I want to reuse my twig variable for the loop outside the loop, but I can't! I've tried like this
{% set b = '' %}
{% set c = '' %}
{% for i,foo in cout %}
{% set b = cout[i] %}
{{ b }}
{% endfor %}
{% for j,bar in efforts %}
{% set c = efforts[j] %}
{{ c }}
{% endfor %}
{{ b }} {{ c }}
But here it's return me the last value of b and c, so I want all value of c and b outside the loop, thanks!
Yes, you're overwriting variable b & c in every loop, that's why you see only the last one.
You have to use merge function here.
{% set b = [] %}
{% for i,foo in cout %}
{% set b = b|merge([cout[i]]) %}
{% endfor %}
As suggested by DarkBee, to output content of b array as a string you can use join function, iterate through array, or access values by keys or any other function provided by Twig.
The merge filter only works with arrays or "Traversable", got "string" as first argument. whenever I change {% set b = [] %} it's return error An exception has been thrown during the rendering of a template ("Notice: Array to string conversion"). 
{{ b }} to something like {{ b | join(',') }} {{ dump(b) }} it's return the array in console but {{ b }} return error{% set b = b ~' '~ count[i] %}?