Restart cycle

Since the same cycle can be written in different ways, there are many approaches to solving your problem. For example, a loop might look like this. Keep in mind that the user may interrupt input, and you should handle this situation.

 n = 0; while ( scanf( "%i", &n ) == 1 && ( n < 1 || n > 3 ) ) { puts( "давай сначала" ); n = 0; } i = n; 

or the last sentence, depending on the context, can be replaced by

 if ( n != 0 ) i = n; 

Or if in real code you do not have such a straightforward assignment as

 if( n == 1 ) i=1; 

and there is some dependence of the value of the variable i on the enumeration of the values ​​of the variable n , then you can write

 int valid = 0; while ( !valid && scanf( "%i", &n ) == 1 ) { valid = 1; switch( n ) { case 1: i = 1; break; case 2: i = 2; break; case 3: i = 3; break; default: puts( "давай сначала" ); valid = 0; } } 

Or

 int valid = 0; while ( !valid && scanf( "%i", &n ) == 1 ) { valid = 1; switch( n ) { case 1: case 2: case 3: i = n; break; default: puts( "давай сначала" ); valid = 0; break; } } 

You can enter enumerations to enter mnemonic names for magic numbers 1, 2, 3. For example, if these values ​​correspond to the items of a menu, then you can write, let's say

 enum { Exit = 0, Add = 1, Division = 2, Multiply = 3 }; i = Exit; while ( i == Exit && scanf( "%i", &n ) == 1 ) { switch( n ) { case Add: case Division: case Multiply: i = n; break; default: puts( "давай сначала" ); break; } }