Logic error

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For logical error in argument, see Fallacy.

In computer programming, a logic error (sometimes called a semantic error) is a bug in a program that causes it to operate incorrectly, but not to terminate abnormally (or crash). A logic error produces unintended or undesired output or other behavior, although it may not immediately be recognized as such.

Logic errors occur in both compiled and interpreted languages. Unlike a program with a syntax error, a program with a logic error is a valid program in the language, though it does not behave as intended.

1 Common causes
2 Debugging logic errors
3 Examples
4 See also

[edit] Common causes

The mistake could be the logical error in a statement (for example, a wrong or incorrect formula), an error in an algorithm, or even the wrong algorithm selected.

[edit] Debugging logic errors

One of the ways to find these type of errors is to output the program's variables to a file or on the screen in order to define the error's location in code. Although this will not work in all cases, for example when calling the wrong subroutine, it is the easiest way to find the problem if the program uses the incorrect results of a bad mathematical calculation.

[edit] Examples

This example in C contains a logic error. After 'scanf', 'money_in_store' is checked instead of 'money'.

#include <stdio.h>
 
int money, money_in_store;
 
int main()
{
   do
   {
      printf("Enter amount of money (0 to exit): ");
      scanf("%d", &money);
      if (money_in_store == 0)   // Should be 'if (money == 0)'
      {
         printf("%d money on exit\n", money_in_store);
         exit(0);
      }
      money_in_store += money;
   }
   while(1);
 
   return 0;
}