## Learn to Use ITensor

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# Reading input from a file

For code maintainability and convenience, it is good practice to read simulation parameters from an external input file rather than hard-coding them into your program.

To make reading parameters easier, ITensor includes a simple input system. Here is an example:

#include "itensor/all.h"

using namespace itensor;

int main(int argc, char* argv[])
{
if(argc < 2)
{
printfln("Usage: %s input_file",argv[0]);
return 0;
}
auto input = InputGroup(argv[1],"input");

auto N = input.getInt("N");
auto t = input.getReal("t",1.);
auto do_print = input.getYesNo("do_print",false);

// ...

return 0;
}


Including the above code at the top of your main makes the program expect the first command line argument to be the name of an input file, which the InputGroup constructor will open and parse.

For example, if your input file is named input_file, then you'd call your program like

./myprogram input_file


The contents of input_file could be as follows

input
{
N = 100
t = 2.0
do_print = yes
}


The "input" argument to the InputGroup constructor says that the inputs will be grouped in a scope labeled input { ... }.

Once the InputGroup object input is constructed, various parameters are read by calling input.getInt, input.getReal, or input.getYesNo. Each of these methods takes an optional second argument, which specifies the default value of the parameter if it is not provided in the file. If no default is given, the parameter must be provided in the file.

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