== vs =
The Program
<![CDATA[
/*****************************************
Program: equal.c.
Author: New C programmers
Date: Every semester
Section: All sections
Email: everyone@umbc.edu
An example of a common, but bad bug.
*****************************************/
#include <stdio.h>
int main()
{
int n ;
/* Set n to be FALSE */
n = 0 ;
/*********************
** Error happens here. I meant to say if n is
** FALSE print "My program works", but instead I
** really set n to be FALSE again because I used =
** (the assignment operator) instead of == for
** comparison for equality. The expression (n = 0)
** evaluates to 0, so instead of the condition
** being TRUE, it is FALSE and the program prints
** "What's wrong??"
********************/
if ( n = 0 )
{
printf("My program works!\n") ;
}
else
{
printf("What's wrong??\n") ;
}
return 0;
}
]]>
The Sample Run
<![CDATA[
What's wrong??
]]>
The Lesson
- A very common bug is using one = instead of
== to test for equality in an if statement. A
seemingly correct program can produce strange results.
Salvation !!
Most newer compilers (including ours at UMBC) will report a
warning message if you use '=' in an if statement.
linux1[108] % gcc -Wall -ansi equal.c
equal.c: In function `main':
equal.c:28: warning: suggest parentheses around assignment used as truth value
linux1[109] %