arrays of int, parsing, changing one element

This is a discussion on arrays of int, parsing, changing one element within the C Programming forums, part of the General Programming Boards category; Dear Fellow coders I have an array of integers in main() and all elements are given a zero value int ...

  1. #1
    Registered User
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    Nov 2001
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    arrays of int, parsing, changing one element

    Dear Fellow coders

    I have an array of integers in main() and all elements are given a zero value
    int counts[10] = {0,0,0,0,0,0,0,0,0,0}

    I am parsing a pointer to this array to another function:
    anotherfuntion(counts);

    1) void anotherfunction(int *num)
    2) {
    3) (num)++; /*Compiles without messages but does not increment value to one*/
    4) printf(“%d”, *num); /*Prints zero*/
    5) }
    from within anotherfunction() I want to increment the value in element 0, (counts[0]) by one and then print it out. So far this has not worked.

    I have tried reading my original study manual, but this does not include an example of incrementing the value inside an element using “++”. It shows how to move along the array using pointer arithmetic.

    I have also tried many variations as follows:
    *num++;
    *(num)++;
    (*(num)++);
    *num+0++;
    *(num+0)++;
    (*num+0)++;
    These produce a message “L value required”. My compiler says “An lvalue is an object locator: an expression that designates an object. An example of an lvalue expression is *P, where P is any expression evaluating to a non-null pointer.
    A modifiable lvalue is an identifier or expression that relates to an object that can be accessed and legally changed in memory.”
    Wow. However, I am sorry to say I do not properly understand this.

    I have performed a step through/watch and seen that line 3 does not increment element zero.

    I am using Borlands DOS Turbo C++ V 3.0 complier

    Any help or guidance appreciated

    Stephanos

  2. #2
    Registered User Sargnagel's Avatar
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    Aug 2002
    Posts
    166
    You must dereference the pointer first - then you can increment the value the pointer is pointing at.
    The following code works fine with Borland Command Line Tools 5.5.1.

    Code:
    #include <stdio.h>
    
    void func(int *num);
    
    int main()
    {
        int array[10] = {0};
    
        func(array);
    	
        return 0;
    }
    
    void func(int *num)
    {
        (*num)++;
        printf("%d", *num);
    }

  3. #3
    Me want cookie! Monster's Avatar
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    Dec 2001
    Posts
    680
    If you want to change other elements in the array you need to pass the number of elements in the array:

    Code:
    #include <stdio.h>
    
    void func(int *array, int nofelements);
    
    void func(int *array, int nofelements)
    {
       int i;
       for(i = 0; i < nofelements; i++)
          array[i] = i;
    }
    
    int main(void)
    {
       int i;
       int array[10] = {0,0,0,0,0,0,0,0,0,0};
    
       func(array, sizeof(array)/sizeof(*array));
    
       for(i = 0; i < sizeof(array)/sizeof(*array); i++)
          printf("array[%d] = %d\n", i, array[i]);
    
       return 0;
    }

  4. #4
    End Of Line Hammer's Avatar
    Join Date
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    Posts
    6,231
    >>int counts[10] = {0,0,0,0,0,0,0,0,0,0};
    This is better written:
    >>int counts[10] = {0};
    All remaining elements are initialised to 0 by default.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

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