how can i increment an element of a 1D array in function in C

Question

if you look at the function i produced below, (intensity) i want to increment that element of intensity_histogram using the pointer so *(pointer+1)++ means i intend to increment element 1 of array_histogram, however the program doesnt allow me to do so, errors are formed, hence my code does not allow me to compile, errors ar at each pointer increment, please help

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

int mean_func (int array[16]);

void intensity(int array[16], int *pointer);

int main()

{
int pix_in[4][4] = {{0,1,2,3},{4,5,6,7},{0,1,2,3},{4,5,6,7}};
int pix_out[16];
int i;
int j;
int n = 0;
int mean;
int intensity_histogram[7];
for(i =0;i <4; i++ )
{//open for
    for(j=0;j<4;j++)
    {//open second for

        pix_out[n] = pix_in[i][j];
        n++;
    }//close second for

}//close for
for(i = 0; i<16; i++)


mean = mean_func(pix_out);

intensity(pix_out, intensity_histogram);
for(i=0;i<8;i++)
printf("%d\n", intensity_histogram[i]);
return 0;
}
int mean_func (int array[16])
{
int sum = 0;
int i;
int j;
int average;
for(i = 0; i<16; i++)
{
    sum = sum + array[i];
}
average = sum/16;
return average;

}
void intensity (int array[16], int *pointer)
{//open intensity
int i;

for(i = 0 ; i<16;i ++)
{//open for

    if(array[i]== 0)
    {//open first if
    *pointer++;
    }//close first if
    else if( array[i]== 1)
    {
        *(pointer+1)++;
    }
    else if( array[i]== 2)
    {
    *(pointer + 2)++;   
    }
    else if( array[i]== 3)
    {
    *(pointer + 3)++;   
    }
    else if( array[i]== 4)
    {
    *(pointer + 4)++;   
    }
    else if( array[i]== 5)
    {
    *(pointer + 5)++;   
    }
    else if( array[i]== 6)
    {
    *(pointer + 6)++;   
    }
    else if( array[i]== 7)
    {
    *(pointer + 7)++;
    }
}//close for


}//close intensity */

score 1 · Accepted Answer · 2013-12-30 07:49:29Z

Although others provided correct solutions to the question, namely instead of *(pointer+1)++;:

(*(pointer+1))++;

or

pointer[1]++;

I want to point out the error:

The increment operator ++ takes precedence over the dereferencing operator *. Therefore the order in which *(pointer+1)++; is evaluated is first pointer+1, the result of which is then applied the increment operator. This is not possible since pointer+1 is a temporary value that is nowhere stored, it is no lvalue. Incrementing it makes no sense and is a syntax error.

Applying brackets, such that the dereferencing happens before the increment, results in correct code, as the dereferenced pointer is an actual variable in memory.

The code also has another issue which you might or might not be aware of. Because of the missing indentations its not obvious, but you are executing the line mean = mean_func(pix_out); 16 times. This seems unintended.

Also all your if ... else if ... constructs can be compactified to:

if(array[i] >= 0 && array[i] <= 7)
    pointer[array[i]]++;

MOHAMED · Accepted Answer · 2013-12-30 09:23:42Z

1

try to use (pointer[1])++ instead of *(pointer+1)++

*(pointer+1)++ is equivalent to *((pointer+1)++) and not equivalent to (*(pointer+1))++.

So the incrementation here is applied on (pointer+1) and (pointer+1) is not an lvalue

the result of (pointer+1) is an rvalue (not an lvalue) so you can not assign to it a value

edited Dec 30, 2013 at 9:23

answered Dec 30, 2013 at 6:58

MOHAMED

44k62 gold badges178 silver badges293 bronze badges

Comments

Jagadesan · Accepted Answer · 2013-12-30 07:02:31Z

0

intensity(pix_out, intensity_histogram); --> invalid argument passing to the function

Because in this function void intensity (int array[16], int *pointer); --> In this function your declaring as int pointer

So you have pass an argument as intensity(pix_out, &intensity_histogram);

This might work

Enjoy your coding.

answered Dec 30, 2013 at 7:02

Jagadesan

112 bronze badges

1 Comment

user3072164 Over a year ago

intensity_histogram is declared as array and is therefore handled as pointer to the first element if [] is omitted. This is no error.

user2760375 · Accepted Answer · 2013-12-30 07:05:59Z

0

    (*(pointer+i))++;

or

pointer[i]++;

this is the problem with the associativity

answered Dec 30, 2013 at 7:05

user2760375

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1 Comment

user2760375 Over a year ago

(*(pointer+i))++; *(pointer+i)++; the difference is in between two is, in first de-reference occur first and then value is incremented while in second due to precedence problem increment is done before de-reference.if you want de-reference first better to associate this deference first by enclosing this in parenthesis so that anyhow de-reference will occur first before increment operation.

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