C Programming - Control Instructions - Discussion
#include<stdio.h>
int main()
{
int i = 5;
while(i-- >= 0)
printf("%d,", i);
i = 5;
printf("\n");
while(i-- >= 0)
printf("%i,", i);
while(i-- >= 0)
printf("%d,", i);
return 0;
}
Step 1: Initially the value of variable i is '5'.
Loop 1: while(i-- >= 0) here i = 5, this statement becomes while(5-- >= 0) Hence the while condition is satisfied and it prints '4'. (variable 'i' is decremented by '1'(one) in previous while condition)
Loop 2: while(i-- >= 0) here i = 4, this statement becomes while(4-- >= 0) Hence the while condition is satisfied and it prints '3'. (variable 'i' is decremented by '1'(one) in previous while condition)
Loop 3: while(i-- >= 0) here i = 3, this statement becomes while(3-- >= 0) Hence the while condition is satisfied and it prints '2'. (variable 'i' is decremented by '1'(one) in previous while condition)
Loop 4: while(i-- >= 0) here i = 2, this statement becomes while(2-- >= 0) Hence the while condition is satisfied and it prints '1'. (variable 'i' is decremented by '1'(one) in previous while condition)
Loop 5: while(i-- >= 0) here i = 1, this statement becomes while(1-- >= 0) Hence the while condition is satisfied and it prints '0'. (variable 'i' is decremented by '1'(one) in previous while condition)
Loop 6: while(i-- >= 0) here i = 0, this statement becomes while(0-- >= 0) Hence the while condition is satisfied and it prints '-1'. (variable 'i' is decremented by '1'(one) in previous while condition)
Loop 7: while(i-- >= 0) here i = -1, this statement becomes while(-1-- >= 0) Hence the while condition is not satisfied and loop exits.
The output of first while loop is 4,3,2,1,0,-1
Step 2: Then the value of variable i is initialized to '5' Then it prints a new line character(\n).
See the above Loop 1 to Loop 7 .
The output of second while loop is 4,3,2,1,0,-1
Step 3: The third while loop, while(i-- >= 0) here i = -1(because the variable 'i' is decremented to '-1' by previous while loop and it never initialized.). This statement becomes while(-1-- >= 0) Hence the while condition is not satisfied and loop exits.
Hence the output of the program is
4,3,2,1,0,-1
4,3,2,1,0,-1
But due to post-decrement property, the value of i will be decremented only after it had been used for comparison.
So at the first iteration, the condition is true because 5>=0 and then i is decremented. Therefore 4 will be printed.
Similarly the loop continues and the value of i keeps on decrementing.
Let us see what what happen when condition of while loop becomes 0 >= 0. At this time, condition is met and i is decremented.
printf("/n %d/",i)
Why does it print i = 4 ? why not i = 5 ? in post decrement, at first time use, the value is unchanged, but in second use it's decremented, but here its decrementing in first use itself .
It is not in the use of first time. The post decrement means it will be decremented after the execution of that particular instruction.
%i is a format specifier for integer same as %d.
In above we are using a post increament so the value printed from 4.
How cum integer value of I is printed if %i is declared. ?