I've been trying to code for subtraction of numbers from n number to 0 (i mean in decrementing order like if n is 5 then 5-4-3-2-1-0) in C language. But I'm not getting the correct answer for that. Here's my code for that, please correct me wherever it is wrong.

#include<stdio.h>

int main(void) {

    int i, n, sum;
    printf("Enter any number:");
    scanf("%d", &n);

    for(i=n; i>=0; i--) { 
        sum -= i;
    }

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

I have done the same code for addition of numbers from 0 to n numbers. I got that right, and according to that code I tried for the subtraction but wasn't getting the right answer. Please help me out, thank you.

CodePudding user response：

The variable sum was not initialized

int i, n, sum;

So it has an indeterminate value. As a result this statement

sum -= i;

invokes undefined behavior.

You need initially to set it to the entered value of the variable n.

Also as the variable n has the signed integer type int then nothing prevents the user to enter a negative number. In this case the for loop will not be executed.

If I have understood correctly the assignment

i mean in decrementing order like if n is 5 then 5-4-3-2-1-0)

then your program can look the following way

#include<stdio.h>

int main(void) 
{
    printf( "Enter any number: " );
    
    int n = 0;
    scanf( "%d", &n );
    
    int sum = n;

    while ( n < 0 ? n   : n-- ) sum -= n;
    
    printf( "sum = %d\n", sum );
}

If to enter the number 5 then the output will be

Enter any number: 5
sum = -5

If to enter the number -5 then the output will be

Enter any number: -5
sum = 5

CodePudding user response：

1. Always check the return value of scanf.
2. sum -= i makes no sense as it makes sum 0 on the first iteration. Simply decrease the control variable.
3. You try to overcomplicate it without any need.

int main(void)
{
    int  start;
    do
    {
        printf("\nEnter number:");
    }while(scanf(" %d", &start) != 1);

    printf("\n");

    for(int current = start; current >= 0; current--) 
        printf("%d%s", current, current ? ", " : "\n");
}

https://godbolt.org/z/6bqnEr7nq

CodePudding user response：

First thing's first, you've got to initialize your variable sum. Furthermore - there is no need for the loop to iterate when i is 0, as it just subtracts 0 off the number. One last thing - you are missing a return statement for your main function. This code would do -

#include<stdio.h>

int main(void) {

    int i, n, sum = 0;
    printf("Enter any number: ");
    scanf("%d", &n);

    for(i = n; i > 0; i--) { 
        sum -= i;
    }

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