Output prediction cannot understand the while statement

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I'm looking for the expalanation of this problem. I cannot understand the while part and why does it print 6.

#include <stdio.h>
#include<stdlib.h>

int main() 
{
    int array[] = {1, 2, 4, 0, 4, 0, 3};
    int *p, sum = 0;

    p = &array[0];

    while (*p++)
        sum += *p;

    printf("%d\n", sum);
    return 0;
}

This is a more readable form of your while loop:

while (*p != 0)
{
  p = p + 1;   // you can put "p++;" here, which has the same effect
  sum += *p;
}

Now you should understand on your own.

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Initially the pointer p points to the first element of the array.

p = &array[0];

However after evaluation the condition of the while statement

while(*p++)

the pointer was incremented and after that it points to the second element of the array that is to 2.

Within the body of the loop there is used the incremented pointer.

So you will have

2 + 4

The forth element of the array is equal to 0. So the loop stops its iterations.

You can consider the condition in the while loop the following way if to rewrite it as a for loop

#include <stdio.h>

int main(void) 
{
     int array[] = {1,2,4,0,4,0,3};
    int *p, sum=0;
    p = &array[0];

    for ( int *tmp = p; ( ++p, *tmp != 0 ); tmp = p )
    {
        sum += *p;
    }

    printf("%d\n",sum);

    return 0;
}

If within the body to use the statement

sum += *tmp;

instead of

sum += *p;

then you will get the expected result equal to 7.

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you are discarding the value from first element of array.

that is the reason why you see answer 6

you sum elements after you already moved to the next element of array

you should increment pointer in the end of iteration

while(*p){
  sum += *p;
  p++;
}

or

for( ; *p ; p++){
  sum += *p;
}

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The loop runs until it encounters a zero value. However you increment your pointer before adding what it points to, to "sum".

So the loop adds the second and third elements to sum and stops at the fourth.

The value of sum is then: 0 + 2 + 4 = 6

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Just modify your while loop as

while(*p)
sum += *p++;

Explanation

++, –– are unary operators used to increment, decrement the value of a variable by 1.

They can be used as postfix or prefix

But this was not just your problem. While loop terminated when it receives 0. Your array contains one of it.

So, instead use

for(int i=0;i<7;i++)  //7 -> size of array
sum += *p++;

o/p - 14

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Output of Java Programs, Explanation: Initialization part of the for loop will be executed only once in the for time error saying error: incompatible types: String cannot be converted to int. 4) The while loop has two important parts: an expression that is tested for a true or false value, and: A) a statement or block that is repeated as long as the expression is true B) a statement or block that is repeated only if the expression is false C) one line of code that is repeated once, if the expression is true

Loops: while and for, For instance, the loop below outputs i while i < 3 : For example, we can omit begin if we don't need to do anything at the loop start. Like here:. In a sentinel-controlled while loop, the body of the loop continues to execute until the EOF symbol is read. True The control variable in a flag-controlled while loop is a bool variable.

Comments
  • Any non-zero number is true condition. Thus your loop runs till first 0.
  • For your own sake (and ours too) please invest a minute or two of your time and format your code
  • the root cause of the problem is the ignoring of the C operator precedence, where the ++ will be performed, then the dereference