CS50 2017 Week1

看视频之前，把官方笔记手抄一遍。

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抄了一整天，关闭另一个破网页时，不小心把这个笔记也关了，也没办法恢复内容了。
算了，这网页也不能手动保存笔记。以后不在这儿写了。在Atom写，写完了再直接复制过来。
那我还复制过来干嘛，直接Word写，比Markdown语言不是方便多了。

2017/11/13

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C

• Last week we used Scratch to better understand ideas and concepts in programming, like loops and functions.
• Today, we’ll get an introduction to C, an older language, as another way of familiarizing ourselves with the tool of programming.
• We can start translating our simple programs from Scratch to C, the code of which is written purely in text.
• The say block in Scratch is a function, which the equivalent in C is the following:

printf("Hello, world\n");

• • Notice that the function itself is called printf, and that the arguments, or parameters, to the function are wrapped inside symmetrical parentheses, ( and ).
  • The double quotes, ", are also symmetrical and surround words, or any sequence of characters, in C. We’ll start calling these sequences of characters s**trings**.

  • The line also ends with a semicolon, which new programmers like us need to remember to include, but will come more naturally with practice!

  • To make this a complete program, we can add the following:

#include <stdio.h>int main(void):{    printf("hello, world\n");}

• • Just like how we need the when green flag clicked block in Scratch to start our program, our C program won’t run unless we write a few lines to set it up.
  • We notice that there’s a int main(void) line, and main is the standard name in C to indicate that it is the default function in a program that should be run.
  • The top line is harder to guess, but include is a keyword that indicates we want to include some other file in our program. stdio.h contains (and we only know from searching online and looking at documentation) the standard input/output library, which means that it deals with input (like from the keyboard) and output (printing characters to the screen). In fact, it contains the code of printf that we are using. There is no equivalent in Scratch, since by default the functions are already defined and created for us.

• A forever block can be translated to C like this:

while (true){    printf("hello world\n");}

• • The while keyboard means that the loop will run as long as the Boolean expression inside the parentheses is true. And since true will always be true, the loop will run forever.

• To repeat something a certain number of times, we can use this:

for (int i=0; i < 50; i++){    printf("hello, world\n");}

• • This is a little harder to figure out, but we can go through step by step. for is another keyword in C that indicates a loop.
  • int i = 0 is an initialization of a variable, which means that we created a variable with the name i, of the type int, or integer, and set its initial value to 0. In C, each variable has a type of value.
  • Then i<50 is the Boolean expression that for loop checks, to determine if it will continue or not. Since this condition is true, the for loop will run the printf line. And since we started i at 0, stopping before i reaches 50 will mean this runs exactly 50 times, as we intended.
  • Finally, i++ is an expression in C that add 1 to the value of i. Then, the for loop will check i < 50, and repeat this process until the Boolean expression is no longer true.

• Last time, we also created a nested set of conditions in Scratch:

  

• In C, the equivalent code will like this:

if (x < y){    printf("x is less than y\n");}else if (x > y){    printf("x is bigger than y\n");}else{    printf("x is equal to y\n");}

• 
  
  • In our code, we assume that x and y have already been initialized or set to some other values beforehand.
  • we use if, else if, and else keywords to denote the forks in the road, based on Boolean expressions. else simply captures all the cases that havn’t fit into a previous condition.
  • Notice that curly braces, { and }, are used to wrap the lines of code that we want to run for each of the conditions if they are true. We also use indentation to make the lines of code more readable.

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未完待续