Decrementing an array and print a value if condition met

Question

I want to decrement a pointer to the last element of an array and check for the condition that if the value of the pointer was smaller than 5, do nothing and go to the next round of loop and if not, that is if the value is equal or bigger than 5, print the value. So, for the array in the example, I want only 6 to be printed, that is the first encounter of a value equal or bigger than 5. I tried the code below but while being compiled with no error, it doesn't print any value. I'd appreciate your comments on this.

#include<stdio.h>

//The is a C program to decrement an array from the last element to the first. 
    int x[11] = {5, 6, -4, -3, -2, -1, 4, 3, 2, 1, -2}; 
    int *lastElem, count;
int main (void) {
    lastElem = &x[10];
    for (count = 0; count < 11; count++)
        if (abs(*lastElem) < 5)
        continue;
        else 
        printf("%d\n", *lastElem--); 
    return 0;
}

the comparisiopn is wrong because the elements are smaller than 5 — Engine, Dec 01 '14 at 12:36

Sourav Ghosh · Accepted Answer · 2014-12-01T13:10:23.470

3

There is a problem in your decrement logic. If the value is less than 5, you're missing the decremet.

check the below code.

#include<stdio.h>

//The is a C programme to decrement an array from the last element to the first. 
    int x[11] = {5, 6, -4, -3, -2, -1, 4, 3, 2, 1, -2};
    int *lastElem, count;
int main (void) {
    lastElem = &x[10];
    for (count = 0; count < 11; count++)
    {
        if (*lastElem >= 5)
            {
            printf("%d\n", *lastElem);
             break ;
             }
        lastElem--;
    }
return 0;
}

EDIT:

As you modified your question to include the absolute value comparions, the changes are like below.

^{Note : When making some major changes [which will change the behaviour and output] to the original question asked, do make a note of that, and if possible, mention the edit properly.}

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

//The is a C programme to decrement an array from the last element to the first. 
int x[11] = {5, -6, -4, -3, -2, -1, 4, 3, 2, 1, -2};
int *lastElem, count;
int main (void) {
        lastElem = &x[10];
        for (count = 0; count < 11; count++)
        {
                if ( abs(*lastElem) >= 5)
                {
                        printf("%d\n", *lastElem);
                        break;
                }
                lastElem--;
        }
        return 0;
}

edited Dec 01 '14 at 13:10

answered Dec 01 '14 at 12:35

Sourav Ghosh

133,132
16
183
261

OP wanted the first occurence of the number greater than 4(6 in OP's case) to be printed and not all the numbers greater than 4 – Spikatrix Dec 01 '14 at 12:43
@coolguy I tried to point out yhe erroneous part. However, will update the answer to accomodate that also. – Sourav Ghosh Dec 01 '14 at 12:48
Sorry, a question, if these numbers were float rather than integers, would the only change be in the printf statement, that is `printf(%.2f\n", *lastElem);` – Lucia Dec 01 '14 at 15:40
@homa that's true but there will be other side effects. `abs ()` operates only on integers, if i'm not wrong. – Sourav Ghosh Dec 01 '14 at 15:43
fabs() works to get the absolute value of floating numbers in c – Lucia Dec 01 '14 at 16:34

score 3 · Answer 2 · answered Dec 01 '14 at 12:35

You never assign another address to lastElem than the inital value (that is the 10th element). Also, if you want to go backwards, set count to 10 and let it count to 0. In each loop you have to assign &x[count] to lastElem (or decrement it, as it is a pointer and the address will be decremented according to the object it points to).

Iharob Al Asimi · Answer 3 · 2014-12-01T12:54:51.413

1

The line printf("%d\n", *lastElem--); is not always executed it is only executed when abs(*lastElem) >= 5. This is how you should do it

#include<stdio.h>

//The is a C programme to decrement an array from the last element to the first. 
int x[11] = {5, 6, -4, -3, -2, -1, 4, 3, 2, 1, -2}; 
int *lastElem, count, value;
int main (void) 
{
    lastElem = &x[10];
    for (count = 0; count < 11; count++)
    {
        value = *(lastElem--);
        if (value < 5)
            continue;
        printf("%d\n", value);
        break;
    }
    return 0;
}

