for(I = 1； I & lt=20; I++) is a cycle. At first, I should be assigned 1, and then whether I is < =20 should be judged. When it is satisfied, the code in curly brackets should be run. After the operation, i++(i variable increases by 1, that is, i = i+ 1, that is, i=2), and then judge whether I is < =20, and run the code in curly braces when it is satisfied. After running, I++run again, and then judge whether I

% 12 1d %d takes shaping as the output, and its corresponding variable is f2. Add 12 1 between% and d for format output, and the output is 12 1 characters.

printf(" \ n ")； Print a carriage return directly, that is, line feed.

f 1 = f 1+F2； F 1 and f2 are two defined variables, and the assignment direction is from right to left, unlike the mathematical operation of 1+ 1=2 from left to right.

That is, first add the values of f 1 and f2 variable (just assigned), and then assign the result to f 1 to overwrite the original value.

Thirdly, f1= f1+F2; In the same way. . . As for why you added this sentence, it depends on what you want to do when you design this program.