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The normal interaction between a person and a computer is for the person to enter information from the keyboard and for the computer to display information on the computer screen. The information that is entered or input is called the input stream and the information that is displayed or output is called the output stream. When Logo starts, the input stream originates from the keyboard and travels to the computer's processor as you type characters. The output stream originates from the computer and is directed to the screen for you to read.

You can use Logo to control the source of input and destination of output as well as its flow. Input and output streams allow you to use the same Logo commands and operations that you use between your keyboard and computer screen to communicate with other devices. For example, you can input characters from the keyboard, a disk file, or a telephone modem and direct the output to another device such as the video screen, a printer, a disk file, or a telephone modem.

STANDARD.INPUT and STANDARD.OUTPUT

Two pre-defined names, STANDARD.INPUT and STANDARD.OUTPUT, control input and output streams in Logo. The values of these names determine the source of the input stream and the destination of the output stream. When Logo starts, the value of both variables is 0, meaning that input comes from the keyboard and output is directed to the computer screen. By changing the value of these names you can re-direct the source of Logo input or the destination of its output.

Other primitives and system names that work with STANDARD.INPUT and STANDARD.OUTPUT to control streams in Logo include:

CLEARINPUT GETBYTE.NO.ECHO READCHAR
CLOSE OPEN READLINE
COPYOFF PEEKBYTE READLIST
COPYON PRINT READQUOTE
CREATE PRINTLINE SHOW
EOF PUTBYTE UNGETBYTE
GETBYTE READ .READ
.WRITE .SEEK

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