This will do it with continue.

edited Dec 01 '14 at 12:54

answered Dec 01 '14 at 12:38

Iharob Al Asimi

52,653
6
59
97

I exactly want this but this does not print any value, I don't understand! – Lucia Dec 01 '14 at 12:42
Yes, but this prints both 6 and 5, but I want it to print the first encounter of the value equal or bigger than 5 and then stop the loop. So, something like the previous code and when found 6, just break the loop. – Lucia Dec 01 '14 at 12:47
You say, you only want `6` to be printed, in that case you should change `if (*lastElem >= 5)` with `if (*lastElem > 5)` – Iharob Al Asimi Dec 01 '14 at 12:47
Yes, but I really am curious why the previous code didn't work as I had thought of the loop with using 'continue' – Lucia Dec 01 '14 at 12:48
@iharob , If you do that,then 5 won't be printed if the array had 5 instead of 6 – Spikatrix Dec 01 '14 at 12:49
@Cool Guy, he said he only wanted 6 to be printed. – Iharob Al Asimi Dec 01 '14 at 12:51
@Homa, it did not work with `continue` because `continue` means you continue looping without executing the code following it. – Iharob Al Asimi Dec 01 '14 at 12:52
@iharob, great, now I understand, thanks for your code! – Lucia Dec 01 '14 at 12:54
@iharob , True. But I'm saying that your code won't work if the first element greater than 4 from the end is 5. And the next number greater than 4 will be printed,if any,provided that it isn't 5 – Spikatrix Dec 01 '14 at 13:11
@Cool Guy Are you sure it won't check it? Change the array and see! – Iharob Al Asimi Dec 01 '14 at 13:21
Oh. Sorry .I became confused with `value = *(lastElem--);` – Spikatrix Dec 01 '14 at 13:43

Vlad from Moscow · Answer 4 · 2014-12-20T09:21:52.333

Here is the only correct code among presented here in other answers that does the task.:)

In fact you need a program that searches backward an alement of an array that satisfies a given condition. And if there is such an element then to output it.

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

int main( void ) 
{
    int a[] = { 5, 6, -4, -3, -2, -1, 4, 3, 2, 1, -2 }; 
    const size_t N = sizeof( a ) / sizeof( *a );
    int *first = a, *last = a + N;

    while ( first != last && ( unsigned int )abs( *( last - 1 ) ) < 5 ) --last;

    if ( first != last ) printf( "%d\n", *--last );

    return 0;
}

The output is

Below there is demonstrated a general approach for such tasks

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

int * find_backward( const int a[], size_t n, int ( *predicate )( int ) )
{
    const int *first = a, *last = a + n;

    while ( first != last && !predicate( *( last -1 ) ) ) --last;

    return ( int * )last;
}

int is_greater_or_equal_to_5( int x )
{
    return 5 <= ( unsigned int )abs( x );
}

int main( void ) 
{
    int a[] = { 5, 6, -4, -3, -2, -1, 4, 3, 2, 1, -2 }; 
    const size_t N = sizeof( a ) / sizeof( *a );


    int *target = find_backward( a, N, is_greater_or_equal_to_5 );  

    if ( target != a ) printf( "%d\n", *--target );

    return 0;
}

Using this approach you can use any integer arrays (even with zero size) and any conditions that are set by means of predicates.

For example if you want to find the last element of an array that is divisible by 3 you can write

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

int * find_backward( const int a[], size_t n, int ( *predicate )( int ) )
{
    const int *first = a, *last = a + n;

    while ( first != last && !predicate( *( last -1 ) ) ) --last;

    return ( int * )last;
}

int divisible_by_3( int x )
{
    return x % 3 == 0;
}

int main( void ) 
{
    int a[] = { 5, 6, -4, -3, -2, -1, 4, 3, 2, 1, -2 }; 
    const size_t N = sizeof( a ) / sizeof( *a );


    int *target = find_backward( a, N, divisible_by_3 );    

    if ( target != a ) printf( "%d\n", *--target );

    return 0;
}

The output is

Take into account that this function allows also to deal with constant arrays.:)

Decrementing an array and print a value if condition met

4 Answers4