# 0813 Loops
Benson Chiu @ FDCS 8th X 7th
<!-- Put the link to this slide here so people can follow -->
slide: https://hackmd.io/@benson-elementary-cpp/rJkf-SMet
---
## A brief example
Input: An interger $n$
Output: A triangle of $n$ levels
---
## A straightforward approach
```python=
n = int(input())
if n == 1:
print("*")
elif if n == 2:
print("*\n**")
......
```
Problem: Maybe n is 1000 or more, it's probably impossible to type that code in a short time
---
## Introduction: For loop
```python=
for n in container:
block
```
e.g.
```python=
n = [0,1,2,3]
for i in n:
print(i)
```
---
## range function
- range(m,n) -> $[m,n)$
```python=
for i in range(0,10):
print(i,end=" ")
#0 1 2 3 4 5 6 7 8 9
```
---
## Break, continue
- Break: leave the loop immediately
- Continue: Skip to the beginning of the next loop
```python=
for i in range(1,20):
if i == 5:
continue
print(i,end=" ")
if i == 10:
break
#1 2 3 4 6 7 8 9 10
```
---
## Multiple for loops
A classic example: multiplication table
```python=
for i in range(1,10):
for j in range(1,10):
print(i,"*",j,"=",i*j,end='\t')
print("\n")
```
---
---
## Introduction: While loop
```python=
while condition:
codeblock
```
https://www.askpython.com/python/python-while-loop
---
```python=
i = 1
while i <= 5:
print(i,end=" ")
i+=1
#1 2 3 4 5
```
You can also use break and continue in while
```python=
i = 1
while i <= 20:
if i == 5:
i+=1
continue
if i == 10:
break
print(i,end=" ")
i+=1
#1 2 3 4 6 7 8 9
```
---
## Back to the triangle problem
```python=
n = int(input())
for i in range(0,n):
for j in range(0,i+1):
print("*",sep='',end='')
print()
```
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