Strange output when using do/while in C programming lang

Question

I am trying to understand the output of this program. If I try to "translate" the code, I believe it should go like this:

while "j" is smaller than 3 - print "Ha" (this loop goes 3 times, so it gives 3 "Ha")
do/while -> j is equal to j - 2 hence print "Hi" while ++j - In the end the program prints out "Hi" 4 times.

How does the program prints it 4 times, how does the condition works here?

#include <stdio.h>

int main() {

    int j = 0;
    while(j++ < 3){
        printf( "Ha ");
    }
    do{
        j -= 2;
        printf( "Hi "); 
    }
    while(++j);
    for(j = 1; j <= 3; j++){
        printf( "Ho ");
    }
    printf("\n");
    return 0;
}

The output is:

Ha Ha Ha Hi Hi Hi Hi Ho Ho Ho

Try using a debugger to step through the code, statement by statement, while monitoring variables and their values. That should tell you the answer. You could also "step" through the code using pen and paper, where you write down all the values of all variables on a new line each time they change. — Some programmer dude, Nov 30 '20 at 15:32

score 1 · Answer 1 · answered Nov 30 '20 at 15:36

The ++j is prefix increment, i.e., the value will be increased and then, the incrased value will be used for the condition check.

I have added a print statement for the ease of understanding:

do{
    j -= 2;
    printf( "Hi "); 
    printf("value of j before the while = %d\n", j);
}
while(++j);

and the output is:

Hi value of j before the while = 2
Hi value of j before the while = 1
Hi value of j before the while = 0
Hi value of j before the while = -1

So, in the

first iteration, in while (++j), j is 2, and ++j is 3
second iteration, 3-2 = 1, and ++j is 2.
third iteration, 2-2 = 0, and ++j is 1.
Fourth iteration, 1-2 = -1, and ++j is 0 - this make the while chec false and the loop ends.

score 1 · Answer 2 · answered Nov 30 '20 at 15:37

After the first while loop

while(j++ < 3){
    printf( "Ha ");
}

j is equal to 4.

So within the first iteration of the do-while loop

do{
    j -= 2;
    printf( "Hi "); 
}
while(++j);

j is equal to 2. After the first iteration j is equal to 3. Within the second iteration j is equal to 1. After the second iteration j is equal to 2. Within the third iteration j is equal to 0. After the third iteration j is equal to 1. Within the forth iteration j is equal to -1. So in this forth iteration in the condition

while(++j);

j is equal to 0 and the control is passed to the next loop. So the do-while loop was executed 4 times.

That is the value of the postfix increment operator is the value of its operand before incrementing. And the value of the pre-increment operator is the value of its operand after incrementing.

score 1 · Answer 3 · answered Nov 30 '20 at 15:38

1

After first loop

j == 4
j -= 2 == 2
Hi
++j == 3
j -= 2 == 1
Hi
++j == 2
j -= 2 == 0
Hi 
++j == 1
j -= 2 == -1
Hi
++j == 0 //end of th loop

answered Nov 30 '20 at 15:38

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Hugo Riffaud de Turckheim · Answer 4 · 2020-11-30T16:07:31.513

The best way to understand what your program does is to watch the value of j at each step of your program.

first j=0 then you enter into the while.

1. j=0 ; j<3? ; yes ; j=j+1 ; it print "Ha".
1. j=1 ; j<3? ; yes ; j=j+1 ; it print "Ha".
1. j=2 ; j<3? ; yes ; j=j+1 ; it print "Ha".
1. j=3 ; j<3? ; no ; j=j+1 ; leave the while statement

j=4

then you leave the while to enter into de do, while() statement. at this moment j=4.

in the do , while() :

1. j=j-2 ; j=2 ; it print "Hi" ; j+=1 ; j=3; j!=0 ? yes
1. j=j-2 ; j=1 ; it print "Hi" ; j+=1 ; j=2; j!=0 ? yes
1. j=j-2 ; j=0 ; it print "Hi" ; j+=1 ; j=1; j!=0 ? yes
1. j=j-2 ; j=1 ; it print "Hi" ; j+=1 ; j=0; j=0 ? no -> leaving the do while()

at the end you enter in the for statement.

++x is called Pre incrementation (the variable is incremented before the expression evaluation)

x++ is the post incrementation (the variable is incremented after the expression evaluation

i'm not sure about this, you need to check this informations.

Strange output when using do/while in C programming lang

4 Answers4