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Interviewers are surprised with my answer to COMP storage?


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tejdeep0274

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Joined: 28 Jan 2015
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PostPosted: Mon May 23, 2022 11:14 am
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In general , COMP is similar to Binary format of storage and 8 bits or one byte can store signed values of -128 to +127 but when it comes to defining the picture class 1 to 4 of comp occupies 2 bytes then s9(1) comp should be able to store -32768 to +32767. But when i tried in online compilers and pass +127 value to s9(1) comp it stores only +7.


with the output as example i told the interviewers that s9(1) comp can store value upto -9 to 9 (tested in online compiler) and to store +123 we need to define s9(3) comp. But interviewers are surprised and i am confused. Please point me in right direction.
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sergeyken

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PostPosted: Mon May 23, 2022 8:33 pm
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Is your question about COBOL, or about the interviewers?
36_2_51.gif
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tejdeep0274

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PostPosted: Mon May 23, 2022 10:08 pm
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About cobol and specifically COMP storage understanding. 36_2_18.gif
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sergeyken

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PostPosted: Mon May 23, 2022 10:43 pm
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1. Learn how to use Code button when posting your samples of code/data to the forum. Never use attachments, and especially - screenshots!

2. The specification PIC S9(1) COMP (that is COMP-1 by default) is different from PIC S9(1). It often depends on both the platform where the program is running, and/or on the COBOL compiler version.

In case of System/Z any variable from PIC S9(1) COMP to PIC S9(4) COMP is compiled into the same IBM halfword binary field (that is DS H), which can actually store the values -32768 to +32767.

There are options in COBOL (do not recall them from my mind, and lazy to RTFM) to control: how to handle the values beyond the initially defined range (e.g. out of the range -9...+9).
It looks like the physically stored value +127, when DISPLAYED, is truncated to its initial definition S9(1).
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Rohit Umarjikar

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PostPosted: Tue May 24, 2022 5:55 am
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Moved to right section
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tejdeep0274

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PostPosted: Tue May 24, 2022 7:47 pm
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Now we are discussing comp storage and for defining NULL POINTER we use S9(4) COMP but in general . but a null pointer holds -1 when null is being fetched. The same -1 can be stored in any PIC b/w S9(1) COMP to S9(4) COMP. When we use only S9(4) COMP. Are we only to use any PIC as between 1 to 4?
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sergeyken

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PostPosted: Tue May 24, 2022 8:28 pm
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tejdeep0274 wrote:
Now we are discussing comp storage and for defining NULL POINTER we use S9(4) COMP but in general . but a null pointer holds -1 when null is being fetched. The same -1 can be stored in any PIC b/w S9(1) COMP to S9(4) COMP. When we use only S9(4) COMP. Are we only to use any PIC as between 1 to 4?

1. This is not NULL POINTER, but rather NULL INDICATOR used in DB2 for unloaded database table fields. They are two different entities!

2. How the mix between POINTER-INDICATOR is related to your original question???

3.
Quote:
Are we only to use any PIC as between 1 to 4?
- please, explain the meaning of this sentence?

You can use various PIC S9(n), and various PIC S9(m) COMP.
If 1<=m<=4 then binary halfword (2 bytes) is allocated for this field.
If m>4, then binary fullword (4 bytes) is allocated for this field.
For PIC S9(n) - exactly n bytes are allocated for this field, as unpacked decimal format.

It is explained in details in any COBOL manual, if you had ever read at least one of them.
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