahr",
  "Jahren"
};

int a = 2;
size_t i;
if (a<=1)
{
  i = 0;
}
else
{
  i = 1;
}

size_t size = strlen(STR_TABLE[i]) + 1;

char rwstr [size]; // or just allocate it "large enough", couple of hundred bytes
memcpy(rwstr, STR_TABLE[i], size);

puts(rwstr);

The issue about buffer overruns doesn't exist since there is no user input.

The compiler will even be able to resolve all of this at compile-time. When I run this on gcc x64 with optimizations on, it just tosses some magic numbers corresponding to the ASCII into a buffer, then call puts.

Answer 4

If you declare something inside a scope, it only exists inside that scope and will effectively go away as soon as that block exits.

To fix this declare it one level up, outside that immediate scope:

int a = 2;
const int LENGTH = 10;       // Define the buffer length clearly
char string[LENGTH] = { 0 }; // Zero out buffer to start

if (a <= 1) {
  strncpy(string, "Jahr", LENGTH - 1);
}
else {
  strncpy(string, "Jahren", LENGTH - 1);
}

printf("%s", string);

Note the use of LENGTH here as a constant so you know how big that buffer is. sizeof(string) can work in some contexts, but if it decays to a pointer when making a function call that information is lost, so it will need to be passed in separately. It's just a good habit to get into. Changing that one value will change all places where it's used, versus having to hunt down to search and replace them all.

If you over-commit on this, it's still safe:

const int LENGTH = 10;
char string[LENGTH] = { 0 };

strncpy(string, "my spoon is too big", LENGTH - 1);

printf("%s", string);

Where here you get my spoon as output.

Tip: Use buffer-overflow resistant functions like strncpy() which will avoid creating a mess if you inadvertently overflow something. Using buffers that are super tiny like 10 bytes long is risky, always be generous with these unless you know specifically how they will be used.