string input parameter for c program

Question

I wrote a function to replace the blank spaces with tab character. But when I tried to implement it using function. I am quite not understanding how to call this function. I need functions

Which takes string from user as input,
Second function which replaces the blank space with tab character,
Function to print the modified string.

I achieved second one:

void SecondFunction()
{
     char string[] = "I am new to c";
     char *p = string;

     for (; *p; ++p)
     {
         if (*p == ' ')
             *p = '\t';
     }
     printf(string);
}

And when I tried to call this function like:

int main()
{
     SecondFunction("Hi s"); 
}

By changing my function to:

void SecondFunction(char* str)
{
    char string[] = str;
    char *p = string;
    ....
    ...etc
}

I get the following error:

 error: invalid initializer
   char string[] = str;
   ^

Please, can anybody help me to write the 3 functions of my requirement?

You can't assign char string[] = str; You could do char *string = str; but you don't need it. Just do char *p = str;. — lurker
– lurker, Commented Mar 21, 2015 at 14:39
i did like void SecondFunction(char* str) { char *p = str; ...bla bla .. printf(str);} It is giving error as "Segmentation fault (core dumped)" — Ajay Philia
– Ajay Philia, Commented Mar 21, 2015 at 14:44
instead of printf(string) you should do printf("%s", string); should your input value have % characters, your program would have undefined behaviour. — Antti Haapala
– Antti Haapala, Commented Mar 21, 2015 at 15:48

flogram_dev · Accepted Answer · 2015-03-21 16:26:55Z

Reading user input

To read input from the user you can use scanf. You need to pass it the memory address of the variable where you want to store the input:
```
char userinput[256]; // make it large enough to hold what the user inputs
scanf("%s", userinput); // array decays to pointer so no '&' here
```
The %s means were reading string input. We could also read an int using %d, like this:
```
int i;
scanf("%d", &i); // note the address-of operator '&' to get the address of i
```
Printing variables

Your SecondFunction is almost correct. To printf a C-string you need to use a syntax similar to when you scanf to a variable:
```
printf("%s", string);
```
Similarly, you could print the int i like this:
```
printf("The number is: %d", i);
```
Copying C-strings

When you tried doing this: char string[] = str, that's not possible. Arrays cannot be assigned or even copy constructed.

Just in case for the future, when you want to copy a C-string, you need to use strcpy:
```
char string[256]; // again, must be large enough to hold the new contents
strcpy(string, str); // copies from str to string
```
So in conclusion, your function could look something like this:
```
void SecondFunction(char* str)
{
    char string[256];
    strcpy(string, str);
    char *p = string;
    for (; *p; ++p)
    {
        if (*p == ' ')
            *p = '\t';
    }
    printf("%s", string);
}
```
Bonus: Why you can't write to the str parameter directly

When you write this: SecondFunction("Hi s"), the string "Hi s" gets stored in a read-only memory segment.

If you then go and try to modify the parameter inside SecondFunction, you get undefined behavior, possibly a segmentation fault.

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