Input¶

So far every program has worked on values written into its source. To work on data from outside, a program reads standard input with input and eof, and its command-line arguments with args.

Reading lines¶

input() returns the next line of standard input without its newline. input("prompt ") writes the prompt first, without a newline, for programs someone runs by hand:

name = input("what is your name? ")
print("hello, " + name)

Once the input runs out, input() returns "". An empty line also gives "", so to read every line, loop until eof() says nothing is left:

// reads "name score" lines from stdin and reports the best score
best_name = ""
best = 0.0
count = 0
total = 0.0
while not eof();
    line = input().strip()
    if line == "";
        cont
    fields = line.split()
    score = float(fields[1])
    total = total + score
    count = count + 1
    if count == 1 or score > best;
        best_name = fields[0]
        best = score
print("read " + str(count) + " scores")
print("best:", best_name, best)
print("average:", total / float(count))

Given this input,

ada 91.5
grace 88

  linus   72.25  
margaret 97

it prints

read 4 scores
best: margaret 97.0
average: 87.1875

Run it with the input redirected from a file, or piped from another program:

stone run scores.st < scores.txt
cat scores.txt | stone run scores.st

Turning text into values¶

Everything read is a str, so a few builtins turn text into other values and back:

line.strip() removes spaces, tabs, and line breaks from both ends.
line.split() breaks a line into words at runs of whitespace, and line.split(",") breaks it at each comma, keeping empty fields.
int("42") and float("2.5") read numbers, allowing whitespace around them. Text that is not a number, such as int("4x"), stops the program with an error naming the text, like invalid literal for int() with base 10: '4x'.
str(value) goes the other way, giving the text print would write, so "n = " + str(3) is "n = 3".

Command-line arguments¶

args() returns the arguments given after the program as a list[str], without the program’s own name:

total = 0
for a in args();
    total = total + int(a)
print(total)

stone run sum.st 1 2 3                       # prints 6
stone build sum.st -o sum && ./sum 1 2 3     # prints 6 too

Everything after the file belongs to the program, even arguments starting with -, so stone run sum.st -5 --x gives it ["-5", "--x"].

Next, Errors.