Problems when printing an array with pointers in C

Question

I'm new to pointers and I'm trying to implement a simple code, in which i read and write an array and I also search for the minimum element between two given indexes. For some reason the write() function prints large values (I'm guessing they are the locations in which values are stored). Also I receive a SIGSECV when I run the poz_minVal function. I know this is a nooby question, but any help will de much appreciated!

The code is as follows:

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

int read(int *v, FILE *f){
    int i,length;
    fscanf(f,"%d",&length);
    v = (int *)malloc(length*sizeof(int));
    for(i=0;i<length;++i)
        fscanf(f,"%d",v+i);
    return length;
}

void write(int *v, int length, FILE *g){
    int i;
    for(i=0;i<length;++i)
        fprintf(g,"%d ",*(v+i));
    fprintf(g,"\n\n");
}

int poz_valMin(int *v, int length,int left,int right){
    int k,mini = *v; //set mini as the 1st element in v
    for(k=left;k<right;++k)
        if(*(v+k) < mini)
            mini = *(v+k);
    return mini;
}

int main(){
    FILE *f = fopen("datein.txt","r");
    FILE *g = fopen("dateout.txt","w");
    int *v, n = read(v,f);
    write(v,n,g);
    fprintf("The minimum value between the indexes 2 and 4 is: %d.\n\n",poz_valMin(v,n,2,4));
    return 0;
}

Instead of *(v+i), it's easier and more idiomatic to write v[i]. — G. Sliepen
– G. Sliepen, Commented Jan 27, 2017 at 11:26

alk · Accepted Answer · 2017-01-27 12:38:38Z

3

Whatever barak manos has said is right, but let me point another way of defining read function.

int * read(FILE *f){
   int *v,length;
   fscanf(f,"%d",&length);
   v = (int *)malloc(length*sizeof(int));
   for(i=0;i<length;++i)
       fscanf(f,"%d",v+i);
   return v;
}

and then in main you should just write v = read(f);
This looks clean and simplifies the understanding.

A tip :
Once you notice that write is producing garbage, you should have printed the array v, and you could have caught the mistake !!

edited Jan 27, 2017 at 12:38

alk

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answered Jan 27, 2017 at 11:26

Prem KTiw

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4 Comments

Paul R Over a year ago

Note that it is redundant and potentially dangerous to cast the result of malloc and friends in C.

theSongbird Over a year ago

@alk This implementation is kinda awsome, thanks a lot for the idea!

alk Over a year ago

@MattB: It's prem ktiw's answer! I just corrected the grammar a bit.

Prem KTiw Over a year ago

@MattB The implementation is just a tweaking in your code only. And you can give a +1 to the answer as well :) Thanks to alk for the correction .

barak manos · Accepted Answer · 2017-01-27 11:23:56Z

2

Function read should take int** v, and assign *v = ....

Changing an input argument inside a function has no effect on the value of that argument outside the function.

answered Jan 27, 2017 at 11:23

barak manos

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Comments

sameerkn · Accepted Answer · 2017-01-27 11:26:39Z

1

//int read(int *v, FILE *f) //<-- in this case v will be treated as local variable inside read() and not the parameter passed from main.
int read(int **v, FILE *f)
{
    int i,length;
    fscanf(f,"%d",&length);
    //v = (int *)malloc(length*sizeof(int));
    int *arr = (int *)malloc(length*sizeof(int));
    for(i=0;i<length;++i)
        //fscanf(f,"%d",v+i);
        fscanf(f,"%d",arr+i);

    *v = arr;  //now you are actually assigning the allocated memory to main::v
    return length;
}

read(&v,f);  //call read like this.

edited Jan 27, 2017 at 11:26

answered Jan 27, 2017 at 11:24

sameerkn

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1 Comment

Paul R Over a year ago

Note that it is redundant and potentially dangerous to cast the result of malloc and friends in C.

user3629249 · Accepted Answer · 2017-01-28 18:49:28Z

the following code:

incorporates the comments to the question, except good variable naming
cleanly compiles
performs the desired operation
performs appropriate error checking

You should get in the habit of having the main() function (in those files that have a main() function) as the first function listed, Then use the appropriate 'prototype' statements for each of the sub functions

Suggest using size_t for values that must never be negative.

#include <stdio.h>  // fopen(), fclose(),
#include <stdlib.h>

size_t read(       int **v, FILE *f);
void   write(      int *v, size_t length, FILE *g );
int    poz_valMin( int *v, int left, int right );


int main( void )
{
    FILE *f = fopen("datein.txt","r");
    if( !f )
    {
        perror( "fopen for read of datain.txt failed" );
        exit( EXIT_FAILURE );
    }

    // implied else, fopen successful

    FILE *g = fopen("dateout.txt","w");
    if( !g )
    {
        perror( "fopen for write to dataout.txt failed" );
        fclose( f ); // cleanup
        exit( EXIT_FAILURE );
    }

    // implied else, fopen for output successful

    int *v;
    size_t n = read(&v,f);

    write(v,n,g);
    fprintf( stdout,
             "The minimum value between the indexes 2 and 4 is: %d.\n\n",
             poz_valMin(v, 2, 4));

    free( v );
    fclose( f );
    fclose( g );
}



size_t read(int **v, FILE *f)
{
    //int i;
    size_t length;

    if( 1 != fscanf(f,"%lu",&length) )
    {
        perror( "fscanf failed" );
        exit( EXIT_FAILURE );
    }

    // implied else, fscanf successful

    *v = malloc(length*sizeof(int));
    if( !v )
    {
        perror( "malloc failed" );
        fclose( f );
        exit( EXIT_FAILURE );
    }

    // implied else, malloc successful

    for(size_t i=0; i<length; ++i)
    {
        if( 1 != fscanf(f,"%d",(*v)+i) )
        {
            perror( "fscanf for array values failed" );
            free( *v );
            fclose( f );
            exit( EXIT_FAILURE );
        }

        // implied else, fscanf successful
    }

    return length;
}


void write(int *v, size_t length, FILE *g)
{
    //int i;

    for( size_t i=0; i<length; ++i )
        fprintf(g,"%d ",*(v+i));

    fprintf(g,"\n\n");
}


int poz_valMin( int *v, int left, int right )
{
    int k;
    int mini = *v; //set mini as the 1st element in v

    for(k=left; k<right; ++k)
        if(*(v+k) < mini)
            mini = *(v+k);

    return mini;
}

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