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POupdate random access file in c
 

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    <p>I want to do the following in C.</p> <p>I have a random access data file containing some records.</p> <p>The records are in the following format:</p> <pre><code>Acct# First Name Last Name Balance 0 "" "" 0.0 0 "" "" 0.0 0 "" "" 0.0 05 Joe Costanza 0.50 0 "" "" 0.0 0 "" "" 0.0 0 "" "" 0.0 19 Jason Bourne 58.00 0 "" "" 0.0 0 "" "" 0.0 42 Andy Der -15.12 0 "" "" 0.0 0 "" "" 0.0 </code></pre> <p>I want to subtract an amount from balance from all the records that have a non zero account number and write the new updated balance to the file for those records.</p> <p>This is what I have tried so far to accomplish the above.</p> <pre><code>#include &lt;stdio.h&gt; struct clientData { int acctNum; char lastName[15]; char firstName[10]; double balance; }; void textFile(FILE *readPtr); int main(void) { FILE *cfPtr; struct clientData client = {0, "", "", 0.0}; double serviceCharge = 5.0; cfPtr = fopen("credit.dat", "rb+"); fread(&amp;client, sizeof(struct clientData), 1, cfPtr); client.balance -= serviceCharge; fseek(cfPtr,(client.acctNum - 1) * sizeof(struct clientData) , SEEK_CUR); fwrite(&amp;client, sizeof(struct clientData), 1, cfPtr); fclose(cfPtr); return 0; } </code></pre> <p>No matter what I try, I just can't write a single updated record back to the file. I have even tried a single record without any while loop or if statement and it still does not work. Can anybody tell me what I am missing.</p> <p>loop:</p> <pre><code>fread( &amp;client, sizeof( struct clientData ), 1, cfPtr); while (!feof(cfPtr)) { if ( client.acctNum != 0 ) { client.balance -= serviceCharge; fseek(cfPtr, (-1 * sizeof(struct clientData)), SEEK_CUR); fwrite( &amp;client, sizeof( struct clientData ), 1, cfPtr); } fread( &amp;client, sizeof( struct clientData ), 1, cfPtr); } </code></pre>
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    1. COYou have `cfPtr` (not declared; opened but not used), `filePtr` (neither declared nor opened, but written to) and `readPtr` (neither declared nor opened, but read from). This code would not compile. Could you please show us the actual code you used, and tell us what it did and what you hoped it would do instead?
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      1. USGareth McCaughan
    2. COIn case the plethora of differently-named file variables indicates that some version of your code tried to open the same file multiple times, reading from one version and writing to another: don't do that, and if you really must do it then flush the file between writing to it and trying to read from it.
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      1. USGareth McCaughan
 

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