Input

Contents

Reading a Complete File

Read(name) : MonStgElt -> MonStgElt
Returns the contents of the file named name as a string. Note that name may be an expression returning a string.
ReadBinary(name) : MonStgElt -> BStgElt
Returns the contents of the file named name as a binary string.

Example IO_Read (H3E9)

In this example we show how Read can be used to import the complete output from a separate C program into a Magma session. We assume that a file mystery.c (the contents of which are shown below) is present in the current directory. We first compile it, from within Magma, and then use it to produce output for the Magma version of our mystery function. 
> Read("mystery.c");
#include <stdio.h>
main(argc, argv)
int     argc;
char    **argv;
{
    int n, i;
    n = atoi(argv[1]);
    for (i = 1; i <= n; i++)
        printf("%d\n", i * i);
    return 0;
}
> System("cc mystery.c -o mystery");
> mysteryMagma := function(n)
>    System("./mystery " cat IntegerToString(n) cat " >outfile");
>    output := Read("outfile");
>    return StringToIntegerSequence(output);
> end function;
> mysteryMagma(5);
[ 1, 4, 9, 16, 25 ]

Reading Command Output

The intrinsics in this section are not currently available on Windows platforms. This will be addressed in a future release.

Pipe(cmd, input) : MonStgElt, MonStgElt -> MonStgElt
Given a shell command cmd and an input string input, run cmd in a new process with input as its input and return the output of this command as a string. Note that for many commands, input should finish with a new line character if it consists of only one line.
Pipe(cmds, inputs) : SeqEnum, SeqEnum -> SeqEnum
Given a sequence of shell commands cmds and a sequence of input strings inputs, run each command in cmds in a new process with the corresponding element of inputs as its input. Returns the sequence of outputs (as strings) of the commands.

Example IO_Pipe (H3E10)

We revisit the mystery program from the earlier example on file reading. Using a pipe means that we no longer need a temporary file. 
> mysteryMagma := function(n)
>    cmd := Sprintf("./mystery %o", n);
>    output := Pipe(cmd, "");
>    return StringToIntegerSequence(output);
> end function;
> mysteryMagma(5);
[ 1, 4, 9, 16, 25 ]

Example IO_Pipes (H3E11)

In this example, we use external commands to apply alphabetic shifts to input words. First, we write a function of n to return the appropriate command to (cyclically) shift each letter forward n places in the alphabet. The UNIX tr command is used for this purpose. 
> letters := "abcdefghijklmnopqrstuvwxyz";
> shiftcmd := func<n | Sprintf("tr 'a-z' '%o-za-%o'", letters[n+1], letters[n])>;
Then we form the commands for shifting by amounts from 1 to 13 and apply them to some well-chosen inputs. 
> cmds := [ shiftcmd(n) : n in [1..13] ];
> inputs :=
> [
>     "steeds", "pyic", "dolt", "pecan", "fizzy", "fusion", "inkier", "talk",
>     "sleep", "cubed", "spots", "road", "nowhere"
> ];
> Pipe(cmds, inputs);
[ tuffet, rake, grow, tiger, kneed, layout, purply, bits, bunny, melon, dazed,
damp, abjurer ]

Interactive Input

read identifier;
read identifier, prompt;
This statement will cause Magma to assign to the given identifier the string of characters appearing (at run-time) on the following line. This allows the user to provide an input string at run-time. If the optional prompt is given (a string), that is printed first.
readi identifier;
readi identifier, prompt;
This statement will cause Magma to assign to the given identifier the literal integer appearing (at run-time) on the following line. This allows the user to specify integer input at run-time. If the optional prompt is given (a string), that is printed first.
V2.28, 13 July 2023