Add Making a calculator (unfinished)

Making-a-calculator-%28unfinished%29.md

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+In this tutorial, we will learn basic string manipulation, control flow and other features of Ground by making a simple calculator program.
+
+Our end goal is to create a program that computes an input, for example `33/5`, `6*3`, `4-10`.
+
+## Getting the input
+
+We will first need to get the input from the user. For this we will use:
+
+```ground
+stdout "What would you like to calculate? "
+stdin &expression
+```
+
+This will store the user's input in the variable `expression`.
+
+## Indexing over a string
+We want to go through the string one character at a time, and act accordingly. First, we will create an `idx` (index) variable, and a loop.
+
+```
+stdout "What would you like to calculate? "
+stdin &expression
+set &idx 0
+
+@loop
+    jump %loop
+```
+
+Now, when do we need to exit the loop? Well, if there are n characters in a string, getting the `idx`th character will be out of range when `idx`=n
+
+```
+stdout "What would you like to calculate? "
+stdin &expression
+set &idx 0
+getstrsize $expression &length
+
+@loop
+    equal $idx $length &cond
+    if $cond %end
+    jump %loop
+    
+@end
+```
+
+This will jump to the end when `idx` and `length` are equal, i.e. we have gone through all the characters in the string.
+
+Now, we just need to get the `idx`th character. Luckily this is pretty simple.
+
+```
+stdout "What would you like to calculate? "
+stdin &expression
+set &idx 0
+getstrsize $expression &length
+
+@loop
+    equal $idx $length &cond
+    if $cond %end
+    getstrcharat $expression $idx &idxchar
+    
+    jump %loop
+    
+@end
+```