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Short-Circuit Evaluation

Short-Circuit Evaluation

The Short-Circuit evaluation consist in check or execute the second argument only if the first argument is not enough to determine the value of the expression.

You can do a short-circuit evaluation with && and || operators.

Example with && operator:

  canOpenFile(filename) && openFile(filename); // If you can open the file then open it.

The example above is equivalent to:

  if ( canOpenFile(filename) ) {
    openFile(filename);
  }

Example with || operator:

  isServerOn || startServer(); // If the server is not on then start it.

The example above is equivalent to:

 if ( !isServerOn ) {
   startServer();
 }

Example with nested if statements

int i, j;
scanf ( "%d %d", &i, &j );
if ( i > 10 && j > 10 ) {
   printf("Both numbers are greater than 10! \n");
}

The above example is equivalent to:

int i, j;
scanf ( "%d %d", &i, &j );
if ( i > 10 ) {
   if ( j > 10 ) {
      printf("Both numbers are greater than 10! \n");
   }
}

Notice when if ( i > 10 ) fails, the statement is false and the check if ( j > 10 ) is never run. if ( i > 10 && j > 10 ) behaves exactly the same way, because if i > 10 is false then the entire statement is automatically false, and there is no need to run an additional check.

Keep it all together in real example

#include <stdio.h>
#include <stdlib.h>

char *getName();

int main(int argc, char *argv[]) { 
   // Get first argument passed via terminal
   char *name = argv[1];

   // If name is not passed via terminal then print message and then get the name
   name || printf("Please give me your name:") && (name = getName()); 

   printf("Hello %s\n", name);
}

char *getName() {
   // Allocate memory 
   char *name = malloc(30);
   
   scanf("%s", name);
   return name;
}

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