How to use format specifier within switch in C

Question

I am trying to use format specifier within switch however compiler returns error: expression result unused

 const char* text;
 const char* input;
 int i=2;
 input = "orange";

 switch(i){
 case 1:
      text = "%s is color", input;
      break;
 case 2:
      text = "%s is fruit", input;
      break;
 default:
      break;
 }

First, you have to allocate memory for strings like: char text[128]; char input[128]; Then, you could use for example sprinf like this: sprintf(text, "%s is color", input); — ziza
– ziza, Commented Jul 5, 2018 at 7:50
It's actually not completely clear what you're trying to achieve. Please tell us what you intend to do with text. — Jabberwocky
– Jabberwocky, Commented Jul 5, 2018 at 7:55
The format specifier functionality is not a language-level thing, it's simply implemented by the printf() family of functions. In general in C if you want something "complicated" (such as converting various values to string representations) to happen, you need to call a function that implements that. — unwind
– unwind, Commented Jul 5, 2018 at 7:55
regarding: text = "%s is color", input; and similar statements: This is nonsense. Your compiler is telling you that. Strongly suggest using sprintf() to format the contents of a char array, then you could assign the address of that array to input or text — user3629249
– user3629249, Commented Jul 5, 2018 at 21:51

Lundin · Accepted Answer · 2018-07-05 07:56:34Z

3

You want to use sprintf or snprintf.

char text[128];
const char* input;
int i=2;
input = "orange";

switch(i){
case 1:
    sprintf(text, "%s is color", input);
    break;
case 2:
    sprintf(text, "%s is fruit", input);
    break;
default:
    break;
}

edited Jul 5, 2018 at 7:56

Lundin

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answered Jul 5, 2018 at 7:51

John Go-Soco

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5 Comments

Lundin Over a year ago

For this to make sense, text must point at allocated memory and it can't be a const char*.

Jabberwocky Over a year ago

This it not goint to work, text is not initialized and probably that's not what the OP wants

Mathieu Over a year ago

What about text?

John Go-Soco Over a year ago

@Lundin Thanks. I spotted it too and was about to fix it.

Aamir Shaikh Over a year ago

text is getting initialised within the switch case, and i need to print it outside it switch case

Jabberwocky · Accepted Answer · 2018-07-05 07:51:30Z

3

You probably want somewhing like this:

 const char* text;
 const char* input;
 int i=2;
 input = "orange";

 switch(i) {
 case 1:
      text = "%s is color";
      break;
 case 2:
      text = "%s is fruit";
      break;
 default:
      break;
 }

 printf(text, input);

answered Jul 5, 2018 at 7:51

Jabberwocky

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2 Comments

Toby Speight Over a year ago

You probably want to assign something to text in the default case (or omit default: and just initialize text at the declaration).

user3629249 Over a year ago

this will cause some compilers, like gcc to output a warning message about the security risk of using a variable pointer for the format string, rather than the actual format literal.

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