Need some help, please!

Question

Hello, I am quite confused about the following pieces of rather elementary loops:

''' int main() {

   int  i;

    i = 0;
while ( i < 4 ) {
        printf( "while: i=%d\n", i );

     ++i; }

    printf( "After while loop, i=%d.\n\n", i );


    return 0;

} '''

and

'''

int main() {

   int  i;

    i = 0;
while ( i++ < 4 ) 
        printf( "while: i=%d\n", i );

    printf( "After while loop, i=%d.\n\n", i );

    return 0;

}

'''

Both of these examples compile correctly, but the results are different:

for the first example, the console output is:

while: i=1
while: i=2
while: i=3
while: i=4
After while loop, i=5.
Program ended with exit code: 0

And for the second example the result is:

while: i=0
while: i=1
while: i=2
while: i=3
After while loop, i=4.
Program ended with exit code: 0

I can imagine it has something to do with postfix vs prefix annotation of ++ (++i vs i++) but I ca't exactly put my finger on it. Could you please try to explain in a step by step manner?

THANK YOU!

Answer 1 · 2014-03-25T00:24:34Z

March 25, 2014 12:24am

A while loop is processed one command at a time. The reason why the results are different is because in program #1 you are auto incrementing before the loop checks the value of i.

while ( i++ < 4 )

i = 0 then you add one to i. i = 1, is i less than 4? Yes, so the program lets the loop run. when it runs again it increments again and checks the value. i = 1 + 1. i is now 2. This runs two more times. Is i less than 4? Yes, because it is equal to four and it runs again. After it has run four times the value is 5. Is i less than 4? No, so it stops the loop and then displays the 5. After while loop, i=5.

In program #2 the program assigns 0 to i then if i is less than 4, execute code block then it runs again. By the time you have run the loop 4 times i will = 4, so the loop reads it again. Is i less than 4? At this point i = 4, which is equal to 4, so it runs through one more time and sets i = 5. The loop checks i again. Is i less than 4? No, so it prints 4 without incrementing, because that only happens when i is less than four in your program. After your loop, the program will print the value of i which is now 4. After while loop, i=4.

Answer 2 · 2014-03-24T23:40:55Z

March 24, 2014 11:40pm

the postfix and prefix annotation of ++ it's important only when you assign the value to a variable like k = i++; vs k = ++i or in a condition like the while loop.

The 2 programs have 2 different output because of the condition in the while loop:

while ( i++ < 4 ) means that the variable "i" is compared to the condition ( < 4 ) with value of 0 , then it is incremented to the value of 1 and the first time the printf is called i is equal to 1

if you try to write while ( ++i < 4 ) the variable "i" is incremented to the value of 1 and then compared to < 4.

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Sebastian Nitu

Sebastian Nitu

Need some help, please!

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Quinton Casburn

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Roberto Alicata

Roberto Alicata