main()
{
int N = 6;
if (N % 2 == 0 && N % 3 == 0)
printf("N has factors 2 and 3\n");
if (N % 2 == 0 && N % 3 != 0)
printf("N has factor 2 but not 3\n");
if (N % 2 != 0 && N % 3 == 0)
printf("N has factor 3 but not 2\n");
if (N % 2 != 0 && N % 3 != 0)
printf("N has neither factors 2 and 3\n");
}
If we look at the above code, and focus in on a certain if expression, we can break it down in a expression tree, which helps us understand how the expression is parsed, and the relative precedences of the operators in the expression.
graph TD
A[&&] --- B[==]
A --- C[==]
B --- D[%]
B --- E(0)
D --- F(N)
D --- G(2)
C --- H[%]
C --- I(0)
H --- J(N)
H --- K(3)
The && symbol represents a logical ‘AND’ expression, which returns a true value if both lhs and rhs are true.
The compiler uses something known as short-circuit evaluation in order to optimise such expressions. In the && expression, the lhs expression is evaluated first, and if it comes out to be false, a false value is immediately returned, without evaluating the rhs expression.
This optimization saves computation time, since the rhs expression doesn’t matter if the lhs expression is false; the && operator requires both to be true for it to return a true value.
int N = 23;
if ( N++ % 3 == 0 && N % 2 == 0)
printf("Expression is true");
Also consider the above code. Since the left subtree is evaluated first, the value of N tested in the right subtree is is not 23, but 24.
#include<stdio.h>
#define A_CUTOFF 90
#define B_CUTOFF 80
#define C_CUTOFF 70
#define D_CUTOFF 60
int main()
{
char grade;
int marks;
printf("Enter student marks: ");
scanf("%d", &marks);
if(marks >= A_CUTOFF)
grade = 'A';
else if(marks >= B_CUTOFF)
grade = 'B';
else if(marks >= C_CUTOFF)
grade = 'C';
else if(marks >= D_CUTOFF)
grade = 'D';
else
grade = 'F';
printf("Student Grade: %c, ", grade);
fflush(stdout);
switch(grade)
{
case 'A':
printf("Excellent\n")
break;
case 'B':
printf("Good\n")
break;
case 'C':
printf("Average\n")
break;
case 'D':
printf("Poor\n")
break;
case 'F':
printf("Failed\n");
break;
default:
printf("Illegal grade\n");
}
}
(talk a bit about above code)
If we compile a program in gcc using the -g flag, it generates extra debugging information for gdb
for eg: gcc -g marks.c
Now we can open up an executable with the debugger by entering the command gdb a.out.
We are greeted with some warranty and licensing information, followed by the gdb prompt:
(gdb)
We can add a break point in front of the main function, where the debugger pauses the program, and then run the program.
(gdb) b main
(gdb) run
(more on gdb